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curs_color(3)                                                    curs_color(3)




NAME

       start_color, has_colors, can_change_color, init_pair, init_color,
       init_extended_pair, init_extended_color, color_content, pair_content,
       extended_color_content, extended_pair_content, reset_color_pairs,
       COLOR_PAIR, PAIR_NUMBER - curses color manipulation routines


SYNOPSIS

       #include <curses.h>

       int start_color(void);

       bool has_colors(void);
       bool can_change_color(void);

       int init_pair(short pair, short f, short b);
       int init_color(short color, short r, short g, short b);
       /* extensions */
       int init_extended_pair(int pair, int f, int b);
       int init_extended_color(int color, int r, int g, int b);

       int color_content(short color, short *r, short *g, short *b);
       int pair_content(short pair, short *f, short *b);
       /* extensions */
       int extended_color_content(int color, int *r, int *g, int *b);
       int extended_pair_content(int pair, int *f, int *b);

       /* extensions */
       void reset_color_pairs(void);

       int COLOR_PAIR(int n);
       PAIR_NUMBER(attrs);


DESCRIPTION

   Overview
       curses supports color attributes on terminals with that capability.  To
       use these routines start_color must be called, usually right after
       initscr.  Colors are always used in pairs (referred to as color-pairs).
       A color-pair consists of a foreground color (for characters) and a
       background color (for the blank field on which the characters are
       displayed).  A programmer initializes a color-pair with the routine
       init_pair.  After it has been initialized, COLOR_PAIR(n) can be used to
       convert the pair to a video attribute.

       If a terminal is capable of redefining colors, the programmer can use the
       routine init_color to change the definition of a color.  The routines
       has_colors and can_change_color return TRUE or FALSE, depending on
       whether the terminal has color capabilities and whether the programmer
       can change the colors.  The routine color_content allows a programmer to
       extract the amounts of red, green, and blue components in an initialized
       color.  The routine pair_content allows a programmer to find out how a
       given color-pair is currently defined.

   Color Rendering
       The curses library combines these inputs to produce the actual foreground
       and background colors shown on the screen:

       o   per-character video attributes (e.g., via waddch),

       o   the window attribute (e.g., by wattrset), and

       o   the background character (e.g., wbkgdset).

       Per-character and window attributes are usually set by a parameter
       containing video attributes including a color pair value.  Some functions
       such as wattr_set use a separate parameter which is the color pair
       number.

       The background character is a special case: it includes a character
       value, just as if it were passed to waddch.

       The curses library does the actual work of combining these color pairs in
       an internal function called from waddch:

       o   If the parameter passed to waddch is blank, and it uses the special
           color pair 0,

           o   curses next checks the window attribute.

           o   If the window attribute does not use color pair 0, curses uses
               the color pair from the window attribute.

           o   Otherwise, curses uses the background character.

       o   If the parameter passed to waddch is not blank, or it does not use
           the special color pair 0, curses prefers the color pair from the
           parameter, if it is nonzero.  Otherwise, it tries the window
           attribute next, and finally the background character.

       Some curses functions such as wprintw call waddch.  Those do not combine
       its parameter with a color pair.  Consequently those calls use only the
       window attribute or the background character.


CONSTANTS

       In <curses.h> the following macros are defined.  These are the standard
       colors (ISO-6429).  curses also assumes that COLOR_BLACK is the default
       background color for all terminals.

             COLOR_BLACK
             COLOR_RED
             COLOR_GREEN
             COLOR_YELLOW
             COLOR_BLUE
             COLOR_MAGENTA
             COLOR_CYAN
             COLOR_WHITE

       Some terminals support more than the eight (8) "ANSI" colors.  There are
       no standard names for those additional colors.


VARIABLES

   COLORS
       is initialized by start_color to the maximum number of colors the
       terminal can support.

   COLOR_PAIRS
       is initialized by start_color to the maximum number of color pairs the
       terminal can support.


FUNCTIONS

   start_color
       The start_color routine requires no arguments.  It must be called if the
       programmer wants to use colors, and before any other color manipulation
       routine is called.  It is good practice to call this routine right after
       initscr.  start_color does this:

       o   It initializes two global variables, COLORS and COLOR_PAIRS
           (respectively defining the maximum number of colors and color-pairs
           the terminal can support).

       o   It initializes the special color pair 0 to the default foreground and
           background colors.  No other color pairs are initialized.

       o   It restores the colors on the terminal to the values they had when
           the terminal was just turned on.

       o   If the terminal supports the initc (initialize_color) capability,
           start_color initializes its internal table representing the red,
           green, and blue components of the color palette.

           The components depend on whether the terminal uses CGA (aka "ANSI")
           or HLS (i.e., the hls (hue_lightness_saturation) capability is set).
           The table is initialized first for eight basic colors (black, red,
           green, yellow, blue, magenta, cyan, and white), using weights that
           depend upon the CGA/HLS choice.  For "ANSI" colors the weights are
           680 or 0 depending on whether the corresponding red, green, or blue
           component is used or not.  That permits using 1000 to represent
           bold/bright colors.  After the initial eight colors (if the terminal
           supports more than eight colors) the components are initialized using
           the same pattern, but with weights of 1000.  SVr4 uses a similar
           scheme, but uses 1000 for the components of the initial eight colors.

           start_color does not attempt to set the terminal's color palette to
           match its built-in table.  An application may use init_color to alter
           the internal table along with the terminal's color.

       These limits apply to color values and color pairs.  Values outside these
       limits are not legal, and may result in a runtime error:

       o   COLORS corresponds to the terminal database's max_colors capability,
           (see terminfo(5)).

       o   color values are expected to be in the range 0 to COLORS-1, inclusive
           (including 0 and COLORS-1).

       o   a special color value -1 is used in certain extended functions to
           denote the default color (see use_default_colors(3X)).

       o   COLOR_PAIRS corresponds to the terminal database's max_pairs
           capability, (see terminfo(5)).

       o   legal color pair values are in the range 1 to COLOR_PAIRS-1,
           inclusive.

       o   color pair 0 is special; it denotes "no color".

           Color pair 0 is assumed to be white on black, but is actually
           whatever the terminal implements before color is initialized.  It
           cannot be modified by the application.

   has_colors
       The has_colors routine requires no arguments.  It returns TRUE if the
       terminal can manipulate colors; otherwise, it returns FALSE.  This
       routine facilitates writing terminal-independent programs.  For example,
       a programmer can use it to decide whether to use color or some other
       video attribute.

   can_change_color
       The can_change_color routine requires no arguments.  It returns TRUE if
       the terminal supports colors and can change their definitions; other, it
       returns FALSE.  This routine facilitates writing terminal-independent
       programs.

   init_pair
       The init_pair routine changes the definition of a color-pair.  It takes
       three arguments: the number of the color-pair to be changed, the
       foreground color number, and the background color number.  For portable
       applications:

       o   The first argument must be a legal color pair value.  If default
           colors are used (see use_default_colors(3X)) the upper limit is
           adjusted to allow for extra pairs which use a default color in
           foreground and/or background.

       o   The second and third arguments must be legal color values.

       If the color-pair was previously initialized, the screen is refreshed and
       all occurrences of that color-pair are changed to the new definition.

       As an extension, ncurses allows you to set color pair 0 via the
       assume_default_colors(3X) routine, or to specify the use of default
       colors (color number -1) if you first invoke the use_default_colors(3X)
       routine.

   init_extended_pair
       Because init_pair uses signed shorts for its parameters, that limits
       color-pairs and color-values to 32767 on modern hardware.  The extension
       init_extended_pair uses ints for the color-pair and color-value, allowing
       a larger number of colors to be supported.

   init_color
       The init_color routine changes the definition of a color.  It takes four
       arguments: the number of the color to be changed followed by three RGB
       values (for the amounts of red, green, and blue components).

       o   The first argument must be a legal color value; default colors are
           not allowed here.  (See the section Colors for the default color
           index.)

       o   Each of the last three arguments must be a value in the range 0
           through 1000.

       When init_color is used, all occurrences of that color on the screen
       immediately change to the new definition.

   init_extended_color
       Because init_color uses signed shorts for its parameters, that limits
       color-values and their red, green, and blue components to 32767 on modern
       hardware.  The extension init_extended_color uses ints for the color
       value and for setting the red, green, and blue components, allowing a
       larger number of colors to be supported.

   color_content
       The color_content routine gives programmers a way to find the intensity
       of the red, green, and blue (RGB) components in a color.  It requires
       four arguments: the color number, and three addresses of shorts for
       storing the information about the amounts of red, green, and blue
       components in the given color.

       o   The first argument must be a legal color value, i.e., 0 through
           COLORS-1, inclusive.

       o   The values that are stored at the addresses pointed to by the last
           three arguments are in the range 0 (no component) through 1000
           (maximum amount of component), inclusive.

   extended_color_content
       Because color_content uses signed shorts for its parameters, that limits
       color-values and their red, green, and blue components to 32767 on modern
       hardware.  The extension extended_color_content uses ints for the color
       value and for returning the red, green, and blue components, allowing a
       larger number of colors to be supported.

   pair_content
       The pair_content routine allows programmers to find out what colors a
       given color-pair consists of.  It requires three arguments: the color-
       pair number, and two addresses of shorts for storing the foreground and
       the background color numbers.

       o   The first argument must be a legal color value, i.e., in the range 1
           through COLOR_PAIRS-1, inclusive.

       o   The values that are stored at the addresses pointed to by the second
           and third arguments are in the range 0 through COLORS, inclusive.

   extended_pair_content
       Because pair_content uses signed shorts for its parameters, that limits
       color-pair and color-values to 32767 on modern hardware.  The extension
       extended_pair_content uses ints for the color pair and for returning the
       foreground and background colors, allowing a larger number of colors to
       be supported.

   reset_color_pairs
       The extension reset_color_pairs tells ncurses to discard all of the
       color-pair information which was set with init_pair.  It also touches the
       current- and standard-screens, allowing an application to switch color
       palettes rapidly.

   PAIR_NUMBER
       PAIR_NUMBER(attrs) extracts the color value from its attrs parameter and
       returns it as a color pair number.

   COLOR_PAIR
       Its inverse COLOR_PAIR(n) converts a color pair number to an attribute.
       Attributes can hold color pairs in the range 0 to 255.  If you need a
       color pair larger than that, you must use functions such as attr_set
       (which pass the color pair as a separate parameter) rather than the
       legacy functions such as attrset.


RETURN VALUE

       The routines can_change_color and has_colors return TRUE or FALSE.

       All other routines return the integer ERR upon failure and an OK (SVr4
       specifies only "an integer value other than ERR") upon successful
       completion.

       X/Open defines no error conditions.  SVr4 does document some error
       conditions which apply in general:

       o   This implementation will return ERR on attempts to use color values
           outside the range 0 to COLORS-1 (except for the default colors
           extension), or use color pairs outside the range 0 to COLOR_PAIRS-1.

           Color values used in init_color must be in the range 0 to 1000.

           An error is returned from all functions if the terminal has not been
           initialized.

           An error is returned from secondary functions such as init_pair if
           start_color was not called.

       o   SVr4 does much the same, except that it returns ERR from pair_content
           if the pair was not initialized using init_pairs and it returns ERR
           from color_content if the terminal does not support changing colors.

           This implementation does not return ERR for either case.

       Specific functions make additional checks:

          init_color
               returns an error if the terminal does not support this feature,
               e.g., if the initialize_color capability is absent from the
               terminal description.

          start_color
               returns an error if the color table cannot be allocated.


NOTES

       In the ncurses implementation, there is a separate color activation flag,
       color palette, color pairs table, and associated COLORS and COLOR_PAIRS
       counts for each screen; the start_color function only affects the current
       screen.  The SVr4/XSI interface is not really designed with this in mind,
       and historical implementations may use a single shared color palette.

       Setting an implicit background color via a color pair affects only
       character cells that a character write operation explicitly touches.  To
       change the background color used when parts of a window are blanked by
       erasing or scrolling operations, see curs_bkgd(3X).

       Several caveats apply on older x86 machines (e.g., i386, i486) with VGA-
       compatible graphics:

       o   COLOR_YELLOW is actually brown.  To get yellow, use COLOR_YELLOW
           combined with the A_BOLD attribute.

       o   The A_BLINK attribute should in theory cause the background to go
           bright.  This often fails to work, and even some cards for which it
           mostly works (such as the Paradise and compatibles) do the wrong
           thing when you try to set a bright "yellow" background (you get a
           blinking yellow foreground instead).

       o   Color RGB values are not settable.


HISTORY

       SVr3.2 introduced color support to curses in 1987.

       SVr4 made internal changes, e.g., moving the storage for the color state
       from SP (the SCREEN structure) to cur_term (the TERMINAL structure), but
       provided the same set of library functions.

       SVr4 curses limits the number of color pairs to 64, reserving color pair
       zero (0) as the terminal's initial uncolored state.  This limit arises
       because the color pair information is a bitfield in the chtype data type
       (denoted by A_COLOR).

       Other implementations of curses had different limits:

       o   PCCurses (1987-1990) provided for only eight (8) colors.

       o   PDCurses (1992-present) inherited the 8-color limitation from
           PCCurses, but changed this to 256 in version 2.5 (2001), along with
           changing chtype from 16-bits to 32-bits.

       o   X/Open Curses (1992-present) added a new structure cchar_t to store
           the character, attributes and color-pair values, allowing increased
           range of color-pairs.  Both color-pairs and color-values used a
           signed short, limiting values to 15 bits.

       o   ncurses (1992-present) uses eight bits for A_COLOR in chtype values.

           Version 5.3 provided a wide-character interface (2002), but left
           color-pairs as part of the attributes-field.

           Since version 6 (2015), ncurses uses a separate int for color-pairs
           in the cchar_t values.  When those color-pair values fit in 8 bits,
           ncurses allows color-pairs to be manipulated via the functions using
           chtype values.

       o   NetBSD curses used 6 bits from 2000 (when colors were first
           supported) until 2004.  At that point, NetBSD changed to use 10 bits.
           As of 2021, that size is unchanged.  Like ncurses before version 6,
           the NetBSD color-pair information is stored in the attributes field
           of cchar_t, limiting the number of color-pairs by the size of the
           bitfield.


PORTABILITY

       This implementation satisfies XSI Curses's minimum maximums for COLORS
       and COLOR_PAIRS.

       The init_pair routine accepts negative values of foreground and
       background color to support the use_default_colors(3X) extension, but
       only if that routine has been first invoked.

       The assumption that COLOR_BLACK is the default background color for all
       terminals can be modified using the assume_default_colors(3X) extension.

       This implementation checks the pointers, e.g., for the values returned by
       color_content and pair_content, and will treat those as optional
       parameters when null.

       X/Open Curses does not specify a limit for the number of colors and color
       pairs which a terminal can support.  However, in its use of short for the
       parameters, it carries over SVr4's implementation detail for the compiled
       terminfo database, which uses signed 16-bit numbers.  This implementation
       provides extended versions of those functions which use short parameters,
       allowing applications to use larger color- and pair-numbers.

       The reset_color_pairs function is an extension of ncurses.


SEE ALSO

       curses(3X), curs_initscr(3X), curs_attr(3X), curs_variables(3X),
       default_colors(3X)



                                                                  curs_color(3)

ncurses 6.4 - Generated Tue Jan 3 13:30:11 CST 2023
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