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libpng(3)                                                            libpng(3)




NAME

       libpng - Portable Network Graphics (PNG) Reference Library 1.5.18


SYNOPSIS

        #include <png.h>

       png_uint_32 png_access_version_number (void);

       void png_benign_error (png_structp png_ptr, png_const_charp error);

       void png_build_grayscale_palette (int bit_depth, png_colorp palette);

       png_voidp png_calloc (png_structp png_ptr, png_alloc_size_t size);

       void   png_chunk_benign_error   (png_structp  png_ptr,  png_const_charp
       error);

       void png_chunk_error (png_structp png_ptr, png_const_charp error);

       void png_chunk_warning (png_structp png_ptr, png_const_charp message);

       void png_convert_from_struct_tm  (png_timep  ptime,  struct  tm  FAR  *
       ttime);

       void png_convert_from_time_t (png_timep ptime, time_t ttime);

       png_charp   png_convert_to_rfc1123   (png_structp   png_ptr,  png_timep
       ptime);

       png_infop png_create_info_struct (png_structp png_ptr);

       png_structp   png_create_read_struct   (png_const_charp   user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp   png_create_read_struct_2  (png_const_charp  user_png_ver,
       png_voidp error_ptr,  png_error_ptr  error_fn,  png_error_ptr  warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       png_structp   png_create_write_struct   (png_const_charp  user_png_ver,
       png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

       png_structp  png_create_write_struct_2  (png_const_charp  user_png_ver,
       png_voidp  error_ptr,  png_error_ptr  error_fn,  png_error_ptr warn_fn,
       png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

       void  png_data_freer  (png_structp  png_ptr,  png_infop  info_ptr,  int
       freer, png_uint_32 mask));

       void    png_destroy_info_struct    (png_structp   png_ptr,   png_infopp
       info_ptr_ptr);

       void  png_destroy_read_struct  (png_structpp  png_ptr_ptr,   png_infopp
       info_ptr_ptr, png_infopp end_info_ptr_ptr);

       void  png_destroy_write_struct  (png_structpp  png_ptr_ptr,  png_infopp
       info_ptr_ptr);

       void png_err (png_structp png_ptr);

       void png_error (png_structp png_ptr, png_const_charp error);

       void png_free (png_structp png_ptr, png_voidp ptr);

       void png_free_chunk_list (png_structp png_ptr);

       void png_free_default (png_structp png_ptr, png_voidp ptr);

       void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);

       png_byte png_get_bit_depth (png_const_structp png_ptr,  png_const_infop
       info_ptr);

       png_uint_32    png_get_bKGD   (png_const_structp   png_ptr,   png_infop
       info_ptr, png_color_16p *background);

       png_byte png_get_channels (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32  png_get_cHRM  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double  *white_x,  double  *white_y,  double  *red_x,  double
       *red_y,  double  *green_x,  double  *green_y,  double  *blue_x,  double
       *blue_y);

       png_uint_32     png_get_cHRM_fixed     (png_const_structp      png_ptr,
       png_const_infop  info_ptr,  png_uint_32 *white_x, png_uint_32 *white_y,
       png_uint_32   *red_x,   png_uint_32   *red_y,   png_uint_32   *green_x,
       png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);

       png_uint_32   png_get_cHRM_XYZ  (png_structp  png_ptr,  png_const_infop
       info_ptr, double *red_X, double *red_Y, double *red_Z, double *green_X,
       double  *green_Y, double *green_Z, double *blue_X, double *blue_Y, dou-
       ble *blue_Z);

       png_uint_32      png_get_cHRM_XYZ_fixed      (png_structp      png_ptr,
       png_const_infop  info_ptr,  png_fixed_point *int_red_X, png_fixed_point
       *int_red_Y, png_fixed_point *int_red_Z,  png_fixed_point  *int_green_X,
       png_fixed_point     *int_green_Y,     png_fixed_point     *int_green_Z,
       png_fixed_point     *int_blue_X,      png_fixed_point      *int_blue_Y,
       png_fixed_point *int_blue_Z);

       png_uint_32 png_get_chunk_cache_max (png_const_structp png_ptr);

       png_alloc_size_t png_get_chunk_malloc_max (png_const_structp png_ptr);

       png_byte png_get_color_type (png_const_structp png_ptr, png_const_infop
       info_ptr);

       png_uint_32     png_get_compression_buffer_size      (png_const_structp
       png_ptr);

       png_byte     png_get_compression_type    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_byte png_get_copyright (png_const_structp png_ptr);

       png_uint_32 png_get_current_row_number (png_const_structp);

       png_byte png_get_current_pass_number (png_const_structp);

       png_voidp png_get_error_ptr (png_const_structp png_ptr);

       png_byte      png_get_filter_type      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_gAMA  (png_const_structp  png_ptr, png_const_infop
       info_ptr, double *file_gamma);

       png_uint_32     png_get_gAMA_fixed     (png_const_structp      png_ptr,
       png_const_infop info_ptr, png_uint_32 *int_file_gamma);

       png_byte png_get_header_ver (png_const_structp png_ptr);

       png_byte png_get_header_version (png_const_structp png_ptr);

       png_uint_32  png_get_hIST  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_uint_16p *hist);

       png_uint_32 png_get_iCCP  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charpp  name, int *compression_type, png_bytepp profile,
       png_uint_32 *proflen);

       png_uint_32  png_get_IHDR  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32   *width,   png_uint_32   *height,   int   *bit_depth,  int
       *color_type, int  *interlace_type,  int  *compression_type,  int  *fil-
       ter_type);

       png_uint_32     png_get_image_height     (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32     png_get_image_width     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_int_32 png_get_int_32 (png_bytep buf);

       png_byte     png_get_interlace_type     (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_const_bytep png_get_io_chunk_name (png_structp png_ptr);

       png_uint_32 png_get_io_chunk_type (png_const_structp png_ptr);

       png_voidp png_get_io_ptr (png_structp png_ptr);

       png_uint_32 png_get_io_state (png_structp png_ptr);

       png_byte png_get_libpng_ver (png_const_structp png_ptr);

       png_voidp png_get_mem_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_oFFs  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,    png_uint_32    *offset_x,   png_uint_32   *offset_y,   int
       *unit_type);

       png_uint_32 png_get_pCAL  (png_const_structp  png_ptr,  png_const_infop
       info_ptr,  png_charp  *purpose,  png_int_32  *X0,  png_int_32  *X1, int
       *type, int *nparams, png_charp *units, png_charpp *params);

       png_uint_32 png_get_pHYs  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

       float     png_get_pixel_aspect_ratio     (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32      png_get_pHYs_dpi      (png_const_structp      png_ptr,
       png_const_infop  info_ptr,  png_uint_32 *res_x, png_uint_32 *res_y, int
       *unit_type);

       png_fixed_point   png_get_pixel_aspect_ratio_fixed   (png_const_structp
       png_ptr, png_const_infop info_ptr);

       png_uint_32    png_get_pixels_per_inch    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_pixels_per_meter   (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_voidp png_get_progressive_ptr (png_const_structp png_ptr);

       png_uint_32  png_get_PLTE  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_colorp *palette, int *num_palette);

       png_byte png_get_rgb_to_gray_status (png_const_structp png_ptr);

       png_uint_32      png_get_rowbytes      (png_const_structp      png_ptr,
       png_const_infop info_ptr);

       png_bytepp  png_get_rows  (png_const_structp  png_ptr,  png_const_infop
       info_ptr);

       png_uint_32   png_get_sBIT   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_color_8p *sig_bit);

       void png_get_sCAL (png_const_structp png_ptr, png_const_infop info_ptr,
       int* unit, double* width, double* height);

       void  png_get_sCAL_fixed  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int* unit, png_fixed_pointp width, png_fixed_pointp height);

       void   png_get_sCAL_s   (png_const_structp   png_ptr,   png_const_infop
       info_ptr, int* unit, png_charpp width, png_charpp height);

       png_bytep  png_get_signature  (png_const_structp   png_ptr,   png_infop
       info_ptr);

       png_uint_32  png_get_sPLT  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_spalette_p *splt_ptr);

       png_uint_32 png_get_sRGB  (png_const_structp  png_ptr,  png_const_infop
       info_ptr, int *file_srgb_intent);

       png_uint_32  png_get_text  (png_const_structp  png_ptr, png_const_infop
       info_ptr, png_textp *text_ptr, int *num_text);

       png_uint_32   png_get_tIME   (png_const_structp   png_ptr,    png_infop
       info_ptr, png_timep *mod_time);

       png_uint_32    png_get_tRNS   (png_const_structp   png_ptr,   png_infop
       info_ptr,  png_bytep  *trans_alpha,   int   *num_trans,   png_color_16p
       *trans_color);

       /* This function is really an inline macro. */

       png_uint_16 png_get_uint_16 (png_bytep buf);

       png_uint_32 png_get_uint_31 (png_structp png_ptr, png_bytep buf);

       /* This function is really an inline macro. */

       png_uint_32 png_get_uint_32 (png_bytep buf);

       png_uint_32    png_get_unknown_chunks    (png_const_structp    png_ptr,
       png_const_infop info_ptr, png_unknown_chunkpp unknowns);

       png_voidp png_get_user_chunk_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_height_max (png_const_structp png_ptr);

       png_voidp png_get_user_transform_ptr (png_const_structp png_ptr);

       png_uint_32 png_get_user_width_max (png_const_structp png_ptr);

       png_uint_32 png_get_valid (png_const_structp  png_ptr,  png_const_infop
       info_ptr, png_uint_32 flag);

       float      png_get_x_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point  png_get_x_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_x_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_x_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_x_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       float      png_get_y_offset_inches      (png_const_structp     png_ptr,
       png_const_infop info_ptr);

       png_fixed_point  png_get_y_offset_inches_fixed  (png_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_microns    (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_int_32    png_get_y_offset_pixels    (png_const_structp    png_ptr,
       png_const_infop info_ptr);

       png_uint_32   png_get_y_pixels_per_inch   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       png_uint_32  png_get_y_pixels_per_meter   (png_const_structp   png_ptr,
       png_const_infop info_ptr);

       int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

       void      png_info_init_3      (png_infopp     info_ptr,     png_size_t
       png_info_struct_size);

       void png_init_io (png_structp png_ptr, FILE *fp);

       void png_longjmp (png_structp png_ptr, int val);

       png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

       png_voidp  png_malloc_default  (png_structp  png_ptr,  png_alloc_size_t
       size);

       png_voidp png_malloc_warn (png_structp png_ptr, png_alloc_size_t size);

       png_uint_32 png_permit_mng_features (png_structp  png_ptr,  png_uint_32
       mng_features_permitted);

       void   png_process_data   (png_structp   png_ptr,  png_infop  info_ptr,
       png_bytep buffer, png_size_t buffer_size);

       png_size_t png_process_data_pause (png_structp, int save);

       png_uint_32 png_process_data_skip (png_structp);

       void  png_progressive_combine_row   (png_structp   png_ptr,   png_bytep
       old_row, png_bytep new_row);

       void png_read_end (png_structp png_ptr, png_infop info_ptr);

       void png_read_image (png_structp png_ptr, png_bytepp image);

       void png_read_info (png_structp png_ptr, png_infop info_ptr);

       void  png_read_png (png_structp png_ptr, png_infop info_ptr, int trans-
       forms, png_voidp params);

       void png_read_row (png_structp png_ptr, png_bytep row,  png_bytep  dis-
       play_row);

       void  png_read_rows  (png_structp  png_ptr,  png_bytepp row, png_bytepp
       display_row, png_uint_32 num_rows);

       void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

       int png_reset_zstream (png_structp png_ptr);

       void png_save_int_32 (png_bytep buf, png_int_32 i);

       void png_save_uint_16 (png_bytep buf, unsigned int i);

       void png_save_uint_32 (png_bytep buf, png_uint_32 i);

       void png_set_add_alpha (png_structp png_ptr,  png_uint_32  filler,  int
       flags);

       void  png_set_alpha_mode  (png_structp  png_ptr,  int mode, double out-
       put_gamma);

       void   png_set_alpha_mode_fixed   (png_structp   png_ptr,   int   mode,
       png_fixed_point output_gamma);

       void   png_set_background  (png_structp  png_ptr,  png_color_16p  back-
       ground_color, int background_gamma_code, int need_expand, double  back-
       ground_gamma);

       void png_set_background_fixed (png_structp png_ptr, png_color_16p back-
       ground_color, int background_gamma_code, int  need_expand,  png_uint_32
       background_gamma);

       void png_set_benign_errors (png_structp png_ptr, int allowed);

       void png_set_bgr (png_structp png_ptr);

       void    png_set_bKGD    (png_structp   png_ptr,   png_infop   info_ptr,
       png_color_16p background);

       void png_set_check_for_invalid_index(png_structp png_ptr, int allowed);

       void  png_set_cHRM  (png_structp  png_ptr,  png_infop  info_ptr, double
       white_x, double white_y, double red_x, double  red_y,  double  green_x,
       double green_y, double blue_x, double blue_y);

       void   png_set_cHRM_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32   white_x,   png_uint_32   white_y,   png_uint_32    red_x,
       png_uint_32    red_y,   png_uint_32   green_x,   png_uint_32   green_y,
       png_uint_32 blue_x, png_uint_32 blue_y);

       void png_set_cHRM_XYZ (png_structp png_ptr, png_infop info_ptr,  double
       red_X, double red_Y, double red_Z, double green_X, double green_Y, dou-
       ble green_Z, double blue_X, double blue_Y, double blue_Z);

       void png_set_cHRM_XYZ_fixed (png_structp png_ptr,  png_infop  info_ptr,
       png_fixed_point  int_red_X,  png_fixed_point int_red_Y, png_fixed_point
       int_red_Z, png_fixed_point  int_green_X,  png_fixed_point  int_green_Y,
       png_fixed_point      int_green_Z,      png_fixed_point      int_blue_X,
       png_fixed_point int_blue_Y, png_fixed_point int_blue_Z);

       void   png_set_chunk_cache_max   (png_structp   png_ptr,    png_uint_32
       user_chunk_cache_max);

       void png_set_compression_level (png_structp png_ptr, int level);

       void    png_set_compression_mem_level    (png_structp    png_ptr,   int
       mem_level);

       void png_set_compression_method (png_structp png_ptr, int method);

       void png_set_compression_strategy (png_structp png_ptr, int strategy);

       void png_set_compression_window_bits  (png_structp  png_ptr,  int  win-
       dow_bits);

       void  png_set_crc_action  (png_structp  png_ptr,  int  crit_action, int
       ancil_action);

       void  png_set_error_fn  (png_structp  png_ptr,   png_voidp   error_ptr,
       png_error_ptr error_fn, png_error_ptr warning_fn);

       void png_set_expand (png_structp png_ptr);

       void png_set_expand_16 (png_structp png_ptr);

       void png_set_expand_gray_1_2_4_to_8 (png_structp png_ptr);

       void  png_set_filler  (png_structp  png_ptr,  png_uint_32  filler,  int
       flags);

       void png_set_filter (png_structp png_ptr, int method, int filters);

       void  png_set_filter_heuristics  (png_structp  png_ptr,   int   heuris-
       tic_method,  int  num_weights,  png_doublep filter_weights, png_doublep
       filter_costs);

       void png_set_filter_heuristics_fixed (png_structp png_ptr, int  heuris-
       tic_method,    int   num_weights,   png_fixed_point_p   filter_weights,
       png_fixed_point_p filter_costs);

       void png_set_flush (png_structp png_ptr, int nrows);

       void png_set_gamma (png_structp png_ptr,  double  screen_gamma,  double
       default_file_gamma);

       void     png_set_gamma_fixed    (png_structp    png_ptr,    png_uint_32
       screen_gamma, png_uint_32 default_file_gamma);

       void png_set_gAMA  (png_structp  png_ptr,  png_infop  info_ptr,  double
       file_gamma);

       void   png_set_gAMA_fixed  (png_structp  png_ptr,  png_infop  info_ptr,
       png_uint_32 file_gamma);

       void png_set_gray_1_2_4_to_8 (png_structp png_ptr);

       void png_set_gray_to_rgb (png_structp png_ptr);

       void   png_set_hIST   (png_structp   png_ptr,    png_infop    info_ptr,
       png_uint_16p hist);

       void    png_set_iCCP    (png_structp   png_ptr,   png_infop   info_ptr,
       png_const_charp name, int  compression_type,  png_const_bytep  profile,
       png_uint_32 proflen);

       int png_set_interlace_handling (png_structp png_ptr);

       void  png_set_invalid  (png_structp  png_ptr,  png_infop  info_ptr, int
       mask);

       void png_set_invert_alpha (png_structp png_ptr);

       void png_set_invert_mono (png_structp png_ptr);

       void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32
       width,  png_uint_32  height,  int bit_depth, int color_type, int inter-
       lace_type, int compression_type, int filter_type);

       void  png_set_keep_unknown_chunks  (png_structp  png_ptr,   int   keep,
       png_bytep chunk_list, int num_chunks);

       jmp_buf*   png_set_longjmp_fn   (png_structp  png_ptr,  png_longjmp_ptr
       longjmp_fn, size_t jmp_buf_size);

       void png_set_chunk_malloc_max  (png_structp  png_ptr,  png_alloc_size_t
       user_chunk_cache_max);

       void  png_set_compression_buffer_size (png_structp png_ptr, png_uint_32
       size);

       void png_set_mem_fn (png_structp png_ptr, png_voidp  mem_ptr,  png_mal-
       loc_ptr malloc_fn, png_free_ptr free_fn);

       void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       offset_x, png_uint_32 offset_y, int unit_type);

       void png_set_packing (png_structp png_ptr);

       void png_set_packswap (png_structp png_ptr);

       void png_set_palette_to_rgb (png_structp png_ptr);

       void png_set_pCAL (png_structp png_ptr, png_infop  info_ptr,  png_charp
       purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp
       units, png_charpp params);

       void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32
       res_x, png_uint_32 res_y, int unit_type);

       void  png_set_progressive_read_fn  (png_structp png_ptr, png_voidp pro-
       gressive_ptr, png_progressive_info_ptr info_fn, png_progressive_row_ptr
       row_fn, png_progressive_end_ptr end_fn);

       void  png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp
       palette, int num_palette);

       void png_set_quantize (png_structp  png_ptr,  png_colorp  palette,  int
       num_palette, int maximum_colors, png_uint_16p histogram, int full_quan-
       tize);

       void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
       read_data_fn);

       void  png_set_read_status_fn  (png_structp png_ptr, png_read_status_ptr
       read_row_fn);

       void   png_set_read_user_chunk_fn   (png_structp   png_ptr,   png_voidp
       user_chunk_ptr, png_user_chunk_ptr read_user_chunk_fn);

       void      png_set_read_user_transform_fn      (png_structp     png_ptr,
       png_user_transform_ptr read_user_transform_fn);

       void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double
       red, double green);

       void  png_set_rgb_to_gray_fixed  (png_structp png_ptr, int error_action
       png_uint_32 red, png_uint_32 green);

       void png_set_rows (png_structp png_ptr, png_infop info_ptr,  png_bytepp
       row_pointers);

       void    png_set_sBIT    (png_structp   png_ptr,   png_infop   info_ptr,
       png_color_8p sig_bit);

       void png_set_sCAL (png_structp png_ptr, png_infop info_ptr,  int  unit,
       double width, double height);

       void  png_set_sCAL_fixed  (png_structp png_ptr, png_infop info_ptr, int
       unit, png_fixed_point width, png_fixed_point height);

       void png_set_sCAL_s (png_structp png_ptr, png_infop info_ptr, int unit,
       png_charp width, png_charp height);

       void png_set_scale_16 (png_structp png_ptr);

       void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

       void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

       void    png_set_sPLT    (png_structp   png_ptr,   png_infop   info_ptr,
       png_spalette_p splt_ptr, int num_spalettes);

       void  png_set_sRGB  (png_structp  png_ptr,  png_infop   info_ptr,   int
       srgb_intent);

       void   png_set_sRGB_gAMA_and_cHRM   (png_structp   png_ptr,   png_infop
       info_ptr, int srgb_intent);

       void png_set_strip_16 (png_structp png_ptr);

       void png_set_strip_alpha (png_structp png_ptr);

       void  png_set_strip_error_numbers  (png_structp  png_ptr,   png_uint_32
       strip_mode);

       void png_set_swap (png_structp png_ptr);

       void png_set_swap_alpha (png_structp png_ptr);

       void  png_set_text  (png_structp png_ptr, png_infop info_ptr, png_textp
       text_ptr, int num_text);

       void png_set_text_compression_level (png_structp png_ptr, int level);

       void  png_set_text_compression_mem_level  (png_structp   png_ptr,   int
       mem_level);

       void png_set_text_compression_strategy (png_structp png_ptr, int strat-
       egy);

       void  png_set_text_compression_window_bits  (png_structp  png_ptr,  int
       window_bits);

       void   png_set_text_compression_method,   (png_structp   png_ptr,   int
       method));

       void png_set_tIME (png_structp png_ptr, png_infop  info_ptr,  png_timep
       mod_time);

       void  png_set_tRNS  (png_structp png_ptr, png_infop info_ptr, png_bytep
       trans_alpha, int num_trans, png_color_16p trans_color);

       void png_set_tRNS_to_alpha (png_structp png_ptr);

       png_uint_32  png_set_unknown_chunks  (png_structp  png_ptr,   png_infop
       info_ptr, png_unknown_chunkp unknowns, int num, int location);

       void  png_set_unknown_chunk_location  (png_structp  png_ptr,  png_infop
       info_ptr, int chunk, int location);

       void    png_set_user_limits    (png_structp    png_ptr,     png_uint_32
       user_width_max, png_uint_32 user_height_max);

       void   png_set_user_transform_info   (png_structp   png_ptr,  png_voidp
       user_transform_ptr, int user_transform_depth, int  user_transform_chan-
       nels);

       void   png_set_write_fn   (png_structp   png_ptr,   png_voidp   io_ptr,
       png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

       void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
       write_row_fn);

       void      png_set_write_user_transform_fn     (png_structp     png_ptr,
       png_user_transform_ptr write_user_transform_fn);

       int  png_sig_cmp   (png_bytep   sig,   png_size_t   start,   png_size_t
       num_to_check);

       void png_start_read_image (png_structp png_ptr);

       void png_warning (png_structp png_ptr, png_const_charp message);

       void   png_write_chunk   (png_structp  png_ptr,  png_bytep  chunk_name,
       png_bytep data, png_size_t length);

       void  png_write_chunk_data  (png_structp   png_ptr,   png_bytep   data,
       png_size_t length);

       void png_write_chunk_end (png_structp png_ptr);

       void  png_write_chunk_start (png_structp png_ptr, png_bytep chunk_name,
       png_uint_32 length);

       void png_write_end (png_structp png_ptr, png_infop info_ptr);

       void png_write_flush (png_structp png_ptr);

       void png_write_image (png_structp png_ptr, png_bytepp image);

       void png_write_info (png_structp png_ptr, png_infop info_ptr);

       void   png_write_info_before_PLTE   (png_structp   png_ptr,   png_infop
       info_ptr);

       void png_write_png (png_structp png_ptr, png_infop info_ptr, int trans-
       forms, png_voidp params);

       void png_write_row (png_structp png_ptr, png_bytep row);

       void png_write_rows (png_structp png_ptr, png_bytepp  row,  png_uint_32
       num_rows);

       void png_write_sig (png_structp png_ptr);



DESCRIPTION

       The  libpng  library supports encoding, decoding, and various manipula-
       tions of the Portable Network Graphics (PNG) format  image  files.   It
       uses  the  zlib(3)  compression  library.   Following  is a copy of the
       libpng-manual.txt file that accompanies libpng.


LIBPNG.TXT

       Libpng-manual.txt - A description on how to use and modify libpng

        libpng version 1.5.18 - February 6, 2014
        Updated and distributed by Glenn Randers-Pehrson
        <glennrp at users.sourceforge.net>
        Copyright (c) 1998-2014 Glenn Randers-Pehrson

        This document is released under the libpng license.
        For conditions of distribution and use, see the disclaimer
        and license in png.h

        Based on:

        libpng versions 0.97, January 1998, through 1.5.18 - February 6, 2014
        Updated and distributed by Glenn Randers-Pehrson
        Copyright (c) 1998-2014 Glenn Randers-Pehrson

        libpng 1.0 beta 6  version 0.96 May 28, 1997
        Updated and distributed by Andreas Dilger
        Copyright (c) 1996, 1997 Andreas Dilger

        libpng 1.0 beta 2 - version 0.88  January 26, 1996
        For conditions of distribution and use, see copyright
        notice in png.h. Copyright (c) 1995, 1996 Guy Eric
        Schalnat, Group 42, Inc.

        Updated/rewritten per request in the libpng FAQ
        Copyright (c) 1995, 1996 Frank J. T. Wojcik
        December 18, 1995 & January 20, 1996



I. Introduction

       This file describes how to use and modify  the  PNG  reference  library
       (known  as  libpng)  for your own use.  In addition to this file, exam-
       ple.c is a good starting point for using the library, as it is  heavily
       commented  and  should  include  everything  most people will need.  We
       assume that libpng is already  installed;  see  the  INSTALL  file  for
       instructions on how to install libpng.

       For  examples  of libpng usage, see the files "example.c", "pngtest.c",
       and the files in the "contrib" directory, all of which are included  in
       the libpng distribution.

       Libpng was written as a companion to the PNG specification, as a way of
       reducing the amount of time and effort it takes to support the PNG file
       format in application programs.

       The  PNG specification (second edition), November 2003, is available as
       a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E)) at
       <http://www.w3.org/TR/2003/REC-PNG-20031110/  The W3C and ISO documents
       have identical technical content.

       The       PNG-1.2       specification       is       available       at
       <http://www.libpng.org/pub/png/documents/>.   It is technically equiva-
       lent to the PNG specification (second edition) but has some  additional
       material.

       The    PNG-1.0    specification    is    available    as    RFC    2083
       <http://www.libpng.org/pub/png/documents/> and as a W3C  Recommendation
       <http://www.w3.org/TR/REC.png.html>.

       Some  additional  chunks  are  described  in the special-purpose public
       chunks documents at <http://www.libpng.org/pub/png/documents/>.

       Other information about PNG, and the latest version of libpng,  can  be
       found at the PNG home page, <http://www.libpng.org/pub/png/>.

       Most  users will not have to modify the library significantly; advanced
       users may want to modify it more.  All attempts were made to make it as
       complete  as possible, while keeping the code easy to understand.  Cur-
       rently, this library only supports C.  Support for other  languages  is
       being considered.

       Libpng has been designed to handle multiple sessions at one time, to be
       easily modifiable, to be portable to  the  vast  majority  of  machines
       (ANSI,  K&R,  16-,  32-,  and 64-bit) available, and to be easy to use.
       The ultimate goal of libpng is to promote the  acceptance  of  the  PNG
       file  format in whatever way possible.  While there is still work to be
       done (see the TODO file), libpng should cover the majority of the needs
       of its users.

       Libpng  uses  zlib  for its compression and decompression of PNG files.
       Further information about zlib, and the latest version of zlib, can  be
       found      at     the     zlib     home     page,     <http://www.info-
       zip.org/pub/infozip/zlib/>.  The zlib compression utility is a  general
       purpose utility that is useful for more than PNG files, and can be used
       without libpng.  See the documentation delivered  with  zlib  for  more
       details.   You  can  usually find the source files for the zlib utility
       wherever you find the libpng source files.

       Libpng is  thread  safe,  provided  the  threads  are  using  different
       instances   of  the  structures.   Each  thread  should  have  its  own
       png_struct and png_info instances, and thus its own image.  Libpng does
       not  protect  itself  against  two threads using the same instance of a
       structure.



II. Structures

       There are two main structures that are important to libpng,  png_struct
       and  png_info.  Both are internal structures that are no longer exposed
       in the libpng interface (as of libpng 1.5.0).

       The png_info structure is designed to provide information about the PNG
       file.  At one time, the fields of png_info were intended to be directly
       accessible to the user.  However, this tended to  cause  problems  with
       applications  using dynamically loaded libraries, and as a result a set
       of interface functions for png_info (the  png_get_*()  and  png_set_*()
       functions)  was developed, and direct access to the png_info fields was
       deprecated..

       The png_struct structure is the object used by the library to decode  a
       single image.  As of 1.5.0 this structure is also not exposed.

       Almost  all  libpng APIs require a pointer to a png_struct as the first
       argument.  Many (in particular  the  png_set  and  png_get  APIs)  also
       require a pointer to png_info as the second argument.  Some application
       visible macros defined in png.h designed for basic data access (reading
       and  writing integers in the PNG format) don't take a png_info pointer,
       but it's almost always safe to assume that a (png_struct*)  has  to  be
       passed to call an API function.

       You can have more than one png_info structure associated with an image,
       as illustrated in pngtest.c, one for information  valid  prior  to  the
       IDAT  chunks  and  another  (called  "end_info" below) for things after
       them.

       The png.h header file is an invaluable reference for  programming  with
       libpng.   And  while I'm on the topic, make sure you include the libpng
       header file:

       #include <png.h>

       and also (as of libpng-1.5.0) the zlib header file, if you need it:

       #include <zlib.h>


   Types
       The png.h header file defines a number of integral types  used  by  the
       APIs.   Most of these are fairly obvious; for example types correspond-
       ing to integers of particular sizes and types for passing color values.

       One exception is how non-integral numbers are handled.  For application
       convenience most APIs that take such numbers have C (double) arguments;
       however,  internally  PNG,  and  libpng, use 32 bit signed integers and
       encode the value by multiplying by 100,000.  As of libpng 1.5.0 a  con-
       venience  macro  PNG_FP_1  is  defined  in  png.h  along  with  a  type
       (png_fixed_point) which is simply (png_int_32).

       All APIs that take (double) arguments also have  a  matching  API  that
       takes the corresponding fixed point integer arguments.  The fixed point
       API has the same name as the floating point one with "_fixed" appended.
       The  actual  range  of  values permitted in the APIs is frequently less
       than the full range of (png_fixed_point) (-21474 to +21474).  When APIs
       require  a  non-negative  argument  the type is recorded as png_uint_32
       above.  Consult the header file and the text below  for  more  informa-
       tion.

       Special  care  must  be take with sCAL chunk handling because the chunk
       itself uses non-integral values encoded as strings  containing  decimal
       floating point numbers.  See the comments in the header file.


   Configuration
       The  main header file function declarations are frequently protected by
       C preprocessing directives of the form:

           #ifdef PNG_feature_SUPPORTED
           declare-function
           #endif
           ...
           #ifdef PNG_feature_SUPPORTED
           use-function
           #endif

       The library can be built without support for  these  APIs,  although  a
       standard  build  will  have all implemented APIs.  Application programs
       should check the feature macros before using an API for maximum  porta-
       bility.   From  libpng 1.5.0 the feature macros set during the build of
       libpng are recorded in the header file "pnglibconf.h" and this file  is
       always included by png.h.

       If you don't need to change the library configuration from the default,
       skip to the next section ("Reading").

       Notice that some of the makefiles in the 'scripts'  directory  and  (in
       1.5.0)  all of the build project files in the 'projects' directory sim-
       ply copy scripts/pnglibconf.h.prebuilt  to  pnglibconf.h.   This  means
       that  these  build systems do not permit easy auto-configuration of the
       library - they only support the default configuration.

       The easiest way to make minor changes to the libpng configuration  when
       auto-configuration  is  supported  is to add definitions to the command
       line using (typically) CPPFLAGS.  For example:

       CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

       will change the internal libpng math implementation for  gamma  correc-
       tion  and  other  arithmetic  calculations to fixed point, avoiding the
       need for fast floating point support.  The result can be  seen  in  the
       generated  pnglibconf.h  -  make  sure  it contains the changed feature
       macro setting.

       If you need to make more extensive configuration changes  -  more  than
       one  or two feature macro settings - you can either add -DPNG_USER_CON-
       FIG to the build command line and put a list of feature macro  settings
       in  pngusr.h  or  you  can set DFA_XTRA (a makefile variable) to a file
       containing the same information in the form of 'option' settings.

       A. Changing pnglibconf.h

       A variety of methods exist to build libpng.  Not all of  these  support
       reconfiguration  of  pnglibconf.h.  To reconfigure pnglibconf.h it must
       either be rebuilt from scripts/pnglibconf.dfa using awk or it  must  be
       edited by hand.

       Hand  editing  is  achieved by copying scripts/pnglibconf.h.prebuilt to
       pnglibconf.h and changing the lines defining  the  supported  features,
       paying   very   close   attention   to   the  'option'  information  in
       scripts/pnglibconf.dfa that describes those features and their require-
       ments.  This is easy to get wrong.

       B. Configuration using DFA_XTRA

       Rebuilding  from  pnglibconf.dfa  is  easy if a functioning 'awk', or a
       later variant such as 'nawk' or 'gawk', is  available.   The  configure
       build  will  automatically  find  an  appropriate awk and build pnglib-
       conf.h.  The scripts/pnglibconf.mak file contains a set of  make  rules
       for  doing  the  same  thing  if configure is not used, and many of the
       makefiles in the scripts directory use this approach.

       When rebuilding simply write a new file containing changed options  and
       set DFA_XTRA to the name of this file.  This causes the build to append
       the new file to the end of scripts/pnglibconf.dfa.  The pngusr.dfa file
       should contain lines of the following forms:

       everything = off

       This  turns  all  optional  features  off.   Include it at the start of
       pngusr.dfa to make it easier to build  a  minimal  configuration.   You
       will  need to turn at least some features on afterward to enable either
       reading or writing code, or both.

       option feature on option feature off

       Enable or disable a single feature.   This  will  automatically  enable
       other features required by a feature that is turned on or disable other
       features that require a feature which is turned off.  Conflicting  set-
       tings will cause an error message to be emitted by awk.

       setting feature default value

       Changes the default value of setting 'feature' to 'value'.  There are a
       small number of settings listed at the top of  pnglibconf.h,  they  are
       documented  in  the source code.  Most of these values have performance
       implications for the library but most of them have no visible effect on
       the API.  Some can also be overridden from the API.

       This  method  of  building  a customized pnglibconf.h is illustrated in
       contrib/pngminim/*.  See the "$(PNGCONF):" target in the  makefile  and
       pngusr.dfa in these directories.

       C. Configuration using PNG_USR_CONFIG

       If  -DPNG_USR_CONFIG  is added to the CFLAGS when pnglibconf.h is built
       the file pngusr.h will automatically be included before the options  in
       scripts/pnglibconf.dfa  are  processed.  Your pngusr.h file should con-
       tain only macro definitions turning features on or off or setting  set-
       tings.

       Apart from the global setting "everything = off" all the options listed
       above can be set using macros in pngusr.h:

       #define PNG_feature_SUPPORTED

       is equivalent to:

       option feature on

       #define PNG_NO_feature

       is equivalent to:

       option feature off

       #define PNG_feature value

       is equivalent to:

       setting feature default value

       Notice that in both cases, pngusr.dfa and pngusr.h, the contents of the
       pngusr file you supply override the contents of scripts/pnglibconf.dfa

       If  confusing  or  incomprehensible  behavior results it is possible to
       examine the intermediate file pnglibconf.dfn to find the  full  set  of
       dependency  information for each setting and option.  Simply locate the
       feature in the file and read the C comments that precede it.

       This method is also illustrated in the contrib/pngminim/* makefiles and
       pngusr.h.



III. Reading

       We'll  now walk you through the possible functions to call when reading
       in a PNG file sequentially, briefly explaining the syntax  and  purpose
       of  each one.  See example.c and png.h for more detail.  While progres-
       sive reading is covered in the next section, you will still  need  some
       of the functions discussed in this section to read a PNG file.


   Setup
       You  will  want  to  do  the  I/O initialization(*) before you get into
       libpng, so if it doesn't work, you don't have much to undo.  Of course,
       you  will also want to insure that you are, in fact, dealing with a PNG
       file.  Libpng provides a simple check to see if a file is a  PNG  file.
       To  use  it, pass in the first 1 to 8 bytes of the file to the function
       png_sig_cmp(), and it will return 0 (false) if the bytes match the cor-
       responding bytes of the PNG signature, or nonzero (true) otherwise.  Of
       course, the more bytes you pass in, the greater  the  accuracy  of  the
       prediction.

       If  you  are intending to keep the file pointer open for use in libpng,
       you must ensure you don't read more than 8 bytes from the beginning  of
       the  file, and you also have to make a call to png_set_sig_bytes_read()
       with the number of bytes you read from the beginning.  Libpng will then
       only check the bytes (if any) that your program didn't read.

       (*):  If you are not using the standard I/O functions, you will need to
       replace them with custom functions.  See the discussion under Customiz-
       ing libpng.


           FILE *fp = fopen(file_name, "rb");
           if (!fp)
           {
              return (ERROR);
           }

           fread(header, 1, number, fp);
           is_png = !png_sig_cmp(header, 0, number);

           if (!is_png)
           {
              return (NOT_PNG);
           }


       Next, png_struct and png_info need to be allocated and initialized.  In
       order to ensure that the size of these structures is correct even  with
       a  dynamically  linked  libpng,  there  are functions to initialize and
       allocate the structures.  We also pass the  library  version,  optional
       pointers  to  error  handling functions, and a pointer to a data struct
       for use by the error functions, if necessary (the pointer and functions
       can  be  NULL  if  the default error handlers are to be used).  See the
       section on Changes to Libpng below  regarding  the  old  initialization
       functions.   The  structure allocation functions quietly return NULL if
       they fail to create the structure, so your application should check for
       that.

           png_structp png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);

           if (!png_ptr)
              return (ERROR);

           png_infop info_ptr = png_create_info_struct(png_ptr);

           if (!info_ptr)
           {
              png_destroy_read_struct(&png_ptr,
                  (png_infopp)NULL, (png_infopp)NULL);
              return (ERROR);
           }

       If  you  want  to use your own memory allocation routines, use a libpng
       that was built with PNG_USER_MEM_SUPPORTED defined,  and  use  png_cre-
       ate_read_struct_2() instead of png_create_read_struct():

           png_structp png_ptr = png_create_read_struct_2
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       The  error handling routines passed to png_create_read_struct() and the
       memory alloc/free routines passed  to  png_create_struct_2()  are  only
       necessary  if  you are not using the libpng supplied error handling and
       memory alloc/free functions.

       When libpng encounters an error, it expects to  longjmp  back  to  your
       routine.   Therefore,  you  will  need  to  call  setjmp  and pass your
       png_jmpbuf(png_ptr).  If you read the file from different routines, you
       will  need to update the longjmp buffer every time you enter a new rou-
       tine that will call a png_*() function.

       See your documentation of setjmp/longjmp for  your  compiler  for  more
       information on setjmp/longjmp.  See the discussion on libpng error han-
       dling in the Customizing Libpng section below for more  information  on
       the  libpng  error  handling.  If an error occurs, and libpng longjmp's
       back to your setjmp, you will want to call png_destroy_read_struct() to
       free any memory.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  &end_info);
              fclose(fp);
              return (ERROR);
           }

       Pass  (png_infopp)NULL  instead  of  &end_info  if you didn't create an
       end_info structure.

       If you would rather avoid the complexity of setjmp/longjmp issues,  you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Now  you  need  to set up the input code.  The default for libpng is to
       use the C function fread().  If you use this, you will need to  pass  a
       valid  FILE  * in the function png_init_io().  Be sure that the file is
       opened in binary mode.  If you wish to handle reading data  in  another
       way,  you  need  not call the png_init_io() function, but you must then
       implement the libpng I/O methods discussed in  the  Customizing  Libpng
       section below.

           png_init_io(png_ptr, fp);

       If  you  had  previously  opened the file and read any of the signature
       from the beginning in order to see if this was a PNG file, you need  to
       let libpng know that there are some bytes missing from the start of the
       file.

           png_set_sig_bytes(png_ptr, number);

       You can change the zlib compression buffer size to be used while  read-
       ing compressed data with

           png_set_compression_buffer_size(png_ptr, buffer_size);

       where  the  default  size  is 8192 bytes.  Note that the buffer size is
       changed immediately and the buffer is reallocated immediately,  instead
       of setting a flag to be acted upon later.

       If  you  want  CRC  errors to be handled in a different manner than the
       default, use

           png_set_crc_action(png_ptr, crit_action, ancil_action);

       The values for png_set_crc_action() say how libpng  is  to  handle  CRC
       errors  in  ancillary  and critical chunks, and whether to use the data
       contained therein.  Note that it is impossible to "discard" data  in  a
       critical chunk.

       Choices for (int) crit_action are
          PNG_CRC_DEFAULT      0  error/quit
          PNG_CRC_ERROR_QUIT   1  error/quit
          PNG_CRC_WARN_USE     3  warn/use data
          PNG_CRC_QUIET_USE    4  quiet/use data
          PNG_CRC_NO_CHANGE    5  use the current value

       Choices for (int) ancil_action are
          PNG_CRC_DEFAULT      0  error/quit
          PNG_CRC_ERROR_QUIT   1  error/quit
          PNG_CRC_WARN_DISCARD 2  warn/discard data
          PNG_CRC_WARN_USE     3  warn/use data
          PNG_CRC_QUIET_USE    4  quiet/use data
          PNG_CRC_NO_CHANGE    5  use the current value


   Setting up callback code
       You  can set up a callback function to handle any unknown chunks in the
       input stream. You must supply the function

           read_chunk_callback(png_structp png_ptr,
                png_unknown_chunkp chunk);
           {
              /* The unknown chunk structure contains your
                 chunk data, along with similar data for any other
                 unknown chunks: */

                  png_byte name[5];
                  png_byte *data;
                  png_size_t size;

              /* Note that libpng has already taken care of
                 the CRC handling */

              /* put your code here.  Search for your chunk in the
                 unknown chunk structure, process it, and return one
                 of the following: */

              return (-n); /* chunk had an error */
              return (0); /* did not recognize */
              return (n); /* success */
           }

       (You can give your function another  name  that  you  like  instead  of
       "read_chunk_callback")

       To inform libpng about your function, use

           png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
               read_chunk_callback);

       This names not only the callback function, but also a user pointer that
       you can retrieve with

           png_get_user_chunk_ptr(png_ptr);

       If you call the png_set_read_user_chunk_fn() function, then all unknown
       chunks  will  be  saved  when read, in case your callback function will
       need one or more of them.

       At this point, you can set up a callback function that will  be  called
       after  each  row has been read, which you can use to control a progress
       meter or the like.  It's demonstrated in pngtest.c.  You must supply  a
       function

           void read_row_callback(png_structp png_ptr,
              png_uint_32 row, int pass);
           {
             /* put your code here */
           }

       (You  can give it another name that you like instead of "read_row_call-
       back")

       To inform libpng about your function, use

           png_set_read_status_fn(png_ptr, read_row_callback);

       When this function is called the row has already been  completely  pro-
       cessed  and  the  'row' and 'pass' refer to the next row to be handled.
       For the non-interlaced case the row that was just handled is simply one
       less than the passed in row number, and pass will always be 0.  For the
       interlaced case the same applies unless the row value is  0,  in  which
       case  the  row  just handled was the last one from one of the preceding
       passes.  Because interlacing may skip a pass you cannot  be  sure  that
       the  preceding  pass  is just 'pass-1', if you really need to know what
       the last pass is record (row,pass) from the callback and use  the  last
       recorded value each time.

       As  with  the  user  transform  you  can  find the output row using the
       PNG_ROW_FROM_PASS_ROW macro.


   Unknown-chunk handling
       Now you get to set the way the library processes unknown chunks in  the
       input  PNG  stream. Both known and unknown chunks will be read.  Normal
       behavior is that known chunks will be parsed into information in  vari-
       ous  info_ptr  members  while  unknown  chunks  will be discarded. This
       behavior can be wasteful if your application will never use some  known
       chunk types. To change this, you can call:

           png_set_keep_unknown_chunks(png_ptr, keep,
               chunk_list, num_chunks);
           keep       - 0: default unknown chunk handling
                        1: ignore; do not keep
                        2: keep only if safe-to-copy
                        3: keep even if unsafe-to-copy

                      You can use these definitions:
                        PNG_HANDLE_CHUNK_AS_DEFAULT   0
                        PNG_HANDLE_CHUNK_NEVER        1
                        PNG_HANDLE_CHUNK_IF_SAFE      2
                        PNG_HANDLE_CHUNK_ALWAYS       3

           chunk_list - list of chunks affected (a byte string,
                        five bytes per chunk, NULL or ' ' if
                        num_chunks is 0)

           num_chunks - number of chunks affected; if 0, all
                        unknown chunks are affected.  If nonzero,
                        only the chunks in the list are affected

       Unknown  chunks  declared  in this way will be saved as raw data onto a
       list of png_unknown_chunk structures.  If  a  chunk  that  is  normally
       known  to  libpng  is named in the list, it will be handled as unknown,
       according to the "keep" directive.  If a chunk is named  in  successive
       instances  of  png_set_keep_unknown_chunks(),  the  final instance will
       take precedence.  The IHDR and IEND  chunks  should  not  be  named  in
       chunk_list;  if they are, libpng will process them normally anyway.  If
       you know that your application will never make use of  some  particular
       chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.

       Here is an example of the usage of png_set_keep_unknown_chunks(), where
       the private "vpAg" chunk will later be processed by a user chunk  call-
       back function:

           png_byte vpAg[5]={118, 112,  65, 103, (png_byte) ' '};

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             png_byte unused_chunks[]=
             {
               104,  73,  83,  84, (png_byte) ' ',   /* hIST */
               105,  84,  88, 116, (png_byte) ' ',   /* iTXt */
               112,  67,  65,  76, (png_byte) ' ',   /* pCAL */
               115,  67,  65,  76, (png_byte) ' ',   /* sCAL */
               115,  80,  76,  84, (png_byte) ' ',   /* sPLT */
               116,  73,  77,  69, (png_byte) ' ',   /* tIME */
             };
           #endif

           ...

           #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
             /* ignore all unknown chunks: */
           #  if PNG_LIBPNG_VER < 10700
             png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);
           #  else
             png_set_keep_unknown_chunks(read_ptr, 1, NULL, 0);
           #  endif

             /* except for vpAg: */
             png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);

             /* also ignore unused known chunks: */
             png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
                (int)png_sizeof(unused_chunks)/5);
           #endif


   User limits
       The  PNG specification allows the width and height of an image to be as
       large as 2^(31-1 (0x7fffffff), or about 2.147 billion rows and columns.
       Since  very  few applications really need to process such large images,
       we have imposed an arbitrary  1-million  limit  on  rows  and  columns.
       Larger  images will be rejected immediately with a png_error() call. If
       you wish to change this limit, you can use

          png_set_user_limits(png_ptr, width_max, height_max);

       to set your own limits, or use width_max = height_max = 0x7fffffffL  to
       allow  all  valid  dimensions (libpng may reject some very large images
       anyway because of potential buffer overflow conditions).

       You should put this statement after you create the  PNG  structure  and
       before  calling png_read_info(), png_read_png(), or png_process_data().

       When writing a  PNG  datastream,  put  this  statement  before  calling
       png_write_info() or png_write_png().

       If you need to retrieve the limits that are being applied, use

          width_max = png_get_user_width_max(png_ptr);
          height_max = png_get_user_height_max(png_ptr);

       The  PNG  specification sets no limit on the number of ancillary chunks
       allowed in a PNG datastream.  You can impose a limit on the total  num-
       ber  of sPLT, tEXt, iTXt, zTXt, and unknown chunks that will be stored,
       with

          png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

       where 0x7fffffffL means unlimited.  You can retrieve this limit with

          chunk_cache_max = png_get_chunk_cache_max(png_ptr);

       You can also set a limit on the amount  of  memory  that  a  compressed
       chunk other than IDAT can occupy, with

          png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

       and you can retrieve the limit with

          chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

       Any  chunks that would cause either of these limits to be exceeded will
       be ignored.


   Information about your system
       If you intend to display the PNG or to incorporate it  in  other  image
       data  you need to tell libpng information about your display or drawing
       surface so that libpng can convert the values in the image to match the
       display.

       From  libpng-1.5.4  this  information can be set before reading the PNG
       file header.  In earlier versions png_set_gamma() existed  but  behaved
       incorrectly  if  called  before  the  PNG file header had been read and
       png_set_alpha_mode() did not exist.

       If you need to support versions prior to libpng-1.5.4 test the  version
       number  as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow
       the procedures described in the appropriate manual page.

       You give libpng the encoding expected by your  system  expressed  as  a
       'gamma'  value.   You  can  also specify a default encoding for the PNG
       file in case the required information is missing  from  the  file.   By
       default  libpng  assumes that the PNG data matches your system, to keep
       this default call:

          png_set_gamma(png_ptr, screen_gamma, 1/screen_gamma/*file gamma*/);

       or you can use the fixed point equivalent:

          png_set_gamma_fixed(png_ptr,                  PNG_FP_1*screen_gamma,
       PNG_FP_1/screen_gamma);

       If you don't know the gamma for your system it is probably 2.2 - a good
       approximation to the IEC  standard  for  display  systems  (sRGB).   If
       images  are too contrasty or washed out you got the value wrong - check
       your system documentation!

       Many systems permit the system gamma to be changed via a  lookup  table
       in  the display driver, a few systems, including older Macs, change the
       response by default.  As of 1.5.4 three special values are available to
       handle common situations:

          PNG_DEFAULT_sRGB:  Indicates  that  the  system  conforms to the IEC
       61966-2-1
                            standard.  This matches almost all systems.
          PNG_GAMMA_MAC_18: Indicates that the system is an older (pre Mac  OS
       10.6)
                            Apple  Macintosh system with the default settings.
          PNG_GAMMA_LINEAR: Just the fixed point value  for  1.0  -  indicates
       that the
                            system expects data with no gamma encoding.

       You  would  use the linear (unencoded) value if you need to process the
       pixel values further because this avoids the need  to  decode  and  re-
       encode each component value whenever arithmetic is performed.  A lot of
       graphics software uses linear values for this reason, often with higher
       precision component values to preserve overall accuracy.

       The  second  thing you may need to tell libpng about is how your system
       handles alpha channel information.  Some, but not all, PNG  files  con-
       tain  an  alpha  channel.  To display these files correctly you need to
       compose the data onto a suitable background, as described  in  the  PNG
       specification.

       Libpng only supports composing onto a single color (using png_set_back-
       ground; see below).  Otherwise you must  do  the  composition  yourself
       and, in this case, you may need to call png_set_alpha_mode:

           #if PNG_LIBPNG_VER >= 10504
              png_set_alpha_mode(png_ptr, mode, screen_gamma);
           #else
              png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
           #endif

       The  screen_gamma  value  is the same as the argument to png_set_gamma;
       however,  how  it   affects   the   output   depends   on   the   mode.
       png_set_alpha_mode()  sets the file gamma default to 1/screen_gamma, so
       normally you don't need to call png_set_gamma.  If you  need  different
       defaults call png_set_gamma() before png_set_alpha_mode() - if you call
       it after it will override the settings made by png_set_alpha_mode().

       The mode is as follows:

           PNG_ALPHA_PNG: The data is encoded according to the PNG  specifica-
       tion.  Red, green and blue, or gray, components are gamma encoded color
       values and are not premultiplied by the alpha value.  The  alpha  value
       is a linear measure of the contribution of the pixel to the correspond-
       ing final output pixel.

       You should normally use this format if you intend to perform color cor-
       rection on the color values; most, maybe all, color correction software
       has no handling for the alpha channel and, anyway, the math  to  handle
       pre-multiplied component values is unnecessarily complex.

       Before you do any arithmetic on the component values you need to remove
       the gamma encoding and multiply out the alpha  channel.   See  the  PNG
       specification  for  more  detail.  It is important to note that when an
       image with an alpha channel is scaled, linear  encoded,  pre-multiplied
       component values must be used!

       The  remaining modes assume you don't need to do any further color cor-
       rection or that if you do, your color  correction  software  knows  all
       about alpha (it probably doesn't!)

           PNG_ALPHA_STANDARD:   The  data  libpng  produces is encoded in the
       standard way assumed by most correctly written graphics software.   The
       gamma  encoding will be removed by libpng and the linear component val-
       ues will be pre-multiplied by the alpha channel.

       With this format the final image must be re-encoded to match  the  dis-
       play  gamma  before  the image is displayed.  If your system doesn't do
       that, yet still seems to  perform  arithmetic  on  the  pixels  without
       decoding them, it is broken - check out the modes below.

       With PNG_ALPHA_STANDARD libpng always produces linear component values,
       whatever screen_gamma you supply.  The screen_gamma value is,  however,
       used  as  a  default  for  the  file gamma if the PNG file has no gamma
       information.

       If you call png_set_gamma() after png_set_alpha_mode() you  will  over-
       ride the linear encoding.  Instead the pre-multiplied pixel values will
       be gamma encoded but the alpha channel will still be linear.  This  may
       actually  match  the  requirements  of  some broken software, but it is
       unlikely.

       While linear 8-bit data is often used it has insufficient precision for
       any  image  with a reasonable dynamic range.  To avoid problems, and if
       your software supports it, use png_set_expand_16() to force all  compo-
       nents to 16 bits.

           PNG_ALPHA_OPTIMIZED:  This  mode  is the same as PNG_ALPHA_STANDARD
       except that completely opaque pixels are gamma encoded according to the
       screen_gamma  value.   Pixels  with alpha less than 1.0 will still have
       linear components.

       Use this format if you have control over your compositing software  and
       so don't do other arithmetic (such as scaling) on the data you get from
       libpng.  Your compositing software can simply copy opaque pixels to the
       output but still has linear values for the non-opaque pixels.

       In  normal  compositing,  where the alpha channel encodes partial pixel
       coverage (as opposed to broad area translucency), the  inaccuracies  of
       the 8-bit representation of non-opaque pixels are irrelevant.

       You  can also try this format if your software is broken; it might look
       better.

           PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however,  all  compo-
       nent values, including the alpha channel are gamma encoded.  This is an
       appropriate format to try if your software, or more likely hardware, is
       totally  broken,  i.e.,  if  it  performs linear arithmetic directly on
       gamma encoded values.

       In most cases of broken software or hardware the bug in the final  dis-
       play  manifests  as a subtle halo around composited parts of the image.
       You may not even perceive this as a halo; the composited  part  of  the
       image  may simply appear separate from the background, as though it had
       been cut out of paper and pasted on afterward.

       If you don't have to deal with bugs in software or hardware, or if  you
       can   fix   them,   there   are   three   recommended   ways  of  using
       png_set_alpha_mode():

          png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
              screen_gamma);

       You can do color correction on the result (libpng  does  not  currently
       support  color correction internally).  When you handle the alpha chan-
       nel you need to undo the gamma encoding and multiply out the alpha.

          png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
              screen_gamma);
          png_set_expand_16(png_ptr);

       If   you   are   using   the   high   level   interface,   don't   call
       png_set_expand_16(); instead pass PNG_TRANSFORM_EXPAND_16 to the inter-
       face.

       With this mode you can't do color correction, but  you  can  do  arith-
       metic,  including  composition and scaling, on the data without further
       processing.

          png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
              screen_gamma);

       You can avoid the expansion to 16-bit components with  this  mode,  but
       you  lose the ability to scale the image or perform other linear arith-
       metic.  All you can do is compose the result onto  a  matching  output.
       Since this mode is libpng-specific you also need to write your own com-
       position software.

       If you don't need, or can't handle, the  alpha  channel  you  can  call
       png_set_background() to remove it by compositing against a fixed color.
       Don't call png_set_strip_alpha() to do this - it  will  leave  spurious
       pixel values in transparent parts of this image.

          png_set_background(png_ptr, &background_color,
              PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);

       The background_color is an RGB or grayscale value according to the data
       format libpng will produce for you.  Because you  don't  yet  know  the
       format  of  the  PNG file, if you call png_set_background at this point
       you must arrange for the format produced by libpng to always have 8-bit
       or  16-bit  components  and  then store the color as an 8-bit or 16-bit
       color as appropriate.  The color contains separate gray and RGB  compo-
       nent values, so you can let libpng produce gray or RGB output according
       to the input format, but low bit depth grayscale images must always  be
       converted  to  at  least  8-bit  format.   (Even  though  low bit depth
       grayscale images can't have an alpha channel they can have a  transpar-
       ent color!)

       You  set  the  transforms  you  need later, either as flags to the high
       level interface or libpng API calls for the low level  interface.   For
       reference the settings and API calls required are:

       8-bit values:
          PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
          png_set_expand(png_ptr); png_set_scale_16(png_ptr);

          If you must get exactly the same inaccurate results
          produced by default in versions prior to libpng-1.5.4,
          use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
          instead.

       16-bit values:
          PNG_TRANSFORM_EXPAND_16
          png_set_expand_16(png_ptr);

       In either case palette image data will be expanded to RGB.  If you just
       want   color   data   you   can   add   PNG_TRANSFORM_GRAY_TO_RGB    or
       png_set_gray_to_rgb(png_ptr) to the list.

       Calling  png_set_background before the PNG file header is read will not
       work prior to libpng-1.5.4.  Because the failure may  result  in  unex-
       pected   warnings  or  errors  it  is  therefore  much  safer  to  call
       png_set_background after the head has been  read.   Unfortunately  this
       means  that prior to libpng-1.5.4 it cannot be used with the high level
       interface.


   The high-level read interface
       At this point there are two ways to  proceed;  through  the  high-level
       read  interface,  or  through  a sequence of low-level read operations.
       You can use the high-level interface if (a) you are willing to read the
       entire image into memory, and (b) the input transformations you want to
       do are limited to the following set:

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
                                       8-bit accurately
           PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
                                       8-bit less accurately
           PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
           PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
                                       samples to bytes
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_EXPAND        Perform set_expand()
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
                                       to RGB (or GA to RGBA)
           PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits

       (This excludes setting a background color, doing gamma  transformation,
       quantizing, and setting filler.)  If this is the case, simply do this:

           png_read_png(png_ptr, info_ptr, png_transforms, NULL)

       where  png_transforms  is  an integer containing the bitwise OR of some
       set  of   transformation   flags.    This   call   is   equivalent   to
       png_read_info(),  followed  the set of transformations indicated by the
       transform mask, then png_read_image(), and finally png_read_end().

       (The final parameter of this call is not yet used.   Someday  it  might
       point to transformation parameters required by some future input trans-
       form.)

       You must use png_transforms and not call any png_set_transform()  func-
       tions when you use png_read_png().

       After  you  have called png_read_png(), you can retrieve the image data
       with

          row_pointers = png_get_rows(png_ptr, info_ptr);

       where row_pointers is an array of pointers to the pixel data  for  each
       row:

          png_bytep row_pointers[height];

       If you know your image size and pixel size ahead of time, you can allo-
       cate row_pointers prior to calling png_read_png() with

          if (height > PNG_UINT_32_MAX/png_sizeof(png_byte))
             png_error (png_ptr,
                 "Image is too tall to process in memory");

          if (width > PNG_UINT_32_MAX/pixel_size)
             png_error (png_ptr,
                 "Image is too wide to process in memory");

          row_pointers = png_malloc(png_ptr,
              height*png_sizeof(png_bytep));

          for (int i=0; i<height, i++)
             row_pointers[i]=NULL;  /* security precaution */

          for (int i=0; i<height, i++)
             row_pointers[i]=png_malloc(png_ptr,
                 width*pixel_size);

          png_set_rows(png_ptr, info_ptr, &row_pointers);

       Alternatively you could allocate your image in one big block and define
       row_pointers[i] to point into the proper places in your block.

       If  you  use png_set_rows(), the application is responsible for freeing
       row_pointers (and row_pointers[i], if they were separately  allocated).

       If  you  don't allocate row_pointers ahead of time, png_read_png() will
       do it, and it'll be free'ed by libpng when you call png_destroy_*().


   The low-level read interface
       If you are going the low-level route, you are now ready to read all the
       file  information up to the actual image data.  You do this with a call
       to png_read_info().

           png_read_info(png_ptr, info_ptr);

       This will process all chunks up to but not including the image data.

       This also copies some of the data from the PNG  file  into  the  decode
       structure  for  use  in  later  transformations.  Important information
       copied in is:

       1) The PNG file gamma from the gAMA chunk.  This overwrites the default
       value    provided    by   an   earlier   call   to   png_set_gamma   or
       png_set_alpha_mode.

       2) Prior to libpng-1.5.4 the background color from a bKGd chunk.   This
       damages  the  information  provided by an earlier call to png_set_back-
       ground resulting in unexpected behavior.  Libpng-1.5.4 no  longer  does
       this.

       3) The number of significant bits in each component value.  Libpng uses
       this to optimize gamma handling by reducing the internal  lookup  table
       sizes.

       4)  The  transparent  color information from a tRNS chunk.  This can be
       modified by a later call to png_set_tRNS.


   Querying the info structure
       Functions are used to get the information from the info_ptr once it has
       been  read.   Note  that  these  fields may not be completely filled in
       until png_read_end() has read the chunk data following the image.

           png_get_IHDR(png_ptr, info_ptr, &width, &height,
              &bit_depth, &color_type, &interlace_type,
              &compression_type, &filter_method);

           width          - holds the width of the image
                            in pixels (up to 2^31).

           height         - holds the height of the image
                            in pixels (up to 2^31).

           bit_depth      - holds the bit depth of one of the
                            image channels.  (valid values are
                            1, 2, 4, 8, 16 and depend also on
                            the color_type.  See also
                            significant bits (sBIT) below).

           color_type     - describes which color/alpha channels
                                are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - (PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7)

           compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                            for PNG 1.0)

           filter_method  - (must be PNG_FILTER_TYPE_BASE
                            for PNG 1.0, and can also be
                            PNG_INTRAPIXEL_DIFFERENCING if
                            the PNG datastream is embedded in
                            a MNG-1.0 datastream)

           Any or all of interlace_type, compression_type, or
           filter_method can be NULL if you are
           not interested in their values.

           Note that png_get_IHDR() returns 32-bit data into
           the application's width and height variables.
           This is an unsafe situation if these are 16-bit
           variables.  In such situations, the
           png_get_image_width() and png_get_image_height()
           functions described below are safer.

           width            = png_get_image_width(png_ptr,
                                info_ptr);

           height           = png_get_image_height(png_ptr,
                                info_ptr);

           bit_depth        = png_get_bit_depth(png_ptr,
                                info_ptr);

           color_type       = png_get_color_type(png_ptr,
                                info_ptr);

           interlace_type   = png_get_interlace_type(png_ptr,
                                info_ptr);

           compression_type = png_get_compression_type(png_ptr,
                                info_ptr);

           filter_method    = png_get_filter_type(png_ptr,
                                info_ptr);

           channels = png_get_channels(png_ptr, info_ptr);

           channels       - number of channels of info for the
                            color type (valid values are 1 (GRAY,
                            PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
                            4 (RGB_ALPHA or RGB + filler byte))

           rowbytes = png_get_rowbytes(png_ptr, info_ptr);

           rowbytes       - number of bytes needed to hold a row

           signature = png_get_signature(png_ptr, info_ptr);

           signature      - holds the signature read from the
                            file (if any).  The data is kept in
                            the same offset it would be if the
                            whole signature were read (i.e. if an
                            application had already read in 4
                            bytes of signature before starting
                            libpng, the remaining 4 bytes would
                            be in signature[4] through signature[7]
                            (see png_set_sig_bytes())).

       These are also important, but their validity  depends  on  whether  the
       chunk    has   been   read.    The   png_get_valid(png_ptr,   info_ptr,
       PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...) functions
       return  non-zero  if  the data has been read, or zero if it is missing.
       The parameters to the png_get_<chunk> are set directly if they are sim-
       ple data types, or a pointer into the info_ptr is returned for any com-
       plex types.

       The colorspace data from gAMA, cHRM, sRGB, iCCP,  and  sBIT  chunks  is
       simply returned to give the application information about how the image
       was encoded.  Libpng itself only does transformations  using  the  file
       gamma  when combining semitransparent pixels with the background color.

           png_get_PLTE(png_ptr, info_ptr, &palette,
                            &num_palette);

           palette        - the palette for the file
                            (array of png_color)

           num_palette    - number of entries in the palette

           png_get_gAMA(png_ptr, info_ptr, &file_gamma);
           png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);

           file_gamma     - the gamma at which the file was
                            written (PNG_INFO_gAMA)

           int_file_gamma - 100,000 times the gamma at which the
                            file is written

           png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
                            &red_y, &green_x, &green_y, &blue_x, &blue_y)
           png_get_cHRM_XYZ(png_ptr,   info_ptr,   &red_X,   &red_Y,   &red_Z,
       &green_X,
                            &green_Y, &green_Z, &blue_X, &blue_Y, &blue_Z)
           png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x, &int_white_y,
                            &int_red_x,        &int_red_y,       &int_green_x,
       &int_green_y,
                            &int_blue_x, &int_blue_y)
           png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
                            &int_red_Z,      &int_green_X,       &int_green_Y,
       &int_green_Z,
                            &int_blue_X, &int_blue_Y, &int_blue_Z)

           {white,red,green,blue}_{x,y}
                            A color space encoding specified using the
                            chromaticities of the end points and the
                            white point. (PNG_INFO_cHRM)

           {red,green,blue}_{X,Y,Z}
                            A  color space encoding specified using the encod-
       ing end
                            points - the CIE tristimulus specification of  the
       intended
                            color  of  the red, green and blue channels in the
       PNG RGB
                            data.  The white point is simply the  sum  of  the
       three end
                            points. (PNG_INFO_cHRM)

           png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

           file_srgb_intent - the rendering intent (PNG_INFO_sRGB)
                            The presence of the sRGB chunk
                            means that the pixel data is in the
                            sRGB color space.  This chunk also
                            implies specific values of gAMA and
                            cHRM.

           png_get_iCCP(png_ptr, info_ptr, &name,
              &compression_type, &profile, &proflen);

           name             - The profile name.

           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.

           profile          - International Color Consortium color
                              profile data. May contain NULs.

           proflen          - length of profile data in bytes.

           png_get_sBIT(png_ptr, info_ptr, &sig_bit);

           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray,
                            red, green, and blue channels,
                            whichever are appropriate for the
                            given color type (png_color_16)

           png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
                            &num_trans, &trans_color);

           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           trans_color    - graylevel or color sample values of
                            the single transparent color for
                            non-paletted images (PNG_INFO_tRNS)

           png_get_hIST(png_ptr, info_ptr, &hist);
                            (PNG_INFO_hIST)

           hist           - histogram of palette (array of
                            png_uint_16)

           png_get_tIME(png_ptr, info_ptr, &mod_time);

           mod_time       - time image was last modified
                           (PNG_VALID_tIME)

           png_get_bKGD(png_ptr, info_ptr, &background);

           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)
                            valid 16-bit red, green and blue
                            values, regardless of color_type

           num_comments   = png_get_text(png_ptr, info_ptr,
                            &text_ptr, &num_text);

           num_comments   - number of comments

           text_ptr       - array of png_text holding image
                            comments

           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt

           text_ptr[i].key   - keyword for comment.  Must contain
                                1-79 characters.

           text_ptr[i].text  - text comments for current
                                keyword.  Can be empty.

           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt

           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt

           text_ptr[i].lang  - language of comment (empty
                                string for unknown).

           text_ptr[i].lang_key  - keyword in UTF-8
                                (empty string for unknown).

           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist when the
           library is built with iTXt chunk support.  Prior to
           libpng-1.4.0 the library was built by default without
           iTXt support. Also note that when iTXt is supported,
           they contain NULL pointers when the "compression"
           field contains PNG_TEXT_COMPRESSION_NONE or
           PNG_TEXT_COMPRESSION_zTXt.

           num_text       - number of comments (same as
                            num_comments; you can put NULL here
                            to avoid the duplication)

           Note while png_set_text() will accept text, language,
           and translated keywords that can be NULL pointers, the
           structure returned by png_get_text will always contain
           regular zero-terminated C strings.  They might be
           empty strings but they will never be NULL pointers.

           num_spalettes = png_get_sPLT(png_ptr, info_ptr,
              &palette_ptr);

           num_spalettes  - number of sPLT chunks read.

           palette_ptr    - array of palette structures holding
                            contents of one or more sPLT chunks
                            read.

           png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
              &unit_type);

           offset_x       - positive offset from the left edge
                            of the screen (can be negative)

           offset_y       - positive offset from the top edge
                            of the screen (can be negative)

           unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
              &unit_type);

           res_x          - pixels/unit physical resolution in
                            x direction

           res_y          - pixels/unit physical resolution in
                            x direction

           unit_type      - PNG_RESOLUTION_UNKNOWN,
                            PNG_RESOLUTION_METER

           png_get_sCAL(png_ptr, info_ptr, &unit, &width,
              &height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

           height      - height of a pixel in physical scale units
                        (width and height are doubles)

           png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
              &height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units
                         (expressed as a string)

           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           num_unknown_chunks = png_get_unknown_chunks(png_ptr,
              info_ptr, &unknowns)

           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks

           unknowns[i].name  - name of unknown chunk

           unknowns[i].data  - data of unknown chunk

           unknowns[i].size  - size of unknown chunk's data

           unknowns[i].location - position of chunk in file

           The value of "i" corresponds to the order in which the
           chunks were read from the PNG file or inserted with the
           png_set_unknown_chunks() function.

           The value of "location" is a bitwise "or" of

                PNG_HAVE_IHDR  (0x01)
                PNG_HAVE_PLTE  (0x02)
                PNG_AFTER_IDAT (0x08)

       The  data  from  the  pHYs chunk can be retrieved in several convenient
       forms:

           res_x = png_get_x_pixels_per_meter(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_meter(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_meter(png_ptr,
              info_ptr)

           res_x = png_get_x_pixels_per_inch(png_ptr,
              info_ptr)

           res_y = png_get_y_pixels_per_inch(png_ptr,
              info_ptr)

           res_x_and_y = png_get_pixels_per_inch(png_ptr,
              info_ptr)

           aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
              info_ptr)

           Each of these returns 0 [signifying "unknown"] if
              the data is not present or if res_x is 0;
              res_x_and_y is 0 if res_x != res_y

           Note that because of the way the resolutions are
              stored internally, the inch conversions won't
              come out to exactly even number.  For example,
              72 dpi is stored as 0.28346 pixels/meter, and
              when this is retrieved it is 71.9988 dpi, so
              be sure to round the returned value appropriately
              if you want to display a reasonable-looking result.

       The data from the oFFs chunk can be  retrieved  in  several  convenient
       forms:

           x_offset = png_get_x_offset_microns(png_ptr, info_ptr);

           y_offset = png_get_y_offset_microns(png_ptr, info_ptr);

           x_offset = png_get_x_offset_inches(png_ptr, info_ptr);

           y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

           Each of these returns 0 [signifying "unknown" if both
              x and y are 0] if the data is not present or if the
              chunk is present but the unit is the pixel.  The
              remark about inexact inch conversions applies here
              as well, because a value in inches can't always be
              converted to microns and back without some loss
              of precision.

       For more information, see the PNG specification for chunk contents.  Be
       careful with trusting rowbytes, as some of  the  transformations  could
       increase  the  space needed to hold a row (expand, filler, gray_to_rgb,
       etc.).  See png_read_update_info(), below.

       A quick word about text_ptr and num_text.  PNG stores comments in  key-
       word/text  pairs,  one  pair  per chunk, with no limit on the number of
       text chunks, and a 2^31 byte limit on their size.  While there are sug-
       gested  keywords,  there is no requirement to restrict the use to these
       strings.  It is strongly suggested that keywords and text  be  sensible
       to humans (that's the point), so don't use abbreviations.  Non-printing
       symbols are not allowed.  See the PNG specification for  more  details.
       There is also no requirement to have text after the keyword.

       Keywords  should be limited to 79 Latin-1 characters without leading or
       trailing spaces, but non-consecutive spaces are allowed within the key-
       word.   It  is  possible  to have the same keyword any number of times.
       The text_ptr is an array of png_text structures, each holding a pointer
       to  a  language  string, a pointer to a keyword and a pointer to a text
       string.  The text string, language code, and translated keyword may  be
       empty  or NULL pointers.  The keyword/text pairs are put into the array
       in the order that they are received.  However, some or all of the  text
       chunks  may  be after the image, so, to make sure you have read all the
       text chunks, don't mess with these until after you read the stuff after
       the  image.   This will be mentioned again below in the discussion that
       goes with png_read_end().


   Input transformations
       After you've read the header information, you can set up the library to
       handle any special transformations of the image data.  The various ways
       to transform the data will be described in the order that  they  should
       occur.   This  is  important,  as  some  of these change the color type
       and/or bit depth of the data, and some  others  only  work  on  certain
       color types and bit depths.

       Transformations  you request are ignored if they don't have any meaning
       for a particular input data format.  However some  transformations  can
       have  an effect as a result of a previous transformation.  If you spec-
       ify a contradictory set of transformations, for example both adding and
       removing the alpha channel, you cannot predict the final result.

       The  color  used  for the transparency values should be supplied in the
       same format/depth as the current image data.  It is stored in the  same
       format/depth  as the image data in a tRNS chunk, so this is what libpng
       expects for this data.

       The color used for the background  value  depends  on  the  need_expand
       argument as described below.

       Data  will  be  decoded into the supplied row buffers packed into bytes
       unless the library has been told to transform it into  another  format.
       For  example, 4 bit/pixel paletted or grayscale data will be returned 2
       pixels/byte with the leftmost pixel in the high-order bits of the byte,
       unless  png_set_packing()  is called.  8-bit RGB data will be stored in
       RGB RGB RGB format unless png_set_filler()  or  png_set_add_alpha()  is
       called to insert filler bytes, either before or after each RGB triplet.
       16-bit RGB data will be returned RRGGBB RRGGBB, with the most  signifi-
       cant byte of the color value first, unless png_set_scale_16() is called
       to transform it to regular RGB RGB  triplets,  or  png_set_filler()  or
       png_set_add  alpha() is called to insert filler bytes, either before or
       after each RRGGBB triplet.  Similarly, 8-bit or 16-bit  grayscale  data
       can    be    modified   with   png_set_filler(),   png_set_add_alpha(),
       png_set_strip_16(), or png_set_scale_16().

       The following code transforms grayscale images of  less  than  8  to  8
       bits,  changes paletted images to RGB, and adds a full alpha channel if
       there is transparency information in a tRNS chunk.  This is most useful
       on  grayscale  images with bit depths of 2 or 4 or if there is a multi-
       ple-image viewing application that wishes to treat all  images  in  the
       same way.

           if (color_type == PNG_COLOR_TYPE_PALETTE)
               png_set_palette_to_rgb(png_ptr);

           if (png_get_valid(png_ptr, info_ptr,
               PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

           if (color_type == PNG_COLOR_TYPE_GRAY &&
               bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

       The  first  two  functions  are  actually aliases for png_set_expand(),
       added in libpng version 1.0.4, with  the  function  names  expanded  to
       improve  code readability.  In some future version they may actually do
       different things.

       As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was added.
       It expands the sample depth without changing tRNS to alpha.

       As  of libpng version 1.5.2, png_set_expand_16() was added.  It behaves
       as png_set_expand(); however,  the  resultant  channels  have  16  bits
       rather  than  8.   Use  this when the output color or gray channels are
       made linear to avoid fairly severe accuracy loss.

          if (bit_depth < 16)
             png_set_expand_16(png_ptr);

       PNG can have files with 16 bits per channel.  If you only can handle  8
       bits per channel, this will strip the pixels down to 8-bit.

           if (bit_depth == 16) #if PNG_LIBPNG_VER >= 10504
              png_set_scale_16(png_ptr); #else
              png_set_strip_16(png_ptr); #endif

       (The  more accurate "png_set_scale_16()" API became available in libpng
       version 1.5.4).

       If you need to process the alpha channel on the image  separately  from
       the  image  data (for example if you convert it to a bitmap mask) it is
       possible to have libpng strip the channel  leaving  just  RGB  or  gray
       data:

           if (color_type & PNG_COLOR_MASK_ALPHA)
              png_set_strip_alpha(png_ptr);

       If you strip the alpha channel you need to find some other way of deal-
       ing with the information.  If, instead, you want to convert  the  image
       to  an opaque version with no alpha channel use png_set_background; see
       below.

       As of libpng version 1.5.2, almost all useful expansions are supported,
       the  major  ommissions  are  conversion  of grayscale to indexed images
       (which can be done trivially in  the  application)  and  conversion  of
       indexed  to  grayscale  (which can be done by a trivial manipulation of
       the palette.)

       In the following table, the 01 means grayscale with depth<8,  31  means
       indexed  with  depth<8,  other  numerals  represent the color type, "T"
       means the tRNS chunk is present, A means an alpha channel  is  present,
       and  O  means  tRNS or alpha is present but all pixels in the image are
       opaque.

         FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
          TO
          01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
          31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
           0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
          0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
          0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
           2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B
          2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
          2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
           3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
          3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
          3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
          4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
          4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
          6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
          6O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +

       Within the matrix,
            "+" identifies entries where 'from' and 'to' are the same.
            "-" means the transformation is not supported.
            "." means nothing is necessary (a tRNS chunk can just be ignored).
            "t" means the transformation is obtained by png_set_tRNS.
            "A" means the transformation is obtained by png_set_add_alpha().
            "X" means the transformation is obtained by png_set_expand().
            "1" means the transformation is obtained by
                png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
                if there is no transparency in the original or the final
                format).
            "C" means the transformation is obtained by png_set_gray_to_rgb().
            "G" means the transformation is obtained by png_set_rgb_to_gray().
            "P" means the transformation is obtained by
                png_set_expand_palette_to_rgb().
            "p" means the transformation is obtained by png_set_packing().
            "Q" means the transformation is obtained by png_set_quantize().
            "T" means the transformation is obtained by
                png_set_tRNS_to_alpha().
            "B" means the transformation is obtained by
                png_set_background(), or png_strip_alpha().

       When  an entry has multiple transforms listed all are required to cause
       the right overall transformation.  When two transforms are separated by
       a comma either will do the job.  When transforms are enclosed in [] the
       transform should do the job but this is  currently  unimplemented  -  a
       different format will result if the suggested transformations are used.

       In PNG files, the alpha channel in an image is the  level  of  opacity.
       If  you  need  the  alpha channel in an image to be the level of trans-
       parency instead of opacity, you can invert the alpha  channel  (or  the
       tRNS chunk data) after it's read, so that 0 is fully opaque and 255 (in
       8-bit or paletted images) or 65535 (in 16-bit images) is  fully  trans-
       parent, with

           png_set_invert_alpha(png_ptr);

       PNG  files pack pixels of bit depths 1, 2, and 4 into bytes as small as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       This  code  expands  to 1 pixel per byte without changing the values of
       the pixels:

           if (bit_depth < 8)
              png_set_packing(png_ptr);

       PNG files have possible bit depths of 1, 2, 4, 8, and 16.   All  pixels
       stored  in  a  PNG image have been "scaled" or "shifted" up to the next
       higher possible bit depth (e.g. from 5 bits/sample in the range  [0,31]
       to  8 bits/sample in the range [0, 255]).  However, it is also possible
       to convert the PNG pixel data back to the original  bit  depth  of  the
       image.   This  call  reduces  the  pixels back down to the original bit
       depth:

           png_color_8p sig_bit;

           if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
              png_set_shift(png_ptr, sig_bit);

       PNG files store 3-color pixels in red, green, blue  order.   This  code
       changes the storage of the pixels to blue, green, red:

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_bgr(png_ptr);

       PNG  files store RGB pixels packed into 3 or 6 bytes. This code expands
       them into 4 or 8 bytes for windowing systems that  need  them  in  this
       format:

           if (color_type == PNG_COLOR_TYPE_RGB)
              png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

       where "filler" is the 8 or 16-bit number to fill with, and the location
       is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending upon whether
       you  want the filler before the RGB or after.  This transformation does
       not affect images that already have full alpha  channels.   To  add  an
       opaque  alpha  channel,  use filler=0xff or 0xffff and PNG_FILLER_AFTER
       which will generate RGBA pixels.

       Note that png_set_filler() does not change the color type.  If you want
       to do that, you can add a true alpha channel with

           if (color_type == PNG_COLOR_TYPE_RGB ||
              color_type == PNG_COLOR_TYPE_GRAY)
              png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);

       where  "filler" contains the alpha value to assign to each pixel.  This
       function was added in libpng-1.2.7.

       If you are reading an image with an alpha channel,  and  you  need  the
       data as ARGB instead of the normal PNG format RGBA:

           if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_swap_alpha(png_ptr);

       For some uses, you may want a grayscale image to be represented as RGB.
       This code will do that conversion:

           if (color_type == PNG_COLOR_TYPE_GRAY ||
               color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
              png_set_gray_to_rgb(png_ptr);

       Conversely, you can convert an  RGB  or  RGBA  image  to  grayscale  or
       grayscale with alpha.

           if (color_type == PNG_COLOR_TYPE_RGB ||
               color_type == PNG_COLOR_TYPE_RGB_ALPHA)
              png_set_rgb_to_gray(png_ptr, error_action,
                 double red_weight, double green_weight);

           error_action = 1: silently do the conversion

           error_action = 2: issue a warning if the original
                             image has any pixel where
                             red != green or red != blue

           error_action = 3: issue an error and abort the
                             conversion if the original
                             image has any pixel where
                             red != green or red != blue

           red_weight:       weight of red component

           green_weight:     weight of green component
                             If either weight is negative, default
                             weights are used.

       In  the  corresponding  fixed point API the red_weight and green_weight
       values are simply scaled by 100,000:

           png_set_rgb_to_gray(png_ptr, error_action,
              png_fixed_point red_weight,
              png_fixed_point green_weight);

       If you have set error_action = 1 or 2, you can later check whether  the
       image  really  was  gray,  after  processing  the  image rows, with the
       png_get_rgb_to_gray_status(png_ptr)  function.   It   will   return   a
       png_byte that is zero if the image was gray or 1 if there were any non-
       gray pixels.  Background and sBIT data will be  silently  converted  to
       grayscale,  using  the  green  channel data for sBIT, regardless of the
       error_action setting.

       The default values come from the PNG file cHRM chunk if present; other-
       wise, the defaults correspond to the ITU-R recommendation 709, and also
       the sRGB color space, as recommended in the  Charles  Poynton's  Colour
       FAQ, <http://www.poynton.com/>, in section 9:

          <http://www.poynton.com/notes/colour_and_gamma/Color-
       FAQ.html#RTFToC9>

           Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

       Previous versions of this document, 1998 through  2002,  recommended  a
       slightly different formula:

           Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

       Libpng uses an integer approximation:

           Y = (6968 * R + 23434 * G + 2366 * B)/32768

       The  calculation is done in a linear colorspace, if the image gamma can
       be determined.

       The png_set_background() function has been described already; it  tells
       libpng  to  composite  images with alpha or simple transparency against
       the supplied background color.   For  compatibility  with  versions  of
       libpng  earlier  than  libpng-1.5.4 it is recommended that you call the
       function after reading the file header, even if you don't want  to  use
       the color in a bKGD chunk, if one exists.

       If  the  PNG  file contains a bKGD chunk (PNG_INFO_bKGD valid), you may
       use this color, or supply another color more suitable for  the  current
       display (e.g., the background color from a web page).  You need to tell
       libpng how the color is represented, both the format of  the  component
       values  in the color (the number of bits) and the gamma encoding of the
       color.  The function takes  two  arguments,  background_gamma_mode  and
       need_expand  to convey this information; however, only two combinations
       are likely to be useful:

           png_color_16 my_background;
           png_color_16p image_background;

           if (png_get_bKGD(png_ptr, info_ptr, &image_background))
              png_set_background(png_ptr, image_background,
                  PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
           else
              png_set_background(png_ptr, &my_background,
                  PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);

       The second call was described above - my_background is in the format of
       the  final,  display,  output produced by libpng.  Because you now know
       the format of the PNG it is possible to avoid the need to choose either
       8-bit or 16-bit output and to retain palette images (the palette colors
       will be modified appropriately and the tRNS chunk  removed.)   However,
       if  you  are doing this, take great care not to ask for transformations
       without checking first that they apply!

       In the first call the background color has the original bit  depth  and
       color  type  of the PNG file.  So, for palette images the color is sup-
       plied as a palette index and for low bit greyscale images the color  is
       a reduced bit value in image_background->gray.

       If  you didn't call png_set_gamma() before reading the file header, for
       example if you need your code to remain compatible with older  versions
       of libpng prior to libpng-1.5.4, this is the place to call it.

       Do not call it if you called png_set_alpha_mode(); doing so will damage
       the   settings   put   in   place   by    png_set_alpha_mode().     (If
       png_set_alpha_mode()   is   supported   then   you   can  certainly  do
       png_set_gamma() before reading the PNG header.)

       This API unconditionally sets the screen and file gamma values,  so  it
       will  override the value in the PNG file unless it is called before the
       PNG file reading starts.  For this reason you must always call it  with
       the PNG file value when you call it in this position:

          if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
             png_set_gamma(png_ptr, screen_gamma, file_gamma);

          else
             png_set_gamma(png_ptr, screen_gamma, 0.45455);

       If  you need to reduce an RGB file to a paletted file, or if a paletted
       file has more entries then will fit on your screen,  png_set_quantize()
       will  do  that.   Note  that  this  is a simple match quantization that
       merely finds the closest color available.  This should work fairly well
       with  optimized palettes, but fairly badly with linear color cubes.  If
       you pass a palette that is larger than maximum_colors,  the  file  will
       reduce  the  number  of colors in the palette so it will fit into maxi-
       mum_colors.  If there is a histogram, libpng will use it to  make  more
       intelligent  choices  when  reducing  the palette.  If there is no his-
       togram, it may not do as good a job.

          if (color_type & PNG_COLOR_MASK_COLOR)
          {
             if (png_get_valid(png_ptr, info_ptr,
                 PNG_INFO_PLTE))
             {
                png_uint_16p histogram = NULL;

                png_get_hIST(png_ptr, info_ptr,
                    &histogram);
                png_set_quantize(png_ptr, palette, num_palette,
                   max_screen_colors, histogram, 1);
             }

             else
             {
                png_color std_color_cube[MAX_SCREEN_COLORS] =
                   { ... colors ... };

                png_set_quantize(png_ptr, std_color_cube,
                   MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
                   NULL,0);
             }
          }

       PNG files describe monochrome as black being zero and white being  one.
       The  following  code  will reverse this (make black be one and white be
       zero):

          if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
             png_set_invert_mono(png_ptr);

       This function can also be  used  to  invert  grayscale  and  gray-alpha
       images:

          if (color_type == PNG_COLOR_TYPE_GRAY ||
              color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
             png_set_invert_mono(png_ptr);

       PNG  files  store  16-bit pixels in network byte order (big-endian, ie.
       most significant bits first).  This code changes  the  storage  to  the
       other  way  (little-endian,  i.e. least significant bits first, the way
       PCs store them):

           if (bit_depth == 16)
              png_set_swap(png_ptr);

       If you are using packed-pixel images (1, 2, or 4 bits/pixel),  and  you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       Finally, you can write your own transformation function if none of  the
       existing  ones  meets  your  needs.  This is done by setting a callback
       with

           png_set_read_user_transform_fn(png_ptr,
               read_transform_fn);

       You must supply the function

           void read_transform_fn(png_structp png_ptr, png_row_infop
               row_info, png_bytep data)

       See pngtest.c for a working example.   Your  function  will  be  called
       after  all of the other transformations have been processed.  Take care
       with interlaced images if you do the interlace yourself - the width  of
       the row is the width in 'row_info', not the overall image width.

       If supported, libpng provides two information routines that you can use
       to find where you are in processing the image:

          png_get_current_pass_number(png_structp png_ptr);
          png_get_current_row_number(png_structp png_ptr);

       Don't try using these outside a transform callback - firstly  they  are
       only supported if user transforms are supported, secondly they may well
       return unexpected results unless the row is actually being processed at
       the moment they are called.

       With  interlaced images the value returned is the row in the input sub-
       image    image.     Use    PNG_ROW_FROM_PASS_ROW(row,     pass)     and
       PNG_COL_FROM_PASS_COL(col,  pass)  to find the output pixel (x,y) given
       an interlaced sub-image pixel (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call-
       back function, and you can inform libpng that your  transform  function
       will change the number of channels or bit depth with the function

           png_set_user_transform_info(png_ptr, user_ptr,
               user_depth, user_channels);

       The  user's  application, not libpng, is responsible for allocating and
       freeing any memory required for the user structure.

       You can retrieve  the  pointer  via  the  function  png_get_user_trans-
       form_ptr().  For example:

           voidp read_user_transform_ptr =
               png_get_user_transform_ptr(png_ptr);

       The  last  thing  to  handle  is interlacing; this is covered in detail
       below, but you must call the function here if you want libpng to handle
       expansion of the interlaced image.

           number_of_passes = png_set_interlace_handling(png_ptr);

       After  setting  the  transformations,  libpng  can update your png_info
       structure to reflect any transformations  you've  requested  with  this
       call.

           png_read_update_info(png_ptr, info_ptr);

       This  is  most  useful to update the info structure's rowbytes field so
       you can use it to allocate your image memory.  This function will  also
       update  your  palette  with  the correct screen_gamma and background if
       these have been  given  with  the  calls  above.   You  may  only  call
       png_read_update_info() once with a particular info_ptr.

       After  you call png_read_update_info(), you can allocate any memory you
       need to hold the image.  The row data is simply raw byte data  for  all
       forms  of  images.  As the actual allocation varies among applications,
       no example will be given.  If you are allocating one large  chunk,  you
       will  need  to  build  an  array of pointers to each row, as it will be
       needed for some of the functions below.

       Remember: Before you call png_read_update_info(), the png_get_*() func-
       tions return the values corresponding to the original PNG image.  After
       you call png_read_update_info the values refer to the image that libpng
       will  output.   Consequently  you  must call all the png_set_ functions
       before you call png_read_update_info().  This is particularly important
       for   png_set_interlace_handling()   -   if   you  are  going  to  call
       png_read_update_info()  you  must   call   png_set_interlace_handling()
       before it unless you want to receive interlaced output.


   Reading image data
       After  you've  allocated memory, you can read the image data.  The sim-
       plest way to do this is in one function call.  If  you  are  allocating
       enough   memory   to   hold   the   whole  image,  you  can  just  call
       png_read_image() and libpng will read in all the image data and put  it
       in  the  memory  area  supplied.   You will need to pass in an array of
       pointers to each row.

       This function automatically handles interlacing, so you don't  need  to
       call       png_set_interlace_handling()      (unless      you      call
       png_read_update_info()) or call this function multiple times, or any of
       that other stuff necessary with png_read_rows().

          png_read_image(png_ptr, row_pointers);

       where row_pointers is:

          png_bytep row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If  you  don't  want  to  read  in the whole image at once, you can use
       png_read_rows() instead.  If there  is  no  interlacing  (check  inter-
       lace_type == PNG_INTERLACE_NONE), this is simple:

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);

       where row_pointers is the same as in the png_read_image() call.

       If  you  are  doing this just one row at a time, you can do this with a
       single row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;
           png_read_row(png_ptr, row_pointer, NULL);

       If the file is interlaced (interlace_type !=  0  in  the  IHDR  chunk),
       things  get  somewhat harder.  The only current (PNG Specification ver-
       sion 1.2) interlacing type for PNG  is  (interlace_type  ==  PNG_INTER-
       LACE_ADAM7);  a  somewhat  complicated  2D  interlace  scheme, known as
       Adam7, that breaks down an image into seven smaller images  of  varying
       size,  based  on an 8x8 grid.  This number is defined (from libpng 1.5)
       as PNG_INTERLACE_ADAM7_PASSES in png.h

       libpng can fill out those images or it can give them to  you  "as  is".
       It  is  almost  always better to have libpng handle the interlacing for
       you.  If you want the images filled out, there are two ways to do that.
       The  one  mentioned in the PNG specification is to expand each pixel to
       cover those pixels  that  have  not  been  read  yet  (the  "rectangle"
       method).   This  results  in  a  blocky image for the first pass, which
       gradually smooths out as more pixels are read.  The other method is the
       "sparkle" method, where pixels are drawn only in their final locations,
       with the rest of the image remaining whatever colors they were initial-
       ized  to  before the start of the read.  The first method usually looks
       better, but tends to be slower, as there are more pixels to put in  the
       rows.

       If,  as  is  likely,  you  want  libpng to expand the images, call this
       before calling png_start_read_image() or png_read_update_info():

           if (interlace_type == PNG_INTERLACE_ADAM7)
              number_of_passes
                  = png_set_interlace_handling(png_ptr);

       This will return the number  of  passes  needed.   Currently,  this  is
       seven,  but  may change if another interlace type is added.  This func-
       tion can be called even if the file is not interlaced,  where  it  will
       return  one  pass.   You  then  need  to  read  the  whole  image 'num-
       ber_of_passes' times.  Each time will distribute the  pixels  from  the
       current  pass  to the correct place in the output image, so you need to
       supply the same rows to png_read_rows in each pass.

       If you are not going to display the image  after  each  pass,  but  are
       going  to  wait  until  the  entire  image  is read in, use the sparkle
       effect.  This effect is faster and the end result of either  method  is
       exactly  the  same.   If you are planning on displaying the image after
       each pass, the "rectangle" effect is generally  considered  the  better
       looking one.

       If  you  only  want  the "sparkle" effect, just call png_read_rows() as
       normal, with the third parameter NULL.  Make sure you  make  pass  over
       the  image number_of_passes times, and you don't change the data in the
       rows between calls.  You can change the locations of the data, just not
       the  data.  Each pass only writes the pixels appropriate for that pass,
       and assumes the data from previous passes is still valid.

           png_read_rows(png_ptr, row_pointers, NULL,
               number_of_rows);

       If you only want the first effect (the  rectangles),  do  the  same  as
       before except pass the row buffer in the third parameter, and leave the
       second parameter NULL.

           png_read_rows(png_ptr, NULL, row_pointers,
               number_of_rows);

       If you don't want libpng to handle the interlacing details,  just  call
       png_read_rows()  PNG_INTERLACE_ADAM7_PASSES  times  to  read in all the
       images.  Each of the images is a valid image by  itself;  however,  you
       will almost certainly need to distribute the pixels from each sub-image
       to the correct place.  This is where everything gets very tricky.

       If you want to retrieve the separate images you must pass  the  correct
       number  of rows to each successive call of png_read_rows().  The calcu-
       lation gets pretty complicated for small images, where some  sub-images
       may  not  even exist because either their width or height ends up zero.
       libpng provides two macros to help you in 1.5 and later versions:

          png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
          png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

       Respectively these tell you the width and height of the sub-image  cor-
       responding  to  the  numbered pass.  'pass' is in in the range 0 to 6 -
       this can be confusing because the  specification  refers  to  the  same
       passes as 1 to 7!  Be careful, you must check both the width and height
       before calling png_read_rows() and not call it for that pass if  either
       is zero.

       You  can,  of  course,  read each sub-image row by row.  If you want to
       produce optimal code to make  a  pixel-by-pixel  transformation  of  an
       interlaced image this is the best approach; read each row of each pass,
       transform it, and write it out to a new interlaced image.

       If you want to de-interlace the image yourself libpng provides  further
       macros  to  help  that tell you where to place the pixels in the output
       image.  Because the interlacing scheme is rectangular - sub-image  pix-
       els  are  always  arranged on a rectangular grid - all you need to know
       for each pass is the starting column and row in the output image of the
       first  pixel  plus  the  spacing  between each pixel.  As of libpng 1.5
       there are four macros to retrieve this information:

          png_uint_32 x = PNG_PASS_START_COL(pass);
          png_uint_32 y = PNG_PASS_START_ROW(pass);
          png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
          png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);

       These allow you to write the obvious loop:

          png_uint_32 input_y = 0;
          png_uint_32 output_y = PNG_PASS_START_ROW(pass);

          while (output_y < output_image_height)
          {
             png_uint_32 input_x = 0;
             png_uint_32 output_x = PNG_PASS_START_COL(pass);

             while (output_x < output_image_width)
             {
                image[output_y][output_x] =
                    subimage[pass][input_y][input_x++];

                output_x += xStep;
             }

             ++input_y;
             output_y += yStep;
          }

       Notice that the steps between successive output rows  and  columns  are
       returned  as shifts.  This is possible because the pixels in the subim-
       ages are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the orig-
       inal  image.  In practice you may need to directly calculate the output
       coordinate given an input  coordinate.   libpng  provides  two  further
       macros for this purpose:

          png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
          png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);

       Finally a pair of macros are provided to tell you if a particular image
       row or column appears in a given pass:

          int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
          int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);

       Bear in mind that you will probably also need to check  the  width  and
       height  of  the  pass in addition to the above to be sure the pass even
       exists!

       With any luck you are convinced by now that you don't want to  do  your
       own  interlace  handling.  In reality normally the only good reason for
       doing this is if you are processing PNG files on a pixel-by-pixel basis
       and  don't  want  to  load the whole file into memory when it is inter-
       laced.

       libpng includes a test program, pngvalid, that illustrates reading  and
       writing  of interlaced images.  If you can't get interlacing to work in
       your code and don't  want  to  leave  it  to  libpng  (the  recommended
       approach), see how pngvalid.c does it.


   Finishing a sequential read
       After  you  are finished reading the image through the low-level inter-
       face, you can finish reading the file.  If you are interested  in  com-
       ments  or  time,  which  may be stored either before or after the image
       data, you should pass the separate png_info struct if you want to  keep
       the comments from before and after the image separate.

           png_infop end_info = png_create_info_struct(png_ptr);

           if (!end_info)
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
              return (ERROR);
           }

          png_read_end(png_ptr, end_info);

       If you are not interested, you should still call png_read_end() but you
       can pass NULL, avoiding the need to create an end_info structure.

          png_read_end(png_ptr, (png_infop)NULL);

       If you don't call png_read_end(), then your file pointer will  be  left
       pointing  to the first chunk after the last IDAT, which is probably not
       what you want if you expect to read something beyond the end of the PNG
       datastream.

       When  you  are  done,  you can free all memory allocated by libpng like
       this:

          png_destroy_read_struct(&png_ptr, &info_ptr,
              &end_info);

       or, if you didn't create an end_info structure,

          png_destroy_read_struct(&png_ptr, &info_ptr,
              (png_infopp)NULL);

       It is also possible to individually  free  the  info_ptr  members  that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)

           mask - identifies data to be freed, a mask
                  containing the bitwise OR of one or
                  more of
                    PNG_FREE_PLTE, PNG_FREE_TRNS,
                    PNG_FREE_HIST, PNG_FREE_ICCP,
                    PNG_FREE_PCAL, PNG_FREE_ROWS,
                    PNG_FREE_SCAL, PNG_FREE_SPLT,
                    PNG_FREE_TEXT, PNG_FREE_UNKN,
                  or simply PNG_FREE_ALL

           seq  - sequence number of item to be freed
                  (-1 for all items)

       This  function  may  be  safely  called  when  the relevant storage has
       already been freed, or has not yet been allocated, or was allocated  by
       the  user  and not by libpng,  and will in those cases do nothing.  The
       "seq" parameter is ignored if only one item of the selected data  type,
       such  as  PLTE, is allowed.  If "seq" is not -1, and multiple items are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or  so  that  it  will  free  data  that was allocated by the user with
       png_malloc() or png_calloc() and passed in via a png_set_*()  function,
       with

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

           mask   - which data elements are affected
                    same choices as in png_free_data()

       This  function  only affects data that has already been allocated.  You
       can call this function after reading the PNG data  but  before  calling
       any   png_set_*()  functions,  to  control  whether  the  user  or  the
       png_set_*() function is responsible for freeing any existing data  that
       might  be present, and again after the png_set_*() functions to control
       whether the user or png_destroy_*() is supposed to free the data.  When
       the user assumes responsibility for libpng-allocated data, the applica-
       tion must use png_free() to  free  it,  and  when  the  user  transfers
       responsibility to libpng for data that the user has allocated, the user
       must have used png_malloc() or png_calloc() to allocate it.

       If you allocated your row_pointers in  a  single  block,  as  suggested
       above in the description of the high level read interface, you must not
       transfer  responsibility  for  freeing  it  to  the   png_set_rows   or
       png_read_destroy  function,  because  they  would  also try to free the
       individual row_pointers[i].

       If you  allocated  text_ptr.text,  text_ptr.lang,  and  text_ptr.trans-
       lated_keyword  separately,  do  not transfer responsibility for freeing
       text_ptr to libpng, because when libpng fills a png_text  structure  it
       combines  these  members  with the key member, and png_free_data() will
       free only text_ptr.key.  Similarly, if you transfer responsibility  for
       free'ing  text_ptr  from  libpng  to your application, your application
       must not separately free those members.

       The png_free_data() function will turn off the "valid"  flag  for  any-
       thing  it frees.  If you need to turn the flag off for a chunk that was
       freed by your application instead of by libpng, you can use

           png_set_invalid(png_ptr, info_ptr, mask);

           mask - identifies the chunks to be made invalid,
                  containing the bitwise OR of one or
                  more of
                    PNG_INFO_gAMA, PNG_INFO_sBIT,
                    PNG_INFO_cHRM, PNG_INFO_PLTE,
                    PNG_INFO_tRNS, PNG_INFO_bKGD,
                    PNG_INFO_hIST, PNG_INFO_pHYs,
                    PNG_INFO_oFFs, PNG_INFO_tIME,
                    PNG_INFO_pCAL, PNG_INFO_sRGB,
                    PNG_INFO_iCCP, PNG_INFO_sPLT,
                    PNG_INFO_sCAL, PNG_INFO_IDAT

       For a more compact example of reading a PNG image, see the  file  exam-
       ple.c.


   Reading PNG files progressively
       The  progressive  reader is slightly different from the non-progressive
       reader.   Instead  of  calling  png_read_info(),  png_read_rows(),  and
       png_read_end(),  you  make  one call to png_process_data(), which calls
       callbacks when it has the info, a row, or the end of  the  image.   You
       set  up  these callbacks with png_set_progressive_read_fn().  You don't
       have to worry about the input/output functions of libpng,  as  you  are
       giving  the  library  the  data directly in png_process_data().  I will
       assume that you have read the section on reading PNG files above, so  I
       will  only  highlight  the differences (although I will show all of the
       code).

       png_structp png_ptr; png_infop info_ptr;

        /*  An example code fragment of how you would
            initialize the progressive reader in your
            application. */
        int
        initialize_png_reader()
        {
           png_ptr = png_create_read_struct
               (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
                user_error_fn, user_warning_fn);

           if (!png_ptr)
               return (ERROR);

           info_ptr = png_create_info_struct(png_ptr);

           if (!info_ptr)
           {
              png_destroy_read_struct(&png_ptr,
                 (png_infopp)NULL, (png_infopp)NULL);
              return (ERROR);
           }

           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                 (png_infopp)NULL);
              return (ERROR);
           }

           /* This one's new.  You can provide functions
              to be called when the header info is valid,
              when each row is completed, and when the image
              is finished.  If you aren't using all functions,
              you can specify NULL parameters.  Even when all
              three functions are NULL, you need to call
              png_set_progressive_read_fn().  You can use
              any struct as the user_ptr (cast to a void pointer
              for the function call), and retrieve the pointer
              from inside the callbacks using the function

                 png_get_progressive_ptr(png_ptr);

              which will return a void pointer, which you have
              to cast appropriately.
            */
           png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
               info_callback, row_callback, end_callback);

           return 0;
        }

        /* A code fragment that you call as you receive blocks
          of data */
        int
        process_data(png_bytep buffer, png_uint_32 length)
        {
           if (setjmp(png_jmpbuf(png_ptr)))
           {
              png_destroy_read_struct(&png_ptr, &info_ptr,
                  (png_infopp)NULL);
              return (ERROR);
           }

           /* This one's new also.  Simply give it a chunk
              of data from the file stream (in order, of
              course).  On machines with segmented memory
              models machines, don't give it any more than
              64K.  The library seems to run fine with sizes
              of 4K. Although you can give it much less if
              necessary (I assume you can give it chunks of
              1 byte, I haven't tried less then 256 bytes
              yet).  When this function returns, you may
              want to display any rows that were generated
              in the row callback if you don't already do
              so there.
            */
           png_process_data(png_ptr, info_ptr, buffer, length);

           /* At this point you can call png_process_data_skip if
              you want to handle data the library will skip yourself;
              it simply returns the number of bytes to skip (and stops
              libpng skipping that number of bytes on the next
              png_process_data call).
           return 0;
        }

        /* This function is called (as set by
           png_set_progressive_read_fn() above) when enough data
           has been supplied so all of the header has been
           read.
        */
        void
        info_callback(png_structp png_ptr, png_infop info)
        {
           /* Do any setup here, including setting any of
              the transformations mentioned in the Reading
              PNG files section.  For now, you _must_ call
              either png_start_read_image() or
              png_read_update_info() after all the
              transformations are set (even if you don't set
              any).  You may start getting rows before
              png_process_data() returns, so this is your
              last chance to prepare for that.

              This is where you turn on interlace handling,
              assuming you don't want to do it yourself.

              If you need to you can stop the processing of
              your original input data at this point by calling
              png_process_data_pause.  This returns the number
              of unprocessed bytes from the last png_process_data
              call - it is up to you to ensure that the next call
              sees these bytes again.  If you don't want to bother
              with this you can get libpng to cache the unread
              bytes by setting the 'save' parameter (see png.h) but
              then libpng will have to copy the data internally.
            */
        }

        /* This function is called when each row of image
           data is complete */
        void
        row_callback(png_structp png_ptr, png_bytep new_row,
           png_uint_32 row_num, int pass)
        {
           /* If the image is interlaced, and you turned
              on the interlace handler, this function will
              be called for every row in every pass.  Some
              of these rows will not be changed from the
              previous pass.  When the row is not changed,
              the new_row variable will be NULL.  The rows
              and passes are called in order, so you don't
              really need the row_num and pass, but I'm
              supplying them because it may make your life
              easier.

              If you did not turn on interlace handling then
              the callback is called for each row of each
              sub-image when the image is interlaced.  In this
              case 'row_num' is the row in the sub-image, not
              the row in the output image as it is in all other
              cases.

              For the non-NULL rows of interlaced images when
              you have switched on libpng interlace handling,
              you must call png_progressive_combine_row()
              passing in the row and the old row.  You can
              call this function for NULL rows (it will just
              return) and for non-interlaced images (it just
              does the memcpy for you) if it will make the
              code easier.  Thus, you can just do this for
              all cases if you switch on interlace handling;
            */

               png_progressive_combine_row(png_ptr, old_row,
                 new_row);

           /* where old_row is what was displayed for
              previously for the row.  Note that the first
              pass (pass == 0, really) will completely cover
              the old row, so the rows do not have to be
              initialized.  After the first pass (and only
              for interlaced images), you will have to pass
              the current row, and the function will combine
              the old row and the new row.

              You can also call png_process_data_pause in this
              callback - see above.
           */
        }

        void
        end_callback(png_structp png_ptr, png_infop info)
        {
           /* This function is called after the whole image
              has been read, including any chunks after the
              image (up to and including the IEND).  You
              will usually have the same info chunk as you
              had in the header, although some data may have
              been added to the comments and time fields.

              Most people won't do much here, perhaps setting
              a flag that marks the image as finished.
            */
        }





IV. Writing

       Much of this is very similar to reading.  However, everything of impor-
       tance is repeated here, so you won't have to constantly look back up in
       the reading section to understand writing.


   Setup
       You will want to do the I/O initialization before you get into  libpng,
       so  if it doesn't work, you don't have anything to undo. If you are not
       using the standard I/O functions, you will need to  replace  them  with
       custom writing functions.  See the discussion under Customizing libpng.

           FILE *fp = fopen(file_name, "wb");

           if (!fp)
              return (ERROR);

       Next, png_struct and png_info need to be allocated and initialized.  As
       these  can be both relatively large, you may not want to store these on
       the stack, unless you have stack space to spare.  Of course,  you  will
       want  to check if they return NULL.  If you are also reading, you won't
       want to  name  your  read  structure  and  your  write  structure  both
       "png_ptr";  you can call them anything you like, such as "read_ptr" and
       "write_ptr".  Look at pngtest.c, for example.

           png_structp png_ptr = png_create_write_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);

           if (!png_ptr)
              return (ERROR);

           png_infop info_ptr = png_create_info_struct(png_ptr);
           if (!info_ptr)
           {
              png_destroy_write_struct(&png_ptr,
                  (png_infopp)NULL);
              return (ERROR);
           }

       If you  want  to  use  your  own  memory  allocation  routines,  define
       PNG_USER_MEM_SUPPORTED  and  use png_create_write_struct_2() instead of
       png_create_write_struct():

           png_structp png_ptr = png_create_write_struct_2
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn, (png_voidp)
               user_mem_ptr, user_malloc_fn, user_free_fn);

       After you have these structures, you will need to set up the error han-
       dling.   When  libpng encounters an error, it expects to longjmp() back
       to your routine.  Therefore, you will need to call  setjmp()  and  pass
       the  png_jmpbuf(png_ptr).   If  you  write the file from different rou-
       tines, you will need to update the png_jmpbuf(png_ptr) every  time  you
       enter  a new routine that will call a png_*() function.  See your docu-
       mentation of setjmp/longjmp for your compiler for more  information  on
       setjmp/longjmp.   See  the  discussion  on libpng error handling in the
       Customizing Libpng section below for more  information  on  the  libpng
       error handling.

           if (setjmp(png_jmpbuf(png_ptr)))
           {
           png_destroy_write_struct(&png_ptr, &info_ptr);
              fclose(fp);
              return (ERROR);
           }
           ...
           return;

       If  you would rather avoid the complexity of setjmp/longjmp issues, you
       can compile libpng with PNG_NO_SETJMP, in which case errors will result
       in a call to PNG_ABORT() which defaults to abort().

       You can #define PNG_ABORT() to a function that does something more use-
       ful than abort(), as long as your function does not return.

       Now you need to set up the output code.  The default for libpng  is  to
       use  the C function fwrite().  If you use this, you will need to pass a
       valid FILE * in the function png_init_io().  Be sure that the  file  is
       opened  in  binary  mode.  Again, if you wish to handle writing data in
       another way, see the discussion on libpng I/O handling in the Customiz-
       ing Libpng section below.

           png_init_io(png_ptr, fp);

       If  you are embedding your PNG into a datastream such as MNG, and don't
       want libpng to write the 8-byte signature, or if you have already writ-
       ten the signature in your application, use

           png_set_sig_bytes(png_ptr, 8);

       to inform libpng that it should not write a signature.


   Write callbacks
       At  this  point, you can set up a callback function that will be called
       after each row has been  written,  which  you  can  use  to  control  a
       progress  meter or the like.  It's demonstrated in pngtest.c.  You must
       supply a function

           void write_row_callback(png_structp png_ptr, png_uint_32 row,
              int pass);
           {
             /* put your code here */
           }

       (You can give it another name that you like instead of "write_row_call-
       back")

       To inform libpng about your function, use

           png_set_write_status_fn(png_ptr, write_row_callback);

       When  this  function is called the row has already been completely pro-
       cessed and it has also been written out.  The 'row' and 'pass' refer to
       the  next  row to be handled.  For the non-interlaced case the row that
       was just handled is simply one less than the passed in row number,  and
       pass will always be 0.  For the interlaced case the same applies unless
       the row value is 0, in which case the row just handled was the last one
       from  one of the preceding passes.  Because interlacing may skip a pass
       you cannot be sure that the preceding pass is  just  'pass-1',  if  you
       really  need  to  know what the last pass is record (row,pass) from the
       callback and use the last recorded value each time.

       As with the user transform you  can  find  the  output  row  using  the
       PNG_ROW_FROM_PASS_ROW macro.

       You  now  have the option of modifying how the compression library will
       run.  The following functions are mainly for testing, but may be useful
       in  some  cases, like if you need to write PNG files extremely fast and
       are willing to give up some compression, or if you want to get the max-
       imum  possible  compression  at  the expense of slower writing.  If you
       have no special needs in this area, let the library do what it wants by
       not  calling  this  function  at all, as it has been tuned to deliver a
       good speed/compression ratio. The second parameter to  png_set_filter()
       is  the filter method, for which the only valid values are 0 (as of the
       July 1999 PNG specification, version 1.2) or 64 (if you are  writing  a
       PNG  datastream that is to be embedded in a MNG datastream).  The third
       parameter is a flag that indicates  which  filter  type(s)  are  to  be
       tested for each scanline.  See the PNG specification for details on the
       specific filter types.


           /* turn on or off filtering, and/or choose
              specific filters.  You can use either a single
              PNG_FILTER_VALUE_NAME or the bitwise OR of one
              or more PNG_FILTER_NAME masks.
            */
           png_set_filter(png_ptr, 0,
              PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
              PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
              PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
              PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
              PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
              PNG_ALL_FILTERS);

       If an application wants to start and stop using particular filters dur-
       ing compression, it should start out with all of the filters (to ensure
       that the previous row of pixels will be  stored  in  case  it's  needed
       later), and then add and remove them after the start of compression.

       If  you  are  writing  a PNG datastream that is to be embedded in a MNG
       datastream, the second parameter can be either 0 or 64.

       The png_set_compression_*() functions interface to the zlib compression
       library,  and  should mostly be ignored unless you really know what you
       are  doing.   The  only  generally  useful  call  is   png_set_compres-
       sion_level()  which changes how much time zlib spends on trying to com-
       press the image data.  See the Compression Library  (zlib.h  and  algo-
       rithm.txt,  distributed  with zlib) for details on the compression lev-
       els.

           #include zlib.h

           /* Set the zlib compression level */
           png_set_compression_level(png_ptr,
               Z_BEST_COMPRESSION);

           /* Set other zlib parameters for compressing IDAT */
           png_set_compression_mem_level(png_ptr, 8);
           png_set_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_compression_window_bits(png_ptr, 15);
           png_set_compression_method(png_ptr, 8);
           png_set_compression_buffer_size(png_ptr, 8192)

           /* Set zlib parameters for text compression
            * If you don't call these, the parameters
            * fall back on those defined for IDAT chunks
            */
           png_set_text_compression_mem_level(png_ptr, 8);
           png_set_text_compression_strategy(png_ptr,
               Z_DEFAULT_STRATEGY);
           png_set_text_compression_window_bits(png_ptr, 15);
           png_set_text_compression_method(png_ptr, 8);


   Setting the contents of info for output
       You now need to fill in the png_info structure with all  the  data  you
       wish  to  write  before the actual image.  Note that the only thing you
       are allowed to write after the image is the text chunks  and  the  time
       chunk  (as  of PNG Specification 1.2, anyway).  See png_write_end() and
       the latest PNG specification for more information on that.  If you wish
       to write them before the image, fill them in now, and flag that data as
       being valid.  If you want to wait until after the data, don't fill them
       until  png_write_end().   For all the fields in png_info and their data
       types, see png.h.  For explanations of what the fields contain, see the
       PNG specification.

       Some of the more important parts of the png_info are:

           png_set_IHDR(png_ptr, info_ptr, width, height,
              bit_depth, color_type, interlace_type,
              compression_type, filter_method)

           width          - holds the width of the image
                            in pixels (up to 2^31).

           height         - holds the height of the image
                            in pixels (up to 2^31).

           bit_depth      - holds the bit depth of one of the
                            image channels.
                            (valid values are 1, 2, 4, 8, 16
                            and depend also on the
                            color_type.  See also significant
                            bits (sBIT) below).

           color_type     - describes which color/alpha
                            channels are present.
                            PNG_COLOR_TYPE_GRAY
                               (bit depths 1, 2, 4, 8, 16)
                            PNG_COLOR_TYPE_GRAY_ALPHA
                               (bit depths 8, 16)
                            PNG_COLOR_TYPE_PALETTE
                               (bit depths 1, 2, 4, 8)
                            PNG_COLOR_TYPE_RGB
                               (bit_depths 8, 16)
                            PNG_COLOR_TYPE_RGB_ALPHA
                               (bit_depths 8, 16)

                            PNG_COLOR_MASK_PALETTE
                            PNG_COLOR_MASK_COLOR
                            PNG_COLOR_MASK_ALPHA

           interlace_type - PNG_INTERLACE_NONE or
                            PNG_INTERLACE_ADAM7

           compression_type - (must be
                            PNG_COMPRESSION_TYPE_DEFAULT)

           filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
                            or, if you are writing a PNG to
                            be embedded in a MNG datastream,
                            can also be
                            PNG_INTRAPIXEL_DIFFERENCING)

       If  you  call  png_set_IHDR(),  the  call must appear before any of the
       other png_set_*() functions, because they might require access to  some
       of  the  IHDR  settings.   The  remaining  png_set_*() functions can be
       called in any order.

       If you wish, you can reset  the  compression_type,  interlace_type,  or
       filter_method  later  by  calling png_set_IHDR() again; if you do this,
       the width, height, bit_depth, and color_type must be the same  in  each
       call.

           png_set_PLTE(png_ptr, info_ptr, palette,
              num_palette);

           palette        - the palette for the file
                            (array of png_color)
           num_palette    - number of entries in the palette

           png_set_gAMA(png_ptr, info_ptr, file_gamma);
           png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);

           file_gamma     - the gamma at which the image was
                            created (PNG_INFO_gAMA)

           int_file_gamma - 100,000 times the gamma at which
                            the image was created

           png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
                            green_x, green_y, blue_x, blue_y)
           png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
                            green_Y, green_Z, blue_X, blue_Y, blue_Z)
           png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
                            int_red_x, int_red_y, int_green_x, int_green_y,
                            int_blue_x, int_blue_y)
           png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
                            int_red_Z, int_green_X, int_green_Y, int_green_Z,
                            int_blue_X, int_blue_Y, int_blue_Z)

           {white,red,green,blue}_{x,y}
                            A  color  space encoding specified using the chro-
       maticities
                            of the end points and the white point.

           {red,green,blue}_{X,Y,Z}
                            A color space encoding specified using the  encod-
       ing end
                            points  - the CIE tristimulus specification of the
       intended
                            color of the red, green and blue channels  in  the
       PNG RGB
                            data.   The  white  point is simply the sum of the
       three end
                            points.

           png_set_sRGB(png_ptr, info_ptr, srgb_intent);

           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of
                            the sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This chunk also implies specific
                            values of gAMA and cHRM.  Rendering
                            intent is the CSS-1 property that
                            has been defined by the International
                            Color Consortium
                            (http://www.color.org).
                            It can be one of
                            PNG_sRGB_INTENT_SATURATION,
                            PNG_sRGB_INTENT_PERCEPTUAL,
                            PNG_sRGB_INTENT_ABSOLUTE, or
                            PNG_sRGB_INTENT_RELATIVE.


           png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
              srgb_intent);

           srgb_intent    - the rendering intent
                            (PNG_INFO_sRGB) The presence of the
                            sRGB chunk means that the pixel
                            data is in the sRGB color space.
                            This function also causes gAMA and
                            cHRM chunks with the specific values
                            that are consistent with sRGB to be
                            written.

           png_set_iCCP(png_ptr, info_ptr, name, compression_type,
                              profile, proflen);

           name             - The profile name.

           compression_type - The compression type; always
                              PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                              You may give NULL to this argument to
                              ignore it.

           profile          - International Color Consortium color
                              profile data. May contain NULs.

           proflen          - length of profile data in bytes.

           png_set_sBIT(png_ptr, info_ptr, sig_bit);

           sig_bit        - the number of significant bits for
                            (PNG_INFO_sBIT) each of the gray, red,
                            green, and blue channels, whichever are
                            appropriate for the given color type
                            (png_color_16)

           png_set_tRNS(png_ptr, info_ptr, trans_alpha,
              num_trans, trans_color);

           trans_alpha    - array of alpha (transparency)
                            entries for palette (PNG_INFO_tRNS)

           num_trans      - number of transparent entries
                            (PNG_INFO_tRNS)

           trans_color    - graylevel or color sample values
                            (in order red, green, blue) of the
                            single transparent color for
                            non-paletted images (PNG_INFO_tRNS)

           png_set_hIST(png_ptr, info_ptr, hist);

           hist           - histogram of palette (array of
                            png_uint_16) (PNG_INFO_hIST)

           png_set_tIME(png_ptr, info_ptr, mod_time);

           mod_time       - time image was last modified
                            (PNG_VALID_tIME)

           png_set_bKGD(png_ptr, info_ptr, background);

           background     - background color (of type
                            png_color_16p) (PNG_VALID_bKGD)

           png_set_text(png_ptr, info_ptr, text_ptr, num_text);

           text_ptr       - array of png_text holding image
                            comments

           text_ptr[i].compression - type of compression used
                        on "text" PNG_TEXT_COMPRESSION_NONE
                                  PNG_TEXT_COMPRESSION_zTXt
                                  PNG_ITXT_COMPRESSION_NONE
                                  PNG_ITXT_COMPRESSION_zTXt
           text_ptr[i].key   - keyword for comment.  Must contain
                        1-79 characters.
           text_ptr[i].text  - text comments for current
                                keyword.  Can be NULL or empty.
           text_ptr[i].text_length - length of text string,
                        after decompression, 0 for iTXt
           text_ptr[i].itxt_length - length of itxt string,
                        after decompression, 0 for tEXt/zTXt
           text_ptr[i].lang  - language of comment (NULL or
                                empty for unknown).
           text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                                or empty for unknown).

           Note that the itxt_length, lang, and lang_key
           members of the text_ptr structure only exist when the
           library is built with iTXt chunk support.  Prior to
           libpng-1.4.0 the library was built by default without
           iTXt support. Also note that when iTXt is supported,
           they contain NULL pointers when the "compression"
           field contains PNG_TEXT_COMPRESSION_NONE or
           PNG_TEXT_COMPRESSION_zTXt.

           num_text       - number of comments

           png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
              num_spalettes);

           palette_ptr    - array of png_sPLT_struct structures
                            to be added to the list of palettes
                            in the info structure.
           num_spalettes  - number of palette structures to be
                            added.

           png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
               unit_type);

           offset_x  - positive offset from the left
                            edge of the screen

           offset_y  - positive offset from the top
                            edge of the screen

           unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

           png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
               unit_type);

           res_x       - pixels/unit physical resolution
                         in x direction

           res_y       - pixels/unit physical resolution
                         in y direction

           unit_type   - PNG_RESOLUTION_UNKNOWN,
                         PNG_RESOLUTION_METER

           png_set_sCAL(png_ptr, info_ptr, unit, width, height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units

           height      - height of a pixel in physical scale units
                         (width and height are doubles)

           png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)

           unit        - physical scale units (an integer)

           width       - width of a pixel in physical scale units
                         expressed as a string

           height      - height of a pixel in physical scale units
                        (width and height are strings like "2.54")

           png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
              num_unknowns)

           unknowns          - array of png_unknown_chunk
                               structures holding unknown chunks
           unknowns[i].name  - name of unknown chunk
           unknowns[i].data  - data of unknown chunk
           unknowns[i].size  - size of unknown chunk's data
           unknowns[i].location - position to write chunk in file
                                  0: do not write chunk
                                  PNG_HAVE_IHDR: before PLTE
                                  PNG_HAVE_PLTE: before IDAT
                                  PNG_AFTER_IDAT: after IDAT

       The "location" member is set automatically according to  what  part  of
       the  output  file  has  already been written.  You can change its value
       after calling png_set_unknown_chunks() as  demonstrated  in  pngtest.c.
       Within  each  of the "locations", the chunks are sequenced according to
       their position in the structure (that is, the value of  "i",  which  is
       the  order  in  which  the chunk was either read from the input file or
       defined with png_set_unknown_chunks).

       A quick word about text and num_text.  text is  an  array  of  png_text
       structures.   num_text  is the number of valid structures in the array.
       Each png_text structure holds a language code, a keyword, a text value,
       and a compression type.

       The  compression  types  have the same valid numbers as the compression
       types of the image data.  Currently, the only  valid  number  is  zero.
       However,  you  can store text either compressed or uncompressed, unlike
       images, which always have to be compressed.  So if you don't  want  the
       text compressed, set the compression type to PNG_TEXT_COMPRESSION_NONE.
       Because tEXt and zTXt chunks don't have a language field, if you  spec-
       ify PNG_TEXT_COMPRESSION_NONE or PNG_TEXT_COMPRESSION_zTXt any language
       code or translated keyword will not be written out.

       Until text gets around a few hundred bytes, it is not worth compressing
       it.   After  the text has been written out to the file, the compression
       type  is  set  to  PNG_TEXT_COMPRESSION_NONE_WR  or   PNG_TEXT_COMPRES-
       SION_zTXt_WR,  so  that  it isn't written out again at the end (in case
       you are calling png_write_end() with the same struct).

       The keywords that are given in the PNG Specification are:

           Title            Short (one line) title or
                            caption for image

           Author           Name of image's creator

           Description      Description of image (possibly long)

           Copyright        Copyright notice

           Creation Time    Time of original image creation
                            (usually RFC 1123 format, see below)

           Software         Software used to create the image

           Disclaimer       Legal disclaimer

           Warning          Warning of nature of content

           Source           Device used to create the image

           Comment          Miscellaneous comment; conversion
                            from other image format

       The keyword-text pairs work like this.  Keywords should be short simple
       descriptions  of  what the comment is about.  Some typical keywords are
       found in the PNG specification, as is some recommendations on keywords.
       You can repeat keywords in a file.  You can even write some text before
       the image and some after.  For example, you may want to put a  descrip-
       tion  of  the  image  before  the image, but leave the disclaimer until
       after, so viewers working over modem connections don't have to wait for
       the disclaimer to go over the modem before they start seeing the image.
       Finally, keywords should be full words,  not  abbreviations.   Keywords
       and  text  are in the ISO 8859-1 (Latin-1) character set (a superset of
       regular ASCII) and can not contain NUL characters, and should not  con-
       tain  control  or  other  unprintable characters.  To make the comments
       widely readable, stick with basic ASCII,  and  avoid  machine  specific
       character  set  extensions  like the IBM-PC character set.  The keyword
       must be present, but you can leave off  the  text  string  on  non-com-
       pressed  pairs.   Compressed pairs must have a text string, as only the
       text string is compressed anyway, so the compression would be  meaning-
       less.

       PNG supports modification time via the png_time structure.  Two conver-
       sion routines are provided, png_convert_from_time_t()  for  time_t  and
       png_convert_from_struct_tm()  for  struct  tm.  The time_t routine uses
       gmtime().  You don't have to use either of these, but if  you  wish  to
       fill in the png_time structure directly, you should provide the time in
       universal time (GMT) if possible instead of your local time.  Note that
       the  year  number  is the full year (e.g. 1998, rather than 98 - PNG is
       year 2000 compliant!), and that months start with 1.

       If you want to store the time  of  the  original  image  creation,  you
       should  use  a plain tEXt chunk with the "Creation Time" keyword.  This
       is necessary because the "creation time" of a  PNG  image  is  somewhat
       vague,  depending  on whether you mean the PNG file, the time the image
       was created in a non-PNG format, a still photo from which the image was
       scanned, or possibly the subject matter itself.  In order to facilitate
       machine-readable dates, it is recommended that the "Creation Time" tEXt
       chunk  use  RFC  1123  format  dates (e.g. "22 May 1997 18:07:10 GMT"),
       although this isn't a requirement.  Unlike the tIME  chunk,  the  "Cre-
       ation  Time"  tEXt chunk is not expected to be automatically changed by
       the software.  To facilitate the use of  RFC  1123  dates,  a  function
       png_convert_to_rfc1123(png_ptr,  png_timep) is provided to convert from
       PNG time to an RFC 1123 format string.


   Writing unknown chunks
       You can use the png_set_unknown_chunks function  to  queue  up  private
       chunks  for writing.  You give it a chunk name, location, raw data, and
       a size.  You also must use png_set_keep_unknown_chunks() to ensure that
       libpng  will  handle them.  That's all there is to it.  The chunks will
       be  written   by   the   next   following   png_write_info_before_PLTE,
       png_write_info, or png_write_end function, depending upon the specified
       location.   Any  chunks  previously  read  into  the  info  structure's
       unknown-chunk  list  will also be written out in a sequence that satis-
       fies the PNG specification's ordering rules.

       Here is an example of writing two private chunks, prVt and miNE:

           #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
           /* Set unknown chunk data */
           png_unknown_chunk unk_chunk[2];
           strcpy((char *) unk_chunk[0].name, "prVt";
           unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
           unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
           unk_chunk[0].location = PNG_HAVE_IHDR;
           strcpy((char *) unk_chunk[1].name, "miNE";
           unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
           unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
           unk_chunk[1].location = PNG_AFTER_IDAT;
           png_set_unknown_chunks(write_ptr, write_info_ptr,
               unk_chunk, 2);
           /* Needed because miNE is not safe-to-copy */
           png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
              (png_bytep) "miNE", 1);
           /* Deal with unknown chunk location bug in 1.5.x and earlier */
           png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
           png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
           # if PNG_LIBPNG_VER < 10500
             /* PNG_AFTER_IDAT  writes  two  copies  of  the  chunk  prior  to
       libpng-1.5.0,
              *  one before IDAT and another after IDAT, so don't use it; only
       use
              * PNG_HAVE_IHDR location.  This call resets the location  previ-
       ously
              *  set  by  assignment  and png_set_unknown_chunk_location() for
       chunk 1.
              */
             png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
           # endif
           #endif


   The high-level write interface
       At this point there are two ways to  proceed;  through  the  high-level
       write  interface,  or through a sequence of low-level write operations.
       You can use the high-level interface if your image data is  present  in
       the  info structure.  All defined output transformations are permitted,
       enabled by the following masks.

           PNG_TRANSFORM_IDENTITY      No transformation
           PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
           PNG_TRANSFORM_PACKSWAP      Change order of packed
                                       pixels to LSB first
           PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
           PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                       sBIT depth
           PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                       to BGRA
           PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                       to AG
           PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                       to transparency
           PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
           PNG_TRANSFORM_STRIP_FILLER        Strip out filler
                                             bytes (deprecated).
           PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
                                             filler bytes
           PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
                                             filler bytes

       If you have valid image  data  in  the  info  structure  (you  can  use
       png_set_rows()  to  put  image  data  in the info structure), simply do
       this:

           png_write_png(png_ptr, info_ptr, png_transforms, NULL)

       where png_transforms is an integer containing the bitwise  OR  of  some
       set   of   transformation   flags.    This   call   is   equivalent  to
       png_write_info(), followed the set of transformations indicated by  the
       transform mask, then png_write_image(), and finally png_write_end().

       (The  final  parameter  of this call is not yet used.  Someday it might
       point to transformation  parameters  required  by  some  future  output
       transform.)

       You  must use png_transforms and not call any png_set_transform() func-
       tions when you use png_write_png().


   The low-level write interface
       If you are going the low-level route instead,  you  are  now  ready  to
       write  all  the  file  information up to the actual image data.  You do
       this with a call to png_write_info().

           png_write_info(png_ptr, info_ptr);

       Note that there is  one  transformation  you  may  need  to  do  before
       png_write_info().   In  PNG files, the alpha channel in an image is the
       level of opacity.  If your data is supplied as a level of transparency,
       you  can  invert  the  alpha  channel before you write it, so that 0 is
       fully transparent and 255 (in 8-bit or paletted images)  or  65535  (in
       16-bit images) is fully opaque, with

           png_set_invert_alpha(png_ptr);

       This  must  appear  before  png_write_info()  instead of later with the
       other transformations because in the case of paletted images  the  tRNS
       chunk  data  has  to  be inverted before the tRNS chunk is written.  If
       your image is not a paletted image, the tRNS data (which in such  cases
       represents  a single color to be rendered as transparent) won't need to
       be changed, and you  can  safely  do  this  transformation  after  your
       png_write_info() call.

       If you need to write a private chunk that you want to appear before the
       PLTE chunk when PLTE is present, you can write  the  PNG  info  in  two
       steps, and insert code to write your own chunk between them:

           png_write_info_before_PLTE(png_ptr, info_ptr);
           png_set_unknown_chunks(png_ptr, info_ptr, ...);
           png_write_info(png_ptr, info_ptr);

       After  you've  written the file information, you can set up the library
       to handle any special transformations of the image data.   The  various
       ways  to  transform  the  data will be described in the order that they
       should occur.  This is important, as some of  these  change  the  color
       type and/or bit depth of the data, and some others only work on certain
       color types and bit depths.  Even though each transformation checks  to
       see  if it has data that it can do something with, you should make sure
       to only enable a transformation if it will be valid for the data.   For
       example, don't swap red and blue on grayscale data.

       PNG  files  store RGB pixels packed into 3 or 6 bytes.  This code tells
       the library to strip input data that has 4 or 8 bytes per pixel down to
       3  or  6  bytes  (or  strip 2 or 4-byte grayscale+filler data to 1 or 2
       bytes per pixel).

           png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

       where the 0 is unused, and the location is either PNG_FILLER_BEFORE  or
       PNG_FILLER_AFTER,  depending  upon whether the filler byte in the pixel
       is stored XRGB or RGBX.

       PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small  as
       they can, resulting in, for example, 8 pixels per byte for 1 bit files.
       If the data is supplied at 1 pixel per byte, use this code, which  will
       correctly pack the pixels into a single byte:

           png_set_packing(png_ptr);

       PNG  files  reduce  possible bit depths to 1, 2, 4, 8, and 16.  If your
       data is of another bit depth, you can write an sBIT chunk into the file
       so that decoders can recover the original data if desired.

           /* Set the true bit depth of the image data */
           if (color_type & PNG_COLOR_MASK_COLOR)
           {
              sig_bit.red = true_bit_depth;
              sig_bit.green = true_bit_depth;
              sig_bit.blue = true_bit_depth;
           }

           else
           {
              sig_bit.gray = true_bit_depth;
           }

           if (color_type & PNG_COLOR_MASK_ALPHA)
           {
              sig_bit.alpha = true_bit_depth;
           }

           png_set_sBIT(png_ptr, info_ptr, &sig_bit);

       If  the  data is stored in the row buffer in a bit depth other than one
       supported by PNG (e.g. 3 bit data in the range 0-7 for  a  4-bit  PNG),
       this  will scale the values to appear to be the correct bit depth as is
       required by PNG.

           png_set_shift(png_ptr, &sig_bit);

       PNG files store 16-bit pixels in network byte  order  (big-endian,  ie.
       most significant bits first).  This code would be used if they are sup-
       plied the other way (little-endian, i.e. least significant bits  first,
       the way PCs store them):

           if (bit_depth > 8)
              png_set_swap(png_ptr);

       If  you  are using packed-pixel images (1, 2, or 4 bits/pixel), and you
       need to change the order the pixels are packed into bytes, you can use:

           if (bit_depth < 8)
              png_set_packswap(png_ptr);

       PNG  files  store  3 color pixels in red, green, blue order.  This code
       would be used if they are supplied as blue, green, red:

           png_set_bgr(png_ptr);

       PNG files describe monochrome as black being zero and white being  one.
       This  code  would be used if the pixels are supplied with this reversed
       (black being one and white being zero):

           png_set_invert_mono(png_ptr);

       Finally, you can write your own transformation function if none of  the
       existing  ones  meets  your  needs.  This is done by setting a callback
       with

           png_set_write_user_transform_fn(png_ptr,
              write_transform_fn);

       You must supply the function

           void write_transform_fn(png_structp png_ptr, png_row_infop
              row_info, png_bytep data)

       See pngtest.c for a working example.   Your  function  will  be  called
       before  any  of  the other transformations are processed.  If supported
       libpng also supplies an information routine that  may  be  called  from
       your callback:

          png_get_current_row_number(png_ptr);
          png_get_current_pass_number(png_ptr);

       This  returns the current row passed to the transform.  With interlaced
       images the value returned is the row in the input sub-image image.  Use
       PNG_ROW_FROM_PASS_ROW(row,  pass)  and PNG_COL_FROM_PASS_COL(col, pass)
       to find the output pixel (x,y)  given  an  interlaced  sub-image  pixel
       (row,col,pass).

       The discussion of interlace handling above contains more information on
       how to use these values.

       You can also set up a pointer to a user structure for use by your call-
       back function.

           png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

       The  user_channels  and  user_depth  parameters  of  this  function are
       ignored when writing; you can set them to zero as shown.

       You can retrieve  the  pointer  via  the  function  png_get_user_trans-
       form_ptr().  For example:

           voidp write_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

       It  is  possible  to have libpng flush any pending output, either manu-
       ally, or automatically after a certain number of lines have been  writ-
       ten.  To flush the output stream a single time call:

           png_write_flush(png_ptr);

       and to have libpng flush the output stream periodically after a certain
       number of scanlines have been written, call:

           png_set_flush(png_ptr, nrows);

       Note  that  the  distance  between  rows  is   from   the   last   time
       png_write_flush()  was  called, or the first row of the image if it has
       never been called.  So if you write 50 lines,  and  then  png_set_flush
       25,  it  will flush the output on the next scanline, and every 25 lines
       thereafter, unless png_write_flush() is called  before  25  more  lines
       have been written.  If nrows is too small (less than about 10 lines for
       a 640 pixel wide RGB image) the image compression may decrease  notice-
       ably  (although  this  may  be  acceptable for real-time applications).
       Infrequent flushing will only degrade the compression performance by  a
       few percent over images that do not use flushing.


   Writing the image data
       That's  it  for the transformations.  Now you can write the image data.
       The simplest way to do this is in one function call.  If you  have  the
       whole  image  in memory, you can just call png_write_image() and libpng
       will write the image.  You will need to pass in an array of pointers to
       each  row.   This  function  automatically  handles interlacing, so you
       don't need to call png_set_interlace_handling() or call  this  function
       multiple   times,   or   any   of   that  other  stuff  necessary  with
       png_write_rows().

           png_write_image(png_ptr, row_pointers);

       where row_pointers is:

           png_byte *row_pointers[height];

       You can point to void or char or whatever you use for pixels.

       If you don't want to write  the  whole  image  at  once,  you  can  use
       png_write_rows()  instead.  If the file is not interlaced, this is sim-
       ple:

           png_write_rows(png_ptr, row_pointers,
              number_of_rows);

       row_pointers is the same as in the png_write_image() call.

       If you are just writing one row at a time, you can do this with a  sin-
       gle row_pointer instead of an array of row_pointers:

           png_bytep row_pointer = row;

           png_write_row(png_ptr, row_pointer);

       When  the  file  is interlaced, things can get a good deal more compli-
       cated.  The only currently (as of the PNG  Specification  version  1.2,
       dated  July  1999)  defined  interlacing  scheme  for  PNG files is the
       "Adam7" interlace scheme, that breaks down an image into seven  smaller
       images of varying size.  libpng will build these images for you, or you
       can do them yourself.  If you want to build them yourself, see the  PNG
       specification for details of which pixels to write when.

       If  you  don't  want libpng to handle the interlacing details, just use
       png_set_interlace_handling() and call png_write_rows() the correct num-
       ber  of times to write all the sub-images (png_set_interlace_handling()
       returns the number of sub-images.)

       If you want libpng to build the sub-images, call this before you  start
       writing any rows:

           number_of_passes = png_set_interlace_handling(png_ptr);

       This  will  return  the  number  of  passes needed.  Currently, this is
       seven, but may change if another interlace type is added.

       Then write the complete image number_of_passes times.

           png_write_rows(png_ptr, row_pointers, number_of_rows);

       Think carefully before you write an interlaced image.   Typically  code
       that  reads  such  images  reads all the image data into memory, uncom-
       pressed, before doing any processing.  Only code that  can  display  an
       image  on  the  fly can take advantage of the interlacing and even then
       the image has to be exactly the correct size  for  the  output  device,
       because  scaling  an  image  requires adjacent pixels and these are not
       available until all the passes have been read.

       If you do write an interlaced image you will hardly ever need to handle
       the  interlacing  yourself.   Call png_set_interlace_handling() and use
       the approach described above.

       The only time it is conceivable that you will really need to  write  an
       interlaced  image  pass-by-pass  is when you have read one pass by pass
       and made some pixel-by-pixel transformation to it, as described in  the
       read  code above.  In this case use the PNG_PASS_ROWS and PNG_PASS_COLS
       macros to determine the size of each sub-image in turn and simply write
       the rows you obtained from the read code.


   Finishing a sequential write
       After you are finished writing the image, you should finish writing the
       file.  If you are interested in writing comments or  time,  you  should
       pass  an  appropriately filled png_info pointer.  If you are not inter-
       ested, you can pass NULL.

           png_write_end(png_ptr, info_ptr);

       When you are done, you can free all memory used by libpng like this:

           png_destroy_write_struct(&png_ptr, &info_ptr);

       It is also possible to individually  free  the  info_ptr  members  that
       point to libpng-allocated storage with the following function:

           png_free_data(png_ptr, info_ptr, mask, seq)

           mask  - identifies data to be freed, a mask
                   containing the bitwise OR of one or
                   more of
                     PNG_FREE_PLTE, PNG_FREE_TRNS,
                     PNG_FREE_HIST, PNG_FREE_ICCP,
                     PNG_FREE_PCAL, PNG_FREE_ROWS,
                     PNG_FREE_SCAL, PNG_FREE_SPLT,
                     PNG_FREE_TEXT, PNG_FREE_UNKN,
                   or simply PNG_FREE_ALL

           seq   - sequence number of item to be freed
                   (-1 for all items)

       This  function  may  be  safely  called  when  the relevant storage has
       already been freed, or has not yet been allocated, or was allocated  by
       the  user  and not by libpng,  and will in those cases do nothing.  The
       "seq" parameter is ignored if only one item of the selected data  type,
       such  as  PLTE, is allowed.  If "seq" is not -1, and multiple items are
       allowed for the data type identified in the mask, such as text or sPLT,
       only the n'th item in the structure is freed, where n is "seq".

       If  you  allocated  data such as a palette that you passed in to libpng
       with png_set_*, you must not free it until  just  before  the  call  to
       png_destroy_write_struct().

       The default behavior is only to free data that was allocated internally
       by libpng.  This can be changed, so that libpng will not free the data,
       or  so  that  it  will  free  data  that was allocated by the user with
       png_malloc() or png_calloc() and passed in via a png_set_*()  function,
       with

           png_data_freer(png_ptr, info_ptr, freer, mask)

           freer  - one of
                      PNG_DESTROY_WILL_FREE_DATA
                      PNG_SET_WILL_FREE_DATA
                      PNG_USER_WILL_FREE_DATA

           mask   - which data elements are affected
                    same choices as in png_free_data()

       For  example,  to  transfer  responsibility  for  some data from a read
       structure to a write structure, you could use

           png_data_freer(read_ptr, read_info_ptr,
              PNG_USER_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

           png_data_freer(write_ptr, write_info_ptr,
              PNG_DESTROY_WILL_FREE_DATA,
              PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

       thereby briefly reassigning responsibility for freeing to the user  but
       immediately  afterwards  reassigning  it once more to the write_destroy
       function.  Having done this, it would then be safe to destroy the  read
       structure  and  continue  to  use  the PLTE, tRNS, and hIST data in the
       write structure.

       This function only affects data that has already been  allocated.   You
       can  call  this function before calling after the png_set_*() functions
       to control whether the user or png_destroy_*() is supposed to free  the
       data.   When the user assumes responsibility for libpng-allocated data,
       the application must use png_free() to  free  it,  and  when  the  user
       transfers  responsibility  to  libpng  for data that the user has allo-
       cated, the user must have used png_malloc() or png_calloc() to allocate
       it.

       If  you  allocated  text_ptr.text,  text_ptr.lang,  and text_ptr.trans-
       lated_keyword separately, do not transfer  responsibility  for  freeing
       text_ptr  to  libpng, because when libpng fills a png_text structure it
       combines these members with the key member,  and  png_free_data()  will
       free  only text_ptr.key.  Similarly, if you transfer responsibility for
       free'ing text_ptr from libpng to  your  application,  your  application
       must  not separately free those members.  For a more compact example of
       writing a PNG image, see the file example.c.



V. Modifying/Customizing libpng:

       There are two issues here.  The first is changing how libpng does stan-
       dard  things  like memory allocation, input/output, and error handling.
       The second deals with more complicated things like adding  new  chunks,
       adding  new  transformations,  and generally changing how libpng works.
       Both of those are compile-time issues;  that  is,  they  are  generally
       determined  at the time the code is written, and there is rarely a need
       to provide the user with a means of changing them.

       Memory allocation, input/output, and error handling

       All of the memory  allocation,  input/output,  and  error  handling  in
       libpng goes through callbacks that are user-settable.  The default rou-
       tines are in pngmem.c,  pngrio.c,  pngwio.c,  and  pngerror.c,  respec-
       tively.  To change these functions, call the appropriate png_set_*_fn()
       function.

       Memory allocation is done through the functions png_malloc(),  png_cal-
       loc(),  and png_free().  The png_malloc() and png_free() functions cur-
       rently just call  the  standard  C  functions  and  png_calloc()  calls
       png_malloc()  and  then clears the newly allocated memory to zero; note
       that png_calloc(png_ptr, size) is not the same  as  the  calloc(number,
       size) function provided by stdlib.h.  There is limited support for cer-
       tain systems with segmented  memory  architectures  and  the  types  of
       pointers declared by png.h match this; you will have to use appropriate
       pointers in your application.  Since it is unlikely that the method  of
       handling  memory  allocation on a platform will change between applica-
       tions, these functions must be modified in the library at compile time.
       If you prefer to use a different method of allocating and freeing data,
       you can use png_create_read_struct_2()  or  png_create_write_struct_2()
       to  register  your  own  functions as described above.  These functions
       also provide a void pointer that can be retrieved via

           mem_ptr=png_get_mem_ptr(png_ptr);

       Your replacement memory functions must have prototypes as follows:

           png_voidp malloc_fn(png_structp png_ptr,
              png_alloc_size_t size);

           void free_fn(png_structp png_ptr, png_voidp ptr);

       Your malloc_fn() must return NULL in case of failure.  The png_malloc()
       function  will normally call png_error() if it receives a NULL from the
       system memory allocator or from your replacement malloc_fn().

       Your free_fn() will never be called with a  NULL  ptr,  since  libpng's
       png_free() checks for NULL before calling free_fn().

       Input/Output  in  libpng  is  done  through png_read() and png_write(),
       which currently just call fread() and fwrite().  The FILE *  is  stored
       in  png_struct  and  is  initialized via png_init_io().  If you wish to
       change the method of I/O, the library supplies callbacks that  you  can
       set  through  the  function png_set_read_fn() and png_set_write_fn() at
       run time, instead of calling the png_init_io() function.   These  func-
       tions  also  provide a void pointer that can be retrieved via the func-
       tion png_get_io_ptr().  For example:

           png_set_read_fn(png_structp read_ptr,
               voidp read_io_ptr, png_rw_ptr read_data_fn)

           png_set_write_fn(png_structp write_ptr,
               voidp write_io_ptr, png_rw_ptr write_data_fn,
               png_flush_ptr output_flush_fn);

           voidp read_io_ptr = png_get_io_ptr(read_ptr);
           voidp write_io_ptr = png_get_io_ptr(write_ptr);

       The replacement I/O functions must have prototypes as follows:

           void user_read_data(png_structp png_ptr,
               png_bytep data, png_size_t length);

           void user_write_data(png_structp png_ptr,
               png_bytep data, png_size_t length);

           void user_flush_data(png_structp png_ptr);

       The user_read_data() function is responsible for detecting and handling
       end-of-data errors.

       Supplying  NULL  for the read, write, or flush functions sets them back
       to using the default C stream functions, which  expect  the  io_ptr  to
       point  to  a standard *FILE structure.  It is probably a mistake to use
       NULL for one of write_data_fn and output_flush_fn but not both of them,
       unless you have built libpng with PNG_NO_WRITE_FLUSH defined.  It is an
       error to read from a write stream, and vice versa.

       Error handling in libpng is done through png_error() and png_warning().
       Errors  handled through png_error() are fatal, meaning that png_error()
       should never return to its caller.   Currently,  this  is  handled  via
       setjmp()   and   longjmp()   (unless  you  have  compiled  libpng  with
       PNG_NO_SETJMP, in which case it is handled via  PNG_ABORT()),  but  you
       could  change this to do things like exit() if you should wish, as long
       as your function does not return.

       On non-fatal errors, png_warning() is called to print  a  warning  mes-
       sage,  and  then  control  returns  to  the  calling  code.  By default
       png_error() and png_warning() print a message on stderr  via  fprintf()
       unless  the library is compiled with PNG_NO_CONSOLE_IO defined (because
       you don't want the messages) or PNG_NO_STDIO defined (because fprintf()
       isn't  available).   If  you  wish  to change the behavior of the error
       functions, you will need to set up your own message  callbacks.   These
       functions are normally supplied at the time that the png_struct is cre-
       ated.  It is also possible to redirect errors and warnings to your  own
       replacement  functions  after  png_create_*_struct() has been called by
       calling:

           png_set_error_fn(png_structp png_ptr,
               png_voidp error_ptr, png_error_ptr error_fn,
               png_error_ptr warning_fn);

           png_voidp error_ptr = png_get_error_ptr(png_ptr);

       If NULL is supplied for either error_fn or warning_fn, then the  libpng
       default  function will be used, calling fprintf() and/or longjmp() if a
       problem is encountered.  The replacement error  functions  should  have
       parameters as follows:

           void user_error_fn(png_structp png_ptr,
               png_const_charp error_msg);

           void user_warning_fn(png_structp png_ptr,
               png_const_charp warning_msg);

       The motivation behind using setjmp() and longjmp() is the C++ throw and
       catch exception handling methods.  This makes the code much  easier  to
       write, as there is no need to check every return code of every function
       call.  However, there are some uncertainties about the status of  local
       variables  after  a  longjmp,  so the user may want to be careful about
       doing anything after setjmp returns non-zero besides returning  itself.
       Consult  your compiler documentation for more details.  For an alterna-
       tive approach,  you  may  wish  to  use  the  "cexcept"  facility  (see
       http://cexcept.sourceforge.net), which is illustrated in pngvalid.c and
       in contrib/visupng.


   Custom chunks
       If you need to read or write custom chunks, you may need to get  deeper
       into  the  libpng code.  The library now has mechanisms for storing and
       writing chunks of unknown type; you can even declare callbacks for cus-
       tom  chunks.   However, this may not be good enough if the library code
       itself needs to know about interactions between your chunk and existing
       `intrinsic' chunks.

       If you need to write a new intrinsic chunk, first read the PNG specifi-
       cation. Acquire a first level of understanding of how  it  works.   Pay
       particular  attention  to  the  sections that describe chunk names, and
       look at how other chunks were designed, so you can do things similarly.
       Second,  check  out  the sections of libpng that read and write chunks.
       Try to find a chunk that is similar to yours and use it as a  template.
       More  details can be found in the comments inside the code.  It is best
       to handle private or unknown chunks in a generic method,  via  callback
       functions,  instead  of  by  modifying libpng functions. This is illus-
       trated in pngtest.c, which uses a callback function to handle a private
       "vpAg"  chunk  and  the  new  "sTER"  chunk,  which are both unknown to
       libpng.

       If you wish to write your own transformation for the data, look through
       the  part of the code that does the transformations, and check out some
       of the simpler ones to get an idea of how they work.   Try  to  find  a
       similar  transformation  to the one you want to add and copy off of it.
       More details can be found in the comments inside the code itself.


   Configuring for 16-bit platforms
       You will want to look into zconf.h to tell zlib (and thus libpng)  that
       it  cannot allocate more then 64K at a time.  Even if you can, the mem-
       ory won't be accessible.  So limit zlib and libpng to 64K  by  defining
       MAXSEG_64K.


   Configuring for DOS
       For  DOS users who only have access to the lower 640K, you will have to
       limit zlib's memory usage via a  png_set_compression_mem_level()  call.
       See zlib.h or zconf.h in the zlib library for more information.


   Configuring for Medium Model
       Libpng's  support for medium model has been tested on most of the popu-
       lar compilers.  Make sure MAXSEG_64K gets defined, USE_FAR_KEYWORD gets
       defined,  and  FAR  gets defined to far in pngconf.h, and you should be
       all set.  Everything in the library (except for  zlib's  structure)  is
       expecting  far data.  You must use the typedefs with the p or pp on the
       end for pointers (or at least look at them and be careful).  Make  note
       that  the rows of data are defined as png_bytepp, which is an "unsigned
       char far * far *".


   Configuring for gui/windowing platforms:
       You will need to write new error and warning functions that use the GUI
       interface,  as  described  previously, and set them to be the error and
       warning functions at the time that png_create_*_struct() is called,  in
       order to have them available during the structure initialization.  They
       can be changed later via png_set_error_fn().  On  some  compilers,  you
       may also have to change the memory allocators (png_malloc, etc.).


   Configuring for compiler xxx:
       All  includes  for libpng are in pngconf.h.  If you need to add, change
       or delete an include, this is the place to do it.   The  includes  that
       are  not  needed  outside libpng are placed in pngpriv.h, which is only
       used by the routines inside libpng itself.  The files in libpng  proper
       only include pngpriv.h and png.h, which %14%in turn includes pngconf.h.
       in turn includes pngconf.h and, as of libpng-1.5.0,  pnglibconf.h.   As
       of  libpng-1.5.0,  pngpriv.h  also  includes three other private header
       files, pngstruct.h, pnginfo.h, and pngdebug.h, which  contain  material
       that previously appeared in the public headers.


   Configuring zlib:
       There  are special functions to configure the compression.  Perhaps the
       most useful one changes the compression  level,  which  currently  uses
       input compression values in the range 0 - 9.  The library normally uses
       the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests  have
       shown  that  for  a large majority of images, compression values in the
       range 3-6 compress nearly as well as higher  levels,  and  do  so  much
       faster.   For  online  applications it may be desirable to have maximum
       speed (Z_BEST_SPEED = 1).  With versions of zlib after v0.99,  you  can
       also specify no compression (Z_NO_COMPRESSION = 0), but this would cre-
       ate files larger than just storing the raw bitmap.  You can specify the
       compression level by calling:

           #include zlib.h
           png_set_compression_level(png_ptr, level);

       Another  useful  one is to reduce the memory level used by the library.
       The memory level defaults to 8, but it can be lowered if you are  short
       on  memory  (running DOS, for example, where you only have 640K).  Note
       that the memory level does have an effect on compression;  among  other
       things,  lower  levels  will  result in sections of incompressible data
       being emitted in smaller stored blocks, with a  correspondingly  larger
       relative overhead of up to 15% in the worst case.

           #include zlib.h
           png_set_compression_mem_level(png_ptr, level);

       The other functions are for configuring zlib.  They are not recommended
       for normal use and may result in writing  an  invalid  PNG  file.   See
       zlib.h for more information on what these mean.

           #include zlib.h
           png_set_compression_strategy(png_ptr,
               strategy);

           png_set_compression_window_bits(png_ptr,
               window_bits);

           png_set_compression_method(png_ptr, method);

           png_set_compression_buffer_size(png_ptr, size);

       As  of  libpng  version  1.5.4, additional APIs became available to set
       these separately for non-IDAT compressed chunks such as zTXt, iTXt, and
       iCCP:

           #include zlib.h
           #if PNG_LIBPNG_VER >= 10504
           png_set_text_compression_level(png_ptr, level);

           png_set_text_compression_mem_level(png_ptr, level);

           png_set_text_compression_strategy(png_ptr,
               strategy);

           png_set_text_compression_window_bits(png_ptr,
               window_bits);

           png_set_text_compression_method(png_ptr, method);
           #endif


   Controlling row filtering
       If you want to control whether libpng uses filtering or not, which fil-
       ters are used, and how it goes about picking row filters, you can  call
       one of these functions.  The selection and configuration of row filters
       can have a significant impact on the size  and  encoding  speed  and  a
       somewhat lesser impact on the decoding speed of an image.  Filtering is
       enabled by default for RGB  and  grayscale  images  (with  and  without
       alpha),  but not for paletted images nor for any images with bit depths
       less than 8 bits/pixel.

       The 'method' parameter sets the main filtering method,  which  is  cur-
       rently  only '0' in the PNG 1.2 specification.  The 'filters' parameter
       sets which filter(s), if any, should be used for each scanline.  Possi-
       ble  values are PNG_ALL_FILTERS and PNG_NO_FILTERS to turn filtering on
       and off, respectively.

       Individual filter types are PNG_FILTER_NONE,  PNG_FILTER_SUB,  PNG_FIL-
       TER_UP,  PNG_FILTER_AVG,  PNG_FILTER_PAETH,  which  can be bitwise ORed
       together with '|' to specify one or more filters to use.  These filters
       are  described  in more detail in the PNG specification.  If you intend
       to change the filter type during the course of writing the  image,  you
       should start with flags set for all of the filters you intend to use so
       that libpng can initialize its internal  structures  appropriately  for
       all  of  the  filter  types.   (Note that this means the first row must
       always be adaptively filtered, because libpng currently does not  allo-
       cate  the  filter buffers until png_write_row() is called for the first
       time.)

           filters = PNG_FILTER_NONE | PNG_FILTER_SUB
                     PNG_FILTER_UP | PNG_FILTER_AVG |
                     PNG_FILTER_PAETH | PNG_ALL_FILTERS;

           png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
              filters);
                     The second parameter can also be
                     PNG_INTRAPIXEL_DIFFERENCING if you are
                     writing a PNG to be embedded in a MNG
                     datastream.  This parameter must be the
                     same as the value of filter_method used
                     in png_set_IHDR().

       It is also possible to influence how  libpng  chooses  from  among  the
       available  filters.   This  is  done  in  one  or both of two ways - by
       telling it how important it is to keep the same filter  for  successive
       rows,  and  by  telling it the relative computational costs of the fil-
       ters.

           double weights[3] = {1.5, 1.3, 1.1},
              costs[PNG_FILTER_VALUE_LAST] =
              {1.0, 1.3, 1.3, 1.5, 1.7};

           png_set_filter_heuristics(png_ptr,
              PNG_FILTER_HEURISTIC_WEIGHTED, 3,
              weights, costs);

       The weights are multiplying factors that indicate to  libpng  that  the
       row  filter  should  be the same for successive rows unless another row
       filter is that many times better than  the  previous  filter.   In  the
       above  example,  if the previous 3 filters were SUB, SUB, NONE, the SUB
       filter could have a "sum of  absolute  differences"  1.5  x  1.3  times
       higher  than  other  filters and still be chosen, while the NONE filter
       could have a sum 1.1 times higher than other filters and still be  cho-
       sen.   Unspecified  weights  are  taken  to  be  1.0, and the specified
       weights should probably be declining  like  those  above  in  order  to
       emphasize recent filters over older filters.

       The  filter costs specify for each filter type a relative decoding cost
       to be considered when selecting row filters.  This means  that  filters
       with  higher costs are less likely to be chosen over filters with lower
       costs, unless their "sum of absolute differences" is that much smaller.
       The  costs do not necessarily reflect the exact computational speeds of
       the various filters, since this would unduly influence the final  image
       size.

       Note  that  the numbers above were invented purely for this example and
       are given only to help explain the function usage.  Little testing  has
       been done to find optimum values for either the costs or the weights.


   Removing unwanted object code
       There  are a bunch of #define's in pngconf.h that control what parts of
       libpng are compiled.  All the defines end in _SUPPORTED.   If  you  are
       never  going  to use a capability, you can change the #define to #undef
       before recompiling libpng and save yourself code and data space, or you
       can  turn  off  individual  capabilities  with  defines that begin with
       PNG_NO_.

       In libpng-1.5.0 and later, the #define's are in pnglibconf.h instead.

       You can also turn all of the transforms and ancillary  chunk  capabili-
       ties  off  en masse with compiler directives that define PNG_NO_READ[or
       WRITE]_TRANSFORMS, or PNG_NO_READ[or  WRITE]_ANCILLARY_CHUNKS,  or  all
       four, along with directives to turn on any of the capabilities that you
       do want.  The PNG_NO_READ[or WRITE]_TRANSFORMS directives  disable  the
       extra  transformations  but  still  leave  the library fully capable of
       reading and writing PNG files with all known public chunks. Use of  the
       PNG_NO_READ[or  WRITE]_ANCILLARY_CHUNKS  directive  produces  a library
       that is incapable of reading or writing ancillary chunks.  If  you  are
       not  using  the  progressive  reading capability, you can turn that off
       with PNG_NO_PROGRESSIVE_READ (don't confuse this with  the  INTERLACING
       capability, which you'll still have).

       All the reading and writing specific code are in separate files, so the
       linker should only grab the files it needs.  However, if  you  want  to
       make  sure, or if you are building a stand alone library, all the read-
       ing files start with "pngr"  and  all  the  writing  files  start  with
       "pngw".   The  files  that  don't match either (like png.c, pngtrans.c,
       etc.)  are used for both reading and writing, and  always  need  to  be
       included.  The progressive reader is in pngpread.c

       If you are creating or distributing a dynamically linked library (a .so
       or DLL file), you should  not  remove  or  disable  any  parts  of  the
       library, as this will cause applications linked with different versions
       of the library to fail if they call functions  not  available  in  your
       library.   The  size  of  the  library  itself  should not be an issue,
       because only those sections that are actually used will be loaded  into
       memory.


   Requesting debug printout
       The  macro definition PNG_DEBUG can be used to request debugging print-
       out.  Set it to an integer value in the range 0 to 3.   Higher  numbers
       result in increasing amounts of debugging information.  The information
       is printed to the "stderr" file, unless another file name is  specified
       in the PNG_DEBUG_FILE macro definition.

       When PNG_DEBUG > 0, the following functions (macros) become available:

          png_debug(level, message)
          png_debug1(level, message, p1)
          png_debug2(level, message, p1, p2)

       in  which  "level"  is compared to PNG_DEBUG to decide whether to print
       the message, "message" is the formatted string to be  printed,  and  p1
       and  p2  are parameters that are to be embedded in the string according
       to printf-style formatting directives.  For example,

          png_debug1(2, "foo=%d", foo);

       is expanded to

          if (PNG_DEBUG > 2)
             fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);

       When PNG_DEBUG is defined but is zero, the macros aren't  defined,  but
       you can still use PNG_DEBUG to control your own debugging:

          #ifdef PNG_DEBUG
              fprintf(stderr, ...
          #endif

       When  PNG_DEBUG  = 1, the macros are defined, but only png_debug state-
       ments having level = 0 will be printed.  There aren't any  such  state-
       ments  in  this  version of libpng, but if you insert some they will be
       printed.



VI. MNG support

       The  MNG  specification  (available  at  http://www.libpng.org/pub/mng)
       allows  certain  extensions  to PNG for PNG images that are embedded in
       MNG datastreams.  Libpng can support  some  of  these  extensions.   To
       enable them, use the png_permit_mng_features() function:

          feature_set = png_permit_mng_features(png_ptr, mask)

          mask is a png_uint_32 containing the bitwise OR of the
               features you want to enable.  These include
               PNG_FLAG_MNG_EMPTY_PLTE
               PNG_FLAG_MNG_FILTER_64
               PNG_ALL_MNG_FEATURES

          feature_set is a png_uint_32 that is the bitwise AND of
             your mask with the set of MNG features that is
             supported by the version of libpng that you are using.

       It  is  an  error to use this function when reading or writing a stand-
       alone PNG file with the PNG 8-byte signature.  The PNG datastream  must
       be  wrapped  in  a  MNG datastream.  As a minimum, it must have the MNG
       8-byte signature and the MHDR and MEND chunks.  Libpng does not provide
       support  for  these or any other MNG chunks; your application must pro-
       vide its own support for them.  You may wish to consider  using  libmng
       (available at http://www.libmng.com) instead.



VII. Changes to Libpng from version 0.88

       It should be noted that versions of libpng later than 0.96 are not dis-
       tributed by the original libpng author, Guy Schalnat,  nor  by  Andreas
       Dilger,  who had taken over from Guy during 1996 and 1997, and distrib-
       uted versions 0.89 through 0.96, but rather by another  member  of  the
       original  PNG  Group, Glenn Randers-Pehrson.  Guy and Andreas are still
       alive and well, but they have moved on to other things.

       The   old   libpng   functions    png_read_init(),    png_write_init(),
       png_info_init(),  png_read_destroy(), and png_write_destroy() have been
       moved to PNG_INTERNAL in version 0.95 to discourage their  use.   These
       functions will be removed from libpng version 1.4.0.

       The preferred method of creating and initializing the libpng structures
       is via  the  png_create_read_struct(),  png_create_write_struct(),  and
       png_create_info_struct()  because  they  isolate the size of the struc-
       tures from the application, allow  version  error  checking,  and  also
       allow  the use of custom error handling routines during the initializa-
       tion, which the old functions do not.  The functions png_read_destroy()
       and  png_write_destroy()  do  not  actually free the memory that libpng
       allocated for these structs, but just reset  the  data  structures,  so
       they   can   be   used   instead   of   png_destroy_read_struct()   and
       png_destroy_write_struct() if you feel there is too much  system  over-
       head allocating and freeing the png_struct for each image read.

       Setting   the   error   callbacks   via   png_set_message_fn()   before
       png_read_init() as was suggested in libpng-0.88 is no longer  supported
       because this caused applications that do not use custom error functions
       to fail if the png_ptr was not initialized to zero.  It is still possi-
       ble to set the error callbacks AFTER png_read_init(), or to change them
       with png_set_error_fn(), which is essentially the  same  function,  but
       with  a new name to force compilation errors with applications that try
       to use the old method.

       Support for the  sCAL,  iCCP,  iTXt,  and  sPLT  chunks  was  added  at
       libpng-1.0.6; however, iTXt support was not enabled by default.

       Starting  with  version  1.0.7,  you  can find out which version of the
       library you are using at run-time:

          png_uint_32 libpng_vn = png_access_version_number();

       The number libpng_vn is constructed from the major version, minor  ver-
       sion  with  leading  zero, and release number with leading zero, (e.g.,
       libpng_vn for version 1.0.7 is 10007).

       Note that this function does not take a png_ptr, so  you  can  call  it
       before you've created one.

       You  can also check which version of png.h you used when compiling your
       application:

          png_uint_32 application_vn = PNG_LIBPNG_VER;



VIII. Changes to Libpng from version 1.0.x to 1.2.x

       Support for user memory management was enabled by default.   To  accom-
       plish   this,   the   functions   png_create_read_struct_2(),  png_cre-
       ate_write_struct_2(),  png_set_mem_fn(),  png_get_mem_ptr(),   png_mal-
       loc_default(), and png_free_default() were added.

       Support  for  the  iTXt chunk has been enabled by default as of version
       1.2.41.

       Support for certain MNG features was enabled.

       Support for numbered error messages was added.  However, we  never  got
       around   to  actually  numbering  the  error  messages.   The  function
       png_set_strip_error_numbers() was added (Note: the prototype  for  this
       function  was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
       builds of libpng-1.2.15.  It was restored in libpng-1.2.36).

       The png_malloc_warn() function was added at libpng-1.2.3.  This  issues
       a  png_warning  and  returns  NULL instead of aborting when it fails to
       acquire the requested memory allocation.

       Support for setting user limits on image width and height  was  enabled
       by       default.        The      functions      png_set_user_limits(),
       png_get_user_width_max(), and png_get_user_height_max() were  added  at
       libpng-1.2.6.

       The png_set_add_alpha() function was added at libpng-1.2.7.

       The    function    png_set_expand_gray_1_2_4_to_8()    was   added   at
       libpng-1.2.9.  Unlike png_set_gray_1_2_4_to_8(), the new function  does
       not expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() func-
       tion is deprecated.

       A number of macro definitions in support of runtime selection of assem-
       bler  code  features  (especially Intel MMX code support) were added at
       libpng-1.2.0:

           PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
           PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
           PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
           PNG_ASM_FLAG_MMX_READ_INTERLACE
           PNG_ASM_FLAG_MMX_READ_FILTER_SUB
           PNG_ASM_FLAG_MMX_READ_FILTER_UP
           PNG_ASM_FLAG_MMX_READ_FILTER_AVG
           PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
           PNG_ASM_FLAGS_INITIALIZED
           PNG_MMX_READ_FLAGS
           PNG_MMX_FLAGS
           PNG_MMX_WRITE_FLAGS
           PNG_MMX_FLAGS

       We added the following functions in support  of  runtime  selection  of
       assembler code features:

           png_get_mmx_flagmask()
           png_set_mmx_thresholds()
           png_get_asm_flags()
           png_get_mmx_bitdepth_threshold()
           png_get_mmx_rowbytes_threshold()
           png_set_asm_flags()

       We  replaced all of these functions with simple stubs in libpng-1.2.20,
       when the Intel assembler code was removed due to a licensing issue.

       These macros are deprecated:

           PNG_READ_TRANSFORMS_NOT_SUPPORTED
           PNG_PROGRESSIVE_READ_NOT_SUPPORTED
           PNG_NO_SEQUENTIAL_READ_SUPPORTED
           PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
           PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
           PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

       They have been replaced, respectively, by:

           PNG_NO_READ_TRANSFORMS
           PNG_NO_PROGRESSIVE_READ
           PNG_NO_SEQUENTIAL_READ
           PNG_NO_WRITE_TRANSFORMS
           PNG_NO_READ_ANCILLARY_CHUNKS
           PNG_NO_WRITE_ANCILLARY_CHUNKS

       PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been deprecated
       since libpng-1.0.16 and libpng-1.2.6.

       The function
           png_check_sig(sig, num) was replaced with
           !png_sig_cmp(sig, 0, num) It has been deprecated since libpng-0.90.

       The function
           png_set_gray_1_2_4_to_8() which also  expands  tRNS  to  alpha  was
       replaced with
           png_set_expand_gray_1_2_4_to_8() which does not. It has been depre-
       cated since libpng-1.0.18 and 1.2.9.



IX. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x

       Private libpng prototypes and macro definitions were moved  from  png.h
       and pngconf.h into a new pngpriv.h header file.

       Functions      png_set_benign_errors(),     png_benign_error(),     and
       png_chunk_benign_error() were added.

       Support for setting the maximum amount of memory that  the  application
       will allocate for reading chunks was added, as a security measure.  The
       functions png_set_chunk_cache_max() and png_get_chunk_cache_max()  were
       added to the library.

       We implemented support for I/O states by adding png_ptr member io_state
       and  functions  png_get_io_chunk_name()   and   png_get_io_state()   in
       pngget.c

       We  added  PNG_TRANSFORM_GRAY_TO_RGB  to the available high-level input
       transforms.

       Checking for and reporting of errors in the IHDR chunk  is  more  thor-
       ough.

       Support for global arrays was removed, to improve thread safety.

       Some obsolete/deprecated macros and functions have been removed.

       Typecasted NULL definitions such as
          #define  png_voidp_NULL             (png_voidp)NULL were eliminated.
       If you used these in your application, just use NULL instead.

       The png_struct and info_struct members "trans" and "trans_values"  were
       changed to "trans_alpha" and "trans_color", respectively.

       The  obsolete,  unused pnggccrd.c and pngvcrd.c files and related make-
       files were removed.

       The PNG_1_0_X and PNG_1_2_X macros were eliminated.

       The PNG_LEGACY_SUPPORTED macro was eliminated.

       Many WIN32_WCE #ifdefs were removed.

       The   functions   png_read_init(info_ptr),    png_write_init(info_ptr),
       png_info_init(info_ptr),  png_read_destroy(),  and  png_write_destroy()
       have been removed.  They have been deprecated since libpng-0.95.

       The png_permit_empty_plte() was removed. It has been  deprecated  since
       libpng-1.0.9.  Use png_permit_mng_features() instead.

       We   removed   the   obsolete  stub  functions  png_get_mmx_flagmask(),
       png_set_mmx_thresholds(),     png_get_asm_flags(),     png_get_mmx_bit-
       depth_threshold(),                    png_get_mmx_rowbytes_threshold(),
       png_set_asm_flags(), and png_mmx_supported()

       We  removed  the  obsolete  png_check_sig(),  png_memcpy_check(),   and
       png_memset_check()  functions.   Instead  use  !png_sig_cmp(), png_mem-
       cpy(), and png_memset(), respectively.

       The function png_set_gray_1_2_4_to_8() was removed. It has been  depre-
       cated  since  libpng-1.0.18  and  1.2.9,  when  it  was  replaced  with
       png_set_expand_gray_1_2_4_to_8()  because  the  former  function   also
       expanded any tRNS chunk to an alpha channel.

       Macros  for  png_get_uint_16,  png_get_uint_32, and png_get_int_32 were
       added and are used by default instead of the  corresponding  functions.
       Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro
       (but  not  the  function)  incorrectly  returned  a   value   of   type
       png_uint_32.

       We changed the prototype for png_malloc() from
           png_malloc(png_structp png_ptr, png_uint_32 size) to
           png_malloc(png_structp png_ptr, png_alloc_size_t size)

       This  also  applies  to  the  prototype  for  the user replacement mal-
       loc_fn().

       The png_calloc() function  was  added  and  is  used  in  place  of  of
       "png_malloc();  memset();"  except  in the case in png_read_png() where
       the array consists of pointers; in this case a "for" loop is used after
       the png_malloc() to set the pointers to NULL, to give robust.  behavior
       in case the  application  runs  out  of  memory  part-way  through  the
       process.

       We  changed  the  prototypes  of  png_get_compression_buffer_size() and
       png_set_compression_buffer_size() to work with  png_size_t  instead  of
       png_uint_32.

       Support  for  numbered  error messages was removed by default, since we
       never got around to actually numbering the error messages. The function
       png_set_strip_error_numbers()  was removed from the library by default.

       The png_zalloc() and png_zfree() functions are no longer exported.  The
       png_zalloc()  function  no  longer  zeroes out the memory that it allo-
       cates.  Applications that called png_zalloc(png_ptr, number, size)  can
       call  png_calloc(png_ptr, number*size) instead, and can call png_free()
       instead of png_zfree().

       Support for dithering was disabled by default in libpng-1.4.0,  because
       it  has  not  been well tested and doesn't actually "dither".  The code
       was not removed, however, and could be enabled by building libpng  with
       PNG_READ_DITHER_SUPPORTED  defined.   In libpng-1.4.2, this support was
       re-enabled, but the function was renamed png_set_quantize() to  reflect
       more  accurately  what  it  actually  does.   At  the  same  time,  the
       PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to  PNG_QUAN-
       TIZE_[RED,GREEN,BLUE]_BITS,  and  PNG_READ_DITHER_SUPPORTED was renamed
       to PNG_READ_QUANTIZE_SUPPORTED.

       We removed the trailing '.' from the warning and error messages.



X. Changes to Libpng from version 1.4.x to 1.5.x

       From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but  not  the
       function) incorrectly returned a value of type png_uint_32.  The incor-
       rect macro was removed from libpng-1.4.5.

       Checking for invalid palette index on write was added at libpng 1.5.10.
       If  a  pixel  contains  an invalid (out-of-range) index libpng issues a
       benign error.  This is enabled by default because this condition is  an
       error  according to the PNG specification, Clause 11.3.2, but the error
       can be ignored in each png_ptr with

          png_set_check_for_invalid_index(png_ptr, allowed);

             allowed  - one of
                        0: disable benign error (accept the
                           invalid data without warning).
                        1: enable benign error (treat the
                           invalid data as an error or a
                           warning).

       If the error is ignored, or if png_benign_error() treats it as a  warn-
       ing,  any invalid pixels are decoded as opaque black by the decoder and
       written as-is by the encoder.

       Retrieving the maximum palette index found was added at  libpng-1.5.15.
       This  statement  must  appear  after png_read_png() or png_read_image()
       while reading, and after  png_write_png()  or  png_write_image()  while
       writing.

          int max_palette = png_get_palette_max(png_ptr, info_ptr);

       This  will return the maximum palette index found in the image, or "-1"
       if the palette was not checked, or "0" if no palette was  found.   Note
       that  this  does  not  account  for any palette index used by ancillary
       chunks such as the bKGD chunk;  you  must  check  those  separately  to
       determine the maximum palette index actually used.

       A. Changes that affect users of libpng

       There  are  no substantial API changes between the non-deprecated parts
       of the 1.4.5 API and the 1.5.0 API; however, the  ability  to  directly
       access  members  of  the main libpng control structures, png_struct and
       png_info, deprecated in earlier versions of libpng, has been completely
       removed from libpng 1.5.

       We  no  longer include zlib.h in png.h.  The include statement has been
       moved to pngstruct.h, where  it  is  not  accessible  by  applications.
       Applications that need access to information in zlib.h will need to add
       the '#include "zlib.h"' directive.  It does not matter whether this  is
       placed prior to or after the '"#include png.h"' directive.

       The png_sprintf(), png_strcpy(), and png_strncpy() macros are no longer
       used and were removed.

       We moved the png_strlen(), png_memcpy(), png_memset(), and png_memcmp()
       macros into a private header file (pngpriv.h) that is not accessible to
       applications.

       In png_get_iCCP, the type of "profile" was changed from  png_charpp  to
       png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.

       There are changes of form in png.h, including new and changed macros to
       declare parts of the API.  Some API functions with arguments  that  are
       pointers  to  data not modified within the function have been corrected
       to declare these arguments with PNG_CONST.

       Much of the internal use of C macros to control the library  build  has
       also  changed and some of this is visible in the exported header files,
       in particular the use of macros to control data and API elements  visi-
       ble  during application compilation may require significant revision to
       application code.  (It is extremely  rare  for  an  application  to  do
       this.)

       Any program that compiled against libpng 1.4 and did not use deprecated
       features or access internal library structures should compile and  work
       against  libpng  1.5,  except  for  the  change  in  the  prototype for
       png_get_iCCP() and png_set_iCCP() API functions mentioned above.

       libpng 1.5.0 adds PNG_ PASS macros to help in the reading  and  writing
       of interlaced images.  The macros return the number of rows and columns
       in each pass and information that can be used to de-interlace  and  (if
       absolutely necessary) interlace an image.

       libpng  1.5.0  adds an API png_longjmp(png_ptr, value).  This API calls
       the application-provided png_longjmp_ptr on the internal, but  applica-
       tion  initialized,  longjmp buffer.  It is provided as a convenience to
       avoid the need to use the png_jmpbuf macro, which had  the  unnecessary
       side effect of resetting the internal png_longjmp_ptr value.

       libpng  1.5.0  includes a complete fixed point API.  By default this is
       present along with the corresponding floating point  API.   In  general
       the  fixed  point API is faster and smaller than the floating point one
       because the PNG file format used fixed point, not floating point.  This
       applies  even  if  the library uses floating point in internal calcula-
       tions.  A new macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals whether
       the library uses floating point arithmetic (the default) or fixed point
       arithmetic internally for performance  critical  calculations  such  as
       gamma  correction.   In  some cases, the gamma calculations may produce
       slightly  different  results.   This  has  changed   the   results   in
       png_rgb_to_gray  and in alpha composition (png_set_background for exam-
       ple). This applies even if the original image was already linear (gamma
       ==  1.0)  and,  therefore,  it is not necessary to linearize the image.
       This is because libpng has *not* been changed  to  optimize  that  case
       correctly, yet.

       Fixed  point support for the sCAL chunk comes with an important caveat;
       the sCAL specification uses a decimal encoding of floating point values
       and  the  accuracy of PNG fixed point values is insufficient for repre-
       sentation of these values. Consequently a "string" API  (png_get_sCAL_s
       and  png_set_sCAL_s) is the only reliable way of reading arbitrary sCAL
       chunks in the absence of either the  floating  point  API  or  internal
       floating point calculations.  Starting with libpng-1.5.0, both of these
       functions are present when PNG_sCAL_SUPPORTED  is  defined.   Prior  to
       libpng-1.5.0,  their  presence  also depended upon PNG_FIXED_POINT_SUP-
       PORTED  being  defined  and  PNG_FLOATING_POINT_SUPPORTED   not   being
       defined.

       Applications no longer need to include the optional distribution header
       file pngusr.h or define the  corresponding  macros  during  application
       build  in  order  to  see  the correct variant of the libpng API.  From
       1.5.0 application code  can  check  for  the  corresponding  _SUPPORTED
       macro:

       #ifdef PNG_INCH_CONVERSIONS_SUPPORTED
          /* code that uses the inch conversion APIs. */ #endif

       This  macro  will only be defined if the inch conversion functions have
       been compiled into libpng.  The full set of macros, and whether or  not
       support  has been compiled in, are available in the header file pnglib-
       conf.h.  This header file is specific to the libpng build.  Notice that
       prior to 1.5.0 the _SUPPORTED macros would always have the default def-
       inition unless reset by pngusr.h or by explicit settings  on  the  com-
       piler  command  line.   These settings may produce compiler warnings or
       errors in 1.5.0 because of macro redefinition.

       Applications can now choose whether to use these macros or to call  the
       corresponding    function    by    defining    PNG_USE_READ_MACROS   or
       PNG_NO_USE_READ_MACROS before including png.h.   Notice  that  this  is
       only  supported  from  1.5.0;  defining PNG_NO_USE_READ_MACROS prior to
       1.5.0 will lead to a link failure.

       Prior to libpng-1.5.4, the zlib compressor used the same set of parame-
       ters  when  compressing the IDAT data and textual data such as zTXt and
       iCCP.  In libpng-1.5.4 we reinitialized the zlib stream for  each  type
       of  data.   We  added  five  png_set_text_*() functions for setting the
       parameters to use with textual data.

       Prior to  libpng-1.5.4,  the  PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
       option  was  off  by default, and slightly inaccurate scaling occurred.
       This option can no longer be turned off, and the choice of accurate  or
       inaccurate  16-to-8 scaling is by using the new png_set_scale_16_to_8()
       API for accurate scaling or the  old  png_set_strip_16_to_8()  API  for
       simple   chopping.    In   libpng-1.5.4,   the   PNG_READ_16_TO_8_ACCU-
       RATE_SCALE_SUPPORTED macro became PNG_READ_SCALE_16_TO_8_SUPPORTED, and
       the  PNG_READ_16_TO_8 macro became PNG_READ_STRIP_16_TO_8_SUPPORTED, to
       enable the two png_set_*_16_to_8() functions separately.

       Prior to libpng-1.5.4, the png_set_user_limits() function could only be
       used  to  reduce  the  width  and  height  limits  from  the  value  of
       PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document said
       that  it could be used to override them.  Now this function will reduce
       or increase the limits.

       Starting in libpng-1.5.10, the user limits can be set en masse with the
       configuration  option  PNG_SAFE_LIMITS_SUPPORTED.   If  this  option is
       enabled, a set of "safe" limits is applied in pngpriv.h.  These can  be
       overridden    by    application    calls    to   png_set_user_limits(),
       png_set_user_chunk_cache_max(), and/or  png_set_user_malloc_max()  that
       increase  or  decrease  the limits.  Also, in libpng-1.5.10 the default
       width and height limits were  increased  from  1,000,000  to  0x7ffffff
       (i.e., made unlimited).  Therefore, the limits are now
                                      default      safe
          png_user_width_max        0x7fffffff    1,000,000
          png_user_height_max       0x7fffffff    1,000,000
          png_user_chunk_cache_max  0 (unlimited)   128
          png_user_chunk_malloc_max 0 (unlimited) 8,000,000

       The  png_set_option()  function  (and  the  "options" member of the png
       struct) was added to libpng-1.5.15.

       B. Changes to the build and configuration of libpng

       Details of internal changes to the library code can  be  found  in  the
       CHANGES  file and in the GIT repository logs.  These will be of no con-
       cern to the vast majority of library users or  builders;  however,  the
       few  who  configure  libpng  to  a  non-default feature set may need to
       change how this is done.

       There should be no need for library builders to alter build scripts  if
       these  use the distributed build support - configure or the makefiles -
       however, users of the makefiles may care to update their build  scripts
       to  build pnglibconf.h where the corresponding makefile does not do so.

       Building libpng with  a  non-default  configuration  has  changed  com-
       pletely.   The  old  method  using pngusr.h should still work correctly
       even though the way pngusr.h is used in the  build  has  been  changed;
       however,  library builders will probably want to examine the changes to
       take advantage of new capabilities and to simplify their build  system.

       B.1 Specific changes to library configuration capabilities

       The  library now supports a complete fixed point implementation and can
       thus be used on systems that have no floating  point  support  or  very
       limited  or  slow  support.   Previously gamma correction, an essential
       part of complete PNG support, required reasonably fast floating  point.

       As  part  of  this  the choice of internal implementation has been made
       independent of the choice of fixed versus floating point APIs  and  all
       the missing fixed point APIs have been implemented.

       The  exact  mechanism  used  to control attributes of API functions has
       changed.  A single set of operating system  independent  macro  defini-
       tions  is  used and operating system specific directives are defined in
       pnglibconf.h

       As part of this the mechanism used to choose procedure  call  standards
       on those systems that allow a choice has been changed.  At present this
       only affects certain Microsoft (DOS, Windows) and IBM (OS/2)  operating
       systems  running  on  Intel  processors.   As before, PNGAPI is defined
       where required to control the exported API functions; however, two  new
       macros,  PNGCBAPI  and PNGCAPI, are used instead for callback functions
       (PNGCBAPI) and (PNGCAPI) for functions that must match a C library pro-
       totype  (currently only png_longjmp_ptr, which must match the C longjmp
       function.)  The new approach is documented in pngconf.h

       Despite these changes, libpng 1.5.0 only supports the native C function
       calling standard on those platforms tested so far (__cdecl on Microsoft
       Windows).  This is because the  support  requirements  for  alternative
       calling  conventions  seem  to no longer exist.  Developers who find it
       necessary to set PNG_API_RULE to 1 should advise the mailing list (png-
       mng-implement)  of  this  and  library  builders who use Openwatcom and
       therefore set PNG_API_RULE to 2 should also contact the mailing list.

       A new test program, pngvalid, is provided in addition to pngtest.  png-
       valid  validates the arithmetic accuracy of the gamma correction calcu-
       lations and includes a number of validations of  the  file  format.   A
       subset  of the full range of tests is run when "make check" is done (in
       the 'configure' build.)  pngvalid also allows  total  allocated  memory
       usage  to be evaluated and performs additional memory overwrite valida-
       tion.

       Many changes to individual feature macros have been made. The following
       are  the changes most likely to be noticed by library builders who con-
       figure libpng:

       1) All feature macros now have consistent naming:

       #define PNG_NO_feature turns the feature off  #define  PNG_feature_SUP-
       PORTED turns the feature on

       pnglibconf.h contains one line for each feature macro which is either:

       #define PNG_feature_SUPPORTED

       if the feature is supported or:

       /*#undef PNG_feature_SUPPORTED*/

       if  it  is  not.   Library code consistently checks for the 'SUPPORTED'
       macro.  It does not, and libpng applications should not, check for  the
       'NO'  macro  which  will not normally be defined even if the feature is
       not supported.  The 'NO' macros are only used internally for setting or
       not setting the corresponding 'SUPPORTED' macros.

       Compatibility with the old names is provided as follows:

       PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED

       And the following definitions disable the corresponding feature:

       PNG_SETJMP_NOT_SUPPORTED  disables  SETJMP PNG_READ_TRANSFORMS_NOT_SUP-
       PORTED disables READ_TRANSFORMS  PNG_NO_READ_COMPOSITED_NODIV  disables
       READ_COMPOSITE_NODIV     PNG_WRITE_TRANSFORMS_NOT_SUPPORTED    disables
       WRITE_TRANSFORMS    PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED    disables
       READ_ANCILLARY_CHUNKS PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables
       WRITE_ANCILLARY_CHUNKS

       Library builders should remove use of the above, inconsistent, names.

       2) Warning and error message formatting was previously  conditional  on
       the  STDIO  feature. The library has been changed to use the CONSOLE_IO
       feature instead. This means that if CONSOLE_IO is disabled the  library
       no  longer  uses  the  printf(3)  functions,  even  though  the default
       read/write implementations use (FILE) style stdio.h functions.

       3) Three feature macros now control the fixed/floating point decisions:

       PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs

       PNG_FIXED_POINT_SUPPORTED  enables  the  fixed  point APIs; however, in
       practice these are normally required internally anyway (because the PNG
       file format is fixed point), therefore in most cases PNG_NO_FIXED_POINT
       merely stops the function from being exported.

       PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal floating
       point implementation or the fixed point one.  Typically the fixed point
       implementation is larger and slower than the floating point implementa-
       tion  on  a  system  that  supports  floating point; however, it may be
       faster on a system which lacks floating point  hardware  and  therefore
       uses a software emulation.

       4)  Added  PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.   This  allows the
       functions to read and  write  ints  to  be  disabled  independently  of
       PNG_USE_READ_MACROS, which allows libpng to be built with the functions
       even though the default is to use the macros - this allows applications
       to  choose  at  app  buildtime whether or not to use macros (previously
       impossible because the functions weren't in the default build.)

       B.2 Changes to the configuration mechanism

       Prior to libpng-1.5.0 library builders who needed to  configure  libpng
       had  either  to modify the exported pngconf.h header file to add system
       specific configuration or had to write feature  selection  macros  into
       pngusr.h  and  cause  this  to  be  included into pngconf.h by defining
       PNG_USER_CONFIG. The latter mechanism  had  the  disadvantage  that  an
       application built without PNG_USER_CONFIG defined would see the unmodi-
       fied, default, libpng API and thus would probably fail to link.

       These mechanisms still work in the configure build and in any  makefile
       build  that  builds pnglibconf.h, although the feature selection macros
       have changed somewhat as described above.  In 1.5.0, however,  pngusr.h
       is  processed  only once, when the exported header file pnglibconf.h is
       built.  pngconf.h no longer includes pngusr.h,  therefore  pngusr.h  is
       ignored  after the build of pnglibconf.h and it is never included in an
       application build.

       The rarely used alternative of adding a list of feature macros  to  the
       CFLAGS  setting in the build also still works; however, the macros will
       be copied to pnglibconf.h and this may produce macro redefinition warn-
       ings when the individual C files are compiled.

       All  configuration  now  only  works  if  pnglibconf.h  is  built  from
       scripts/pnglibconf.dfa.   This  requires  the   program   awk.    Brian
       Kernighan  (the original author of awk) maintains C source code of that
       awk and this and all known later implementations (often called by  sub-
       tly  different names - nawk and gawk for example) are adequate to build
       pnglibconf.h.  The Sun Microsystems (now Oracle) program  'awk'  is  an
       earlier version and does not work; this may also apply to other systems
       that have a functioning awk called 'nawk'.

       Configuration options are  now  documented  in  scripts/pnglibconf.dfa.
       This  file also includes dependency information that ensures a configu-
       ration is consistent; that is, if a feature is switched  off  dependent
       features  are also removed.  As a recommended alternative to using fea-
       ture macros in pngusr.h a system builder  may  also  define  equivalent
       options  in  pngusr.dfa (or, indeed, any file) and add that to the con-
       figuration by setting DFA_XTRA to the file name.  The makefiles in con-
       trib/pngminim  illustrate  how to do this, and a case where pngusr.h is
       still required.



XI. Detecting libpng

       The png_get_io_ptr() function has been present since  libpng-0.88,  has
       never changed, and is unaffected by conditional compilation macros.  It
       is the best choice for use in configure scripts for detecting the pres-
       ence  of  any libpng version since 0.88.  In an autoconf "configure.in"
       you could use

           AC_CHECK_LIB(png, png_get_io_ptr, ...



XII. Source code repository

       Since about February 2009, version 1.2.34, libpng has been under  "git"
       source  control.   The  git  repository  was  built  from  old  libpng-
       x.y.z.tar.gz files going back to version 0.70.  You can access the  git
       repository (read only) at

           git://git.code.sf.net/p/libpng/code

       or  you can browse it with a web browser by selecting the "code" button
       at

           https://sourceforge.net/projects/libpng/

       Patches can be sent to glennrp at users.sourceforge.net or to  png-mng-
       implement at lists.sourceforge.net or you can upload them to the libpng
       bug tracker at

           http://libpng.sourceforge.net

       We also accept patches built from the tar  or  zip  distributions,  and
       simple verbal discriptions of bug fixes, reported either to the Source-
       Forge bug tracker, to the  png-mng-implement  at  lists.sf.net  mailing
       list, or directly to glennrp.



XIII. Coding style

       Our coding style is similar to the "Allman" style, with curly braces on
       separate lines:

           if (condition)
           {
              action;
           }

           else if (another condition)
           {
              another action;
           }

       The braces can be omitted from simple one-line actions:

           if (condition)
              return (0);

       We use 3-space indentation, except for continued statements  which  are
       usually  indented the same as the first line of the statement plus four
       more spaces.

       For macro definitions we use 2-space indentation,  always  leaving  the
       "#" in the first column.

           #ifndef PNG_NO_FEATURE
           #  ifndef PNG_FEATURE_SUPPORTED
           #    define PNG_FEATURE_SUPPORTED
           #  endif
           #endif

       Comments  appear  with  the leading "/*" at the same indentation as the
       statement that follows the comment:

           /* Single-line comment */
           statement;

           /* This is a multiple-line
            * comment.
            */
           statement;

       Very short comments can be placed after the end  of  the  statement  to
       which they pertain:

           statement;    /* comment */

       We  don't use C++ style ("//") comments. We have, however, used them in
       the past in some now-abandoned MMX assembler code.

       Functions and their curly braces are not indented, and  exported  func-
       tions are marked with PNGAPI:

        /* This is a public function that is visible to
         * application programmers. It does thus-and-so.
         */
        void PNGAPI
        png_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The  prototypes  for  all exported functions appear in png.h, above the
       comment that says

           /* Maintainer: Put new public prototypes here ... */

       We mark all non-exported functions with "/* PRIVATE */"":

        void /* PRIVATE */
        png_non_exported_function(png_ptr, png_info, foo)
        {
           body;
        }

       The prototypes for non-exported functions (except for those in pngtest)
       appear in pngpriv.h above the comment that says

         /* Maintainer: Put new private prototypes here ^ */

       To  avoid  polluting  the  global  namespace, the names of all exported
       functions and variables begin with "png_", and all publicly  visible  C
       preprocessor  macros  begin  with  "PNG".  We request that applications
       that use libpng *not* begin any of their own  symbols  with  either  of
       these strings.

       We  put  a  space  after  each  comma and after each semicolon in "for"
       statements, and we put spaces before and after each C  binary  operator
       and  after  "for"  or  "while",  and  before "?".  We don't put a space
       between a typecast and the expression being cast, nor  do  we  put  one
       between a function name and the left parenthesis that follows it:

           for (i = 2; i > 0; --i)
              y[i] = a(x) + (int)b;

       We  prefer  #ifdef and #ifndef to #if defined() and #if !defined() when
       there is only one macro being tested.  We always use  parentheses  with
       "defined".

       We prefer to express integers that are used as bit masks in hex format,
       with an even  number  of  lower-case  hex  digits  (e.g.,  0x00,  0xff,
       0x0100).

       We do not use the TAB character for indentation in the C sources.

       Lines do not exceed 80 characters.

       Other rules can be inferred by inspecting the libpng source.



XIV. Y2K Compliance in libpng

       February 6, 2014

       Since  the  PNG  Development  group is an ad-hoc body, we can't make an
       official declaration.

       This is your unofficial assurance that libpng  from  version  0.71  and
       upward  through 1.5.18 are Y2K compliant.  It is my belief that earlier
       versions were also Y2K compliant.

       Libpng only has two year fields.  One is a 2-byte unsigned integer that
       will  hold years up to 65535.  The other holds the date in text format,
       and will hold years up to 9999.

       The integer is
           "png_uint_16 year" in png_time_struct.

       The string is
           "char time_buffer[29]" in png_struct.  This will no longer be  used
       in libpng-1.6.x and will be removed from libpng-1.7.0.

       There are seven time-related functions:

           png_convert_to_rfc_1123() in png.c
             (formerly png_convert_to_rfc_1152() in error)
           png_convert_from_struct_tm() in pngwrite.c, called
             in pngwrite.c
           png_convert_from_time_t() in pngwrite.c
           png_get_tIME() in pngget.c
           png_handle_tIME() in pngrutil.c, called in pngread.c
           png_set_tIME() in pngset.c
           png_write_tIME() in pngwutil.c, called in pngwrite.c

       All appear to handle dates properly in a Y2K environment.  The png_con-
       vert_from_time_t() function calls gmtime() to convert from system clock
       time,  which  returns  (year  - 1900), which we properly convert to the
       full 4-digit year.  There is  a  possibility  that  applications  using
       libpng are not passing 4-digit years into the png_convert_to_rfc_1123()
       function, or that they are incorrectly  passing  only  a  2-digit  year
       instead  of  "year  - 1900" into the png_convert_from_struct_tm() func-
       tion, but this is not under our control.  The libpng documentation  has
       always  stated that it works with 4-digit years, and the APIs have been
       documented as such.

       The tIME chunk itself is also Y2K compliant.  It uses a 2-byte unsigned
       integer to hold the year, and can hold years as large as 65535.

       zlib, upon which libpng depends, is also Y2K compliant.  It contains no
       date-related code.


          Glenn Randers-Pehrson
          libpng maintainer
          PNG Development Group



NOTE

       Note about libpng version numbers:

       Due to various miscommunications, unforeseen code incompatibilities and
       occasional  factors  outside the authors' control, version numbering on
       the library has not always been consistent  and  straightforward.   The
       following  table  summarizes matters since version 0.89c, which was the
       first widely used release:

        source             png.h  png.h  shared-lib
        version            string   int  version
        -------            ------  ----- ----------
        0.89c ("beta 3")  0.89       89  1.0.89
        0.90  ("beta 4")  0.90       90  0.90
        0.95  ("beta 5")  0.95       95  0.95
        0.96  ("beta 6")  0.96       96  0.96
        0.97b ("beta 7")  1.00.97    97  1.0.1
        0.97c             0.97       97  2.0.97
        0.98              0.98       98  2.0.98
        0.99              0.99       98  2.0.99
        0.99a-m           0.99       99  2.0.99
        1.00              1.00      100  2.1.0
        1.0.0             1.0.0     100  2.1.0
        1.0.0   (from here on, the  100  2.1.0
        1.0.1    png.h string is  10001  2.1.0
        1.0.1a-e identical to the 10002  from here on, the
        1.0.2    source version)  10002  shared library is 2.V
        1.0.2a-b                  10003  where V is the source
        1.0.1                     10001  code version except as
        1.0.1a-e                  10002  2.1.0.1a-e   noted.
        1.0.2                     10002  2.1.0.2
        1.0.2a-b                  10003  2.1.0.2a-b
        1.0.3                     10003  2.1.0.3
        1.0.3a-d                  10004  2.1.0.3a-d
        1.0.4                     10004  2.1.0.4
        1.0.4a-f                  10005  2.1.0.4a-f
        1.0.5 (+ 2 patches)       10005  2.1.0.5
        1.0.5a-d                  10006  2.1.0.5a-d
        1.0.5e-r                  10100  2.1.0.5e-r
        1.0.5s-v                  10006  2.1.0.5s-v
        1.0.6 (+ 3 patches)       10006  2.1.0.6
        1.0.6d-g                  10007  2.1.0.6d-g
        1.0.6h                    10007  10.6h
        1.0.6i                    10007  10.6i
        1.0.6j                    10007  2.1.0.6j
        1.0.7beta11-14    DLLNUM  10007  2.1.0.7beta11-14
        1.0.7beta15-18       1    10007  2.1.0.7beta15-18
        1.0.7rc1-2           1    10007  2.1.0.7rc1-2
        1.0.7                1    10007  2.1.0.7
        1.0.8beta1-4         1    10008  2.1.0.8beta1-4
        1.0.8rc1             1    10008  2.1.0.8rc1
        1.0.8                1    10008  2.1.0.8
        1.0.9beta1-6         1    10009  2.1.0.9beta1-6
        1.0.9rc1             1    10009  2.1.0.9rc1
        1.0.9beta7-10        1    10009  2.1.0.9beta7-10
        1.0.9rc2             1    10009  2.1.0.9rc2
        1.0.9                1    10009  2.1.0.9
        1.0.10beta1          1    10010  2.1.0.10beta1
        1.0.10rc1            1    10010  2.1.0.10rc1
        1.0.10               1    10010  2.1.0.10
        1.0.11beta1-3        1    10011  2.1.0.11beta1-3
        1.0.11rc1            1    10011  2.1.0.11rc1
        1.0.11               1    10011  2.1.0.11
        1.0.12beta1-2        2    10012  2.1.0.12beta1-2
        1.0.12rc1            2    10012  2.1.0.12rc1
        1.0.12               2    10012  2.1.0.12
        1.1.0a-f             -    10100  2.1.1.0a-f abandoned
        1.2.0beta1-2         2    10200  2.1.2.0beta1-2
        1.2.0beta3-5         3    10200  3.1.2.0beta3-5
        1.2.0rc1             3    10200  3.1.2.0rc1
        1.2.0                3    10200  3.1.2.0
        1.2.1beta-4          3    10201  3.1.2.1beta1-4
        1.2.1rc1-2           3    10201  3.1.2.1rc1-2
        1.2.1                3    10201  3.1.2.1
        1.2.2beta1-6        12    10202  12.so.0.1.2.2beta1-6
        1.0.13beta1         10    10013  10.so.0.1.0.13beta1
        1.0.13rc1           10    10013  10.so.0.1.0.13rc1
        1.2.2rc1            12    10202  12.so.0.1.2.2rc1
        1.0.13              10    10013  10.so.0.1.0.13
        1.2.2               12    10202  12.so.0.1.2.2
        1.2.3rc1-6          12    10203  12.so.0.1.2.3rc1-6
        1.2.3               12    10203  12.so.0.1.2.3
        1.2.4beta1-3        13    10204  12.so.0.1.2.4beta1-3
        1.2.4rc1            13    10204  12.so.0.1.2.4rc1
        1.0.14              10    10014  10.so.0.1.0.14
        1.2.4               13    10204  12.so.0.1.2.4
        1.2.5beta1-2        13    10205  12.so.0.1.2.5beta1-2
        1.0.15rc1           10    10015  10.so.0.1.0.15rc1
        1.0.15              10    10015  10.so.0.1.0.15
        1.2.5               13    10205  12.so.0.1.2.5
        1.2.6beta1-4        13    10206  12.so.0.1.2.6beta1-4
        1.2.6rc1-5          13    10206  12.so.0.1.2.6rc1-5
        1.0.16              10    10016  10.so.0.1.0.16
        1.2.6               13    10206  12.so.0.1.2.6
        1.2.7beta1-2        13    10207  12.so.0.1.2.7beta1-2
        1.0.17rc1           10    10017  12.so.0.1.0.17rc1
        1.2.7rc1            13    10207  12.so.0.1.2.7rc1
        1.0.17              10    10017  12.so.0.1.0.17
        1.2.7               13    10207  12.so.0.1.2.7
        1.2.8beta1-5        13    10208  12.so.0.1.2.8beta1-5
        1.0.18rc1-5         10    10018  12.so.0.1.0.18rc1-5
        1.2.8rc1-5          13    10208  12.so.0.1.2.8rc1-5
        1.0.18              10    10018  12.so.0.1.0.18
        1.2.8               13    10208  12.so.0.1.2.8
        1.2.9beta1-3        13    10209  12.so.0.1.2.9beta1-3
        1.2.9beta4-11       13    10209  12.so.0.9[.0]
        1.2.9rc1            13    10209  12.so.0.9[.0]
        1.2.9               13    10209  12.so.0.9[.0]
        1.2.10beta1-7       13    10210  12.so.0.10[.0]
        1.2.10rc1-2         13    10210  12.so.0.10[.0]
        1.2.10              13    10210  12.so.0.10[.0]
        1.4.0beta1-6        14    10400  14.so.0.0[.0]
        1.2.11beta1-4       13    10210  12.so.0.11[.0]
        1.4.0beta7-8        14    10400  14.so.0.0[.0]
        1.2.11              13    10211  12.so.0.11[.0]
        1.2.12              13    10212  12.so.0.12[.0]
        1.4.0beta9-14       14    10400  14.so.0.0[.0]
        1.2.13              13    10213  12.so.0.13[.0]
        1.4.0beta15-36      14    10400  14.so.0.0[.0]
        1.4.0beta37-87      14    10400  14.so.14.0[.0]
        1.4.0rc01           14    10400  14.so.14.0[.0]
        1.4.0beta88-109     14    10400  14.so.14.0[.0]
        1.4.0rc02-08        14    10400  14.so.14.0[.0]
        1.4.0               14    10400  14.so.14.0[.0]
        1.4.1beta01-03      14    10401  14.so.14.1[.0]
        1.4.1rc01           14    10401  14.so.14.1[.0]
        1.4.1beta04-12      14    10401  14.so.14.1[.0]
        1.4.1               14    10401  14.so.14.1[.0]
        1.4.2               14    10402  14.so.14.2[.0]
        1.4.3               14    10403  14.so.14.3[.0]
        1.4.4               14    10404  14.so.14.4[.0]
        1.5.0beta01-58      15    10500  15.so.15.0[.0]
        1.5.0rc01-07        15    10500  15.so.15.0[.0]
        1.5.0               15    10500  15.so.15.0[.0]
        1.5.1beta01-11      15    10501  15.so.15.1[.0]
        1.5.1rc01-02        15    10501  15.so.15.1[.0]
        1.5.1               15    10501  15.so.15.1[.0]
        1.5.2beta01-03      15    10502  15.so.15.2[.0]
        1.5.2rc01-03        15    10502  15.so.15.2[.0]
        1.5.2               15    10502  15.so.15.2[.0]
        1.5.3beta01-10      15    10503  15.so.15.3[.0]
        1.5.3rc01-02        15    10503  15.so.15.3[.0]
        1.5.3beta11         15    10503  15.so.15.3[.0]
        1.5.3 [omitted]
        1.5.4beta01-08      15    10504  15.so.15.4[.0]
        1.5.4rc01           15    10504  15.so.15.4[.0]
        1.5.4               15    10504  15.so.15.4[.0]
        1.5.5beta01-08      15    10505  15.so.15.5[.0]
        1.5.5rc01           15    10505  15.so.15.5[.0]
        1.5.5               15    10505  15.so.15.5[.0]
        1.5.6beta01-07      15    10506  15.so.15.6[.0]
        1.5.6rc01-03        15    10506  15.so.15.6[.0]
        1.5.6               15    10506  15.so.15.6[.0]
        1.5.7beta01-05      15    10507  15.so.15.7[.0]
        1.5.7rc01-03        15    10507  15.so.15.7[.0]
        1.5.7               15    10507  15.so.15.7[.0]
        1.5.8beta01         15    10508  15.so.15.8[.0]
        1.5.8rc01           15    10508  15.so.15.8[.0]
        1.5.8               15    10508  15.so.15.8[.0]
        1.5.9beta01-02      15    10509  15.so.15.9[.0]
        1.5.9rc01           15    10509  15.so.15.9[.0]
        1.5.9               15    10509  15.so.15.9[.0]
        1.5.10beta01-05     15    10510  15.so.15.10[.0]
        1.5.10              15    10510  15.so.15.10[.0]
        1.5.11beta01        15    10511  15.so.15.11[.0]
        1.5.11rc01-05       15    10511  15.so.15.11[.0]
        1.5.11              15    10511  15.so.15.11[.0]
        1.5.12              15    10512  15.so.15.12[.0]
        1.5.13beta01-02     15    10513  15.so.15.13[.0]
        1.5.13rc01          15    10513  15.so.15.13[.0]
        1.5.13              15    10513  15.so.15.13[.0]
        1.5.14beta01-08     15    10514  15.so.15.14[.0]
        1.5.14rc01-03       15    10514  15.so.15.14[.0]
        1.5.14              15    10514  15.so.15.14[.0]
        1.5.15beta01-09     15    10515  15.so.15.15[.0]
        1.5.15rc01          15    10515  15.so.15.15[.0]
        1.5.15              15    10515  15.so.15.15[.0]
        1.5.16beta01-06     15    10516  15.so.15.16[.0]
        1.5.16rc01          15    10516  15.so.15.16[.0]
        1.5.16              15    10516  15.so.15.16[.0]
        1.5.17beta01        15    10517  15.so.15.17[.0]
        1.5.17rc01-03       15    10517  15.so.15.17[.0]
        1.5.17              15    10517  15.so.15.17[.0]
        1.5.18beta01-05     15    10518  15.so.15.18[.0]
        1.5.18rc01-02       15    10518  15.so.15.18[.0]
        1.5.18              15    10518  15.so.15.18[.0]

       Henceforth the source version will match the shared-library  minor  and
       patch numbers; the shared-library major version number will be used for
       changes  in  backward  compatibility,   as   it   is   intended.    The
       PNG_PNGLIB_VER  macro, which is not used within libpng but is available
       for applications, is an unsigned integer of the form xyyzz  correspond-
       ing  to the source version x.y.z (leading zeros in y and z).  Beta ver-
       sions were given the previous public  release  number  plus  a  letter,
       until  version 1.0.6j; from then on they were given the upcoming public
       release number plus "betaNN" or "rcN".



SEE ALSO

       png(5), libpngpf(3), zlib(3), deflate(5), and zlib(5)


       libpng:

              http://libpng.sourceforge.net  (follow  the   [DOWNLOAD]   link)
              http://www.libpng.org/pub/png


       zlib:

              (generally) at the same location as libpng or at
              ftp://ftp.info-zip.org/pub/infozip/zlib


       PNGspecification:RFC2083

              (generally) at the same location as libpng or at
              ftp://ds.internic.net/rfc/rfc2083.txt
              or (as a W3C Recommendation) at
              http://www.w3.org/TR/REC-png.html


       In the case of any inconsistency between the PNG specification and this
       library, the specification takes precedence.



AUTHORS

       This man page: Glenn Randers-Pehrson <glennrp at users.sourceforge.net>

       The  contributing authors would like to thank all those who helped with
       testing, bug fixes, and patience.  This  wouldn't  have  been  possible
       without all of you.

       Thanks to Frank J. T. Wojcik for helping with the documentation.

       Libpng  version 1.5.18 - February 6, 2014: Initially created in 1995 by
       Guy Eric Schalnat, then of Group  42,  Inc.   Currently  maintained  by
       Glenn Randers-Pehrson (glennrp at users.sourceforge.net).

       Supported by the PNG development group
       png-mng-implement  at  lists.sf.net  (subscription required; visit png-
       mng-implement at lists.sourceforge.net  (subscription  required;  visit
       https://lists.sourceforge.net/lists/listinfo/png-mng-implement  to sub-
       scribe).



COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:

       (This copy of the libpng notices is provided for your convenience.   In
       case  of  any discrepancy between this copy and the notices in the file
       png.h that is included in the libpng  distribution,  the  latter  shall
       prevail.)

       If you modify libpng you may insert additional notices immediately fol-
       lowing this sentence.

       This code is released under the libpng license.

       libpng versions 1.2.6, August 15, 2004,  through  1.5.18,  February  6,
       2014,  are  Copyright (c) 2004,2006-2007 Glenn Randers-Pehrson, and are
       distributed  according  to  the  same   disclaimer   and   license   as
       libpng-1.2.5  with  the  following individual added to the list of Con-
       tributing Authors

          Cosmin Truta

       libpng versions 1.0.7, July 1, 2000, through 1.2.5 - October  3,  2002,
       are  Copyright (c) 2000-2002 Glenn Randers-Pehrson, and are distributed
       according to the same disclaimer and license as libpng-1.0.6  with  the
       following individuals added to the list of Contributing Authors

          Simon-Pierre Cadieux
          Eric S. Raymond
          Gilles Vollant

       and with the following additions to the disclaimer:

          There is no warranty against interference with your
          enjoyment of the library or against infringement.
          There is no warranty that our efforts or the library
          will fulfill any of your particular purposes or needs.
          This library is provided with all faults, and the entire
          risk of satisfactory quality, performance, accuracy, and
          effort is with the user.

       libpng  versions 0.97, January 1998, through 1.0.6, March 20, 2000, are
       Copyright (c) 1998, 1999 Glenn Randers-Pehrson Distributed according to
       the  same  disclaimer  and  license  as libpng-0.96, with the following
       individuals added to the list of Contributing Authors:

          Tom Lane
          Glenn Randers-Pehrson
          Willem van Schaik

       libpng versions 0.89, June 1996, through 0.96, May 1997, are  Copyright
       (c)  1996,  1997  Andreas Dilger Distributed according to the same dis-
       claimer and license as  libpng-0.88,  with  the  following  individuals
       added to the list of Contributing Authors:

          John Bowler
          Kevin Bracey
          Sam Bushell
          Magnus Holmgren
          Greg Roelofs
          Tom Tanner

       libpng  versions  0.5,  May 1995, through 0.88, January 1996, are Copy-
       right (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.

       For the purposes of this copyright and license, "Contributing  Authors"
       is defined as the following set of individuals:

          Andreas Dilger
          Dave Martindale
          Guy Eric Schalnat
          Paul Schmidt
          Tim Wegner

       The  PNG  Reference  Library  is  supplied  "AS  IS".  The Contributing
       Authors and Group  42,  Inc.  disclaim  all  warranties,  expressed  or
       implied, including, without limitation, the warranties of merchantabil-
       ity and of fitness for any purpose.  The Contributing Authors and Group
       42,  Inc.   assume  no liability for direct, indirect, incidental, spe-
       cial, exemplary, or consequential damages, which may  result  from  the
       use of the PNG Reference Library, even if advised of the possibility of
       such damage.

       Permission is hereby granted to use, copy, modify, and distribute  this
       source  code, or portions hereof, for any purpose, without fee, subject
       to the following restrictions:

       1. The origin of this source code must not be misrepresented.

       2. Altered versions must be plainly marked as such and
          must not be misrepresented as being the original source.

       3. This Copyright notice may not be removed or altered from
          any source or altered source distribution.

       The Contributing Authors and Group 42, Inc. specifically permit,  with-
       out  fee,  and  encourage the use of this source code as a component to
       supporting the PNG file format in commercial products.  If you use this
       source  code  in a product, acknowledgment is not required but would be
       appreciated.


       A "png_get_copyright" function is  available,  for  convenient  use  in
       "about" boxes and the like:

          printf("%s",png_get_copyright(NULL));

       Also,  the PNG logo (in PNG format, of course) is supplied in the files
       "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).

       Libpng is OSI Certified  Open  Source  Software.   OSI  Certified  Open
       Source is a certification mark of the Open Source Initiative.

       Glenn Randers-Pehrson glennrp at users.sourceforge.net February 6, 2014





                               February 6, 2014                      libpng(3)

libpng 1.5.18 - Generated Fri Feb 28 15:43:51 CST 2014