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User Fonts

User Fonts — Font support with font data provided by the user

Types and Values

Description

The user-font feature allows the cairo user to provide drawings for glyphs in a font. This is most useful in implementing fonts in non-standard formats, like SVG fonts and Flash fonts, but can also be used by games and other application to draw "funky" fonts.

Functions

cairo_user_scaled_font_init_func_t ()

cairo_status_t
(*cairo_user_scaled_font_init_func_t) (cairo_scaled_font_t *scaled_font,
                                       cairo_t *cr,
                                       cairo_font_extents_t *extents);

cairo_user_scaled_font_init_func_t is the type of function which is called when a scaled-font needs to be created for a user font-face.

The cairo context cr is not used by the caller, but is prepared in font space, similar to what the cairo contexts passed to the render_glyph method will look like. The callback can use this context for extents computation for example. After the callback is called, cr is checked for any error status.

The extents argument is where the user font sets the font extents for scaled_font . It is in font space, which means that for most cases its ascent and descent members should add to 1.0. extents is preset to hold a value of 1.0 for ascent, height, and max_x_advance, and 0.0 for descent and max_y_advance members.

The callback is optional. If not set, default font extents as described in the previous paragraph will be used.

Note that scaled_font is not fully initialized at this point and trying to use it for text operations in the callback will result in deadlock.

Parameters

scaled_font

the scaled-font being created

 

cr

a cairo context, in font space

 

extents

font extents to fill in, in font space

 

Returns

CAIRO_STATUS_SUCCESS upon success, or an error status on error.

Since: 1.8


cairo_user_scaled_font_render_glyph_func_t ()

cairo_status_t
(*cairo_user_scaled_font_render_glyph_func_t)
                               (cairo_scaled_font_t *scaled_font,
                                unsigned long  glyph,
                                cairo_t *cr,
                                cairo_text_extents_t *extents);

cairo_user_scaled_font_render_glyph_func_t is the type of function which is called when a user scaled-font needs to render a glyph.

The callback is mandatory, and expected to draw the glyph with code glyph to the cairo context cr . cr is prepared such that the glyph drawing is done in font space. That is, the matrix set on cr is the scale matrix of scaled_font , The extents argument is where the user font sets the font extents for scaled_font . However, if user prefers to draw in user space, they can achieve that by changing the matrix on cr . All cairo rendering operations to cr are permitted, however, the result is undefined if any source other than the default source on cr is used. That means, glyph bitmaps should be rendered using cairo_mask() instead of cairo_paint().

Other non-default settings on cr include a font size of 1.0 (given that it is set up to be in font space), and font options corresponding to scaled_font .

The extents argument is preset to have x_bearing, width, and y_advance of zero, y_bearing set to -font_extents.ascent, height to font_extents.ascent+font_extents.descent, and x_advance to font_extents.max_x_advance. The only field user needs to set in majority of cases is x_advance. If the width field is zero upon the callback returning (which is its preset value), the glyph extents are automatically computed based on the drawings done to cr . This is in most cases exactly what the desired behavior is. However, if for any reason the callback sets the extents, it must be ink extents, and include the extents of all drawing done to cr in the callback.

Parameters

scaled_font

user scaled-font

 

glyph

glyph code to render

 

cr

cairo context to draw to, in font space

 

extents

glyph extents to fill in, in font space

 

Returns

CAIRO_STATUS_SUCCESS upon success, or CAIRO_STATUS_USER_FONT_ERROR or any other error status on error.

Since: 1.8


cairo_user_scaled_font_text_to_glyphs_func_t ()

cairo_status_t
(*cairo_user_scaled_font_text_to_glyphs_func_t)
                               (cairo_scaled_font_t *scaled_font,
                                const char *utf8,
                                int utf8_len,
                                cairo_glyph_t **glyphs,
                                int *num_glyphs,
                                cairo_text_cluster_t **clusters,
                                int *num_clusters,
                                cairo_text_cluster_flags_t *cluster_flags);

cairo_user_scaled_font_text_to_glyphs_func_t is the type of function which is called to convert input text to an array of glyphs. This is used by the cairo_show_text() operation.

Using this callback the user-font has full control on glyphs and their positions. That means, it allows for features like ligatures and kerning, as well as complex shaping required for scripts like Arabic and Indic.

The num_glyphs argument is preset to the number of glyph entries available in the glyphs buffer. If the glyphs buffer is NULL, the value of num_glyphs will be zero. If the provided glyph array is too short for the conversion (or for convenience), a new glyph array may be allocated using cairo_glyph_allocate() and placed in glyphs . Upon return, num_glyphs should contain the number of generated glyphs. If the value glyphs points at has changed after the call, the caller will free the allocated glyph array using cairo_glyph_free(). The caller will also free the original value of glyphs , so the callback shouldn't do so. The callback should populate the glyph indices and positions (in font space) assuming that the text is to be shown at the origin.

If clusters is not NULL, num_clusters and cluster_flags are also non-NULL, and cluster mapping should be computed. The semantics of how cluster array allocation works is similar to the glyph array. That is, if clusters initially points to a non-NULL value, that array may be used as a cluster buffer, and num_clusters points to the number of cluster entries available there. If the provided cluster array is too short for the conversion (or for convenience), a new cluster array may be allocated using cairo_text_cluster_allocate() and placed in clusters . In this case, the original value of clusters will still be freed by the caller. Upon return, num_clusters should contain the number of generated clusters. If the value clusters points at has changed after the call, the caller will free the allocated cluster array using cairo_text_cluster_free().

The callback is optional. If num_glyphs is negative upon the callback returning or if the return value is CAIRO_STATUS_USER_FONT_NOT_IMPLEMENTED, the unicode_to_glyph callback is tried. See cairo_user_scaled_font_unicode_to_glyph_func_t.

Note: While cairo does not impose any limitation on glyph indices, some applications may assume that a glyph index fits in a 16-bit unsigned integer. As such, it is advised that user-fonts keep their glyphs in the 0 to 65535 range. Furthermore, some applications may assume that glyph 0 is a special glyph-not-found glyph. User-fonts are advised to use glyph 0 for such purposes and do not use that glyph value for other purposes.

Parameters

scaled_font

the scaled-font being created

 

utf8

a string of text encoded in UTF-8

 

utf8_len

length of utf8 in bytes

 

glyphs

pointer to array of glyphs to fill, in font space

 

num_glyphs

pointer to number of glyphs

 

clusters

pointer to array of cluster mapping information to fill, or NULL

 

num_clusters

pointer to number of clusters

 

cluster_flags

pointer to location to store cluster flags corresponding to the output clusters

 

Returns

CAIRO_STATUS_SUCCESS upon success, CAIRO_STATUS_USER_FONT_NOT_IMPLEMENTED if fallback options should be tried, or CAIRO_STATUS_USER_FONT_ERROR or any other error status on error.

Since: 1.8


cairo_user_scaled_font_unicode_to_glyph_func_t ()

cairo_status_t
(*cairo_user_scaled_font_unicode_to_glyph_func_t)
                               (cairo_scaled_font_t *scaled_font,
                                unsigned long  unicode,
                                unsigned long *glyph_index);

cairo_user_scaled_font_unicode_to_glyph_func_t is the type of function which is called to convert an input Unicode character to a single glyph. This is used by the cairo_show_text() operation.

This callback is used to provide the same functionality as the text_to_glyphs callback does (see cairo_user_scaled_font_text_to_glyphs_func_t) but has much less control on the output, in exchange for increased ease of use. The inherent assumption to using this callback is that each character maps to one glyph, and that the mapping is context independent. It also assumes that glyphs are positioned according to their advance width. These mean no ligatures, kerning, or complex scripts can be implemented using this callback.

The callback is optional, and only used if text_to_glyphs callback is not set or fails to return glyphs. If this callback is not set or if it returns CAIRO_STATUS_USER_FONT_NOT_IMPLEMENTED, an identity mapping from Unicode code-points to glyph indices is assumed.

Note: While cairo does not impose any limitation on glyph indices, some applications may assume that a glyph index fits in a 16-bit unsigned integer. As such, it is advised that user-fonts keep their glyphs in the 0 to 65535 range. Furthermore, some applications may assume that glyph 0 is a special glyph-not-found glyph. User-fonts are advised to use glyph 0 for such purposes and do not use that glyph value for other purposes.

Parameters

scaled_font

the scaled-font being created

 

unicode

input unicode character code-point

 

glyph_index

output glyph index

 

Returns

CAIRO_STATUS_SUCCESS upon success, CAIRO_STATUS_USER_FONT_NOT_IMPLEMENTED if fallback options should be tried, or CAIRO_STATUS_USER_FONT_ERROR or any other error status on error.

Since: 1.8


cairo_user_font_face_create ()

cairo_font_face_t *
cairo_user_font_face_create (void);

Creates a new user font-face.

Use the setter functions to associate callbacks with the returned user font. The only mandatory callback is render_glyph.

After the font-face is created, the user can attach arbitrary data (the actual font data) to it using cairo_font_face_set_user_data() and access it from the user-font callbacks by using cairo_scaled_font_get_font_face() followed by cairo_font_face_get_user_data().

Returns

a newly created cairo_font_face_t. Free with cairo_font_face_destroy() when you are done using it.

Since: 1.8


cairo_user_font_face_set_init_func ()

void
cairo_user_font_face_set_init_func (cairo_font_face_t *font_face,
                                    cairo_user_scaled_font_init_func_t init_func);

Sets the scaled-font initialization function of a user-font. See cairo_user_scaled_font_init_func_t for details of how the callback works.

The font-face should not be immutable or a CAIRO_STATUS_USER_FONT_IMMUTABLE error will occur. A user font-face is immutable as soon as a scaled-font is created from it.

Parameters

font_face

A user font face

 

init_func

The init callback, or NULL

 

Since: 1.8


cairo_user_font_face_get_init_func ()

cairo_user_scaled_font_init_func_t
cairo_user_font_face_get_init_func (cairo_font_face_t *font_face);

Gets the scaled-font initialization function of a user-font.

Parameters

font_face

A user font face

 

Returns

The init callback of font_face or NULL if none set or an error has occurred.

Since: 1.8


cairo_user_font_face_set_render_glyph_func ()

void
cairo_user_font_face_set_render_glyph_func
                               (cairo_font_face_t *font_face,
                                cairo_user_scaled_font_render_glyph_func_t render_glyph_func);

Sets the glyph rendering function of a user-font. See cairo_user_scaled_font_render_glyph_func_t for details of how the callback works.

The font-face should not be immutable or a CAIRO_STATUS_USER_FONT_IMMUTABLE error will occur. A user font-face is immutable as soon as a scaled-font is created from it.

The render_glyph callback is the only mandatory callback of a user-font. If the callback is NULL and a glyph is tried to be rendered using font_face , a CAIRO_STATUS_USER_FONT_ERROR will occur.

Parameters

font_face

A user font face

 

render_glyph_func

The render_glyph callback, or NULL

 

Since: 1.8


cairo_user_font_face_get_render_glyph_func ()

cairo_user_scaled_font_render_glyph_func_t
cairo_user_font_face_get_render_glyph_func
                               (cairo_font_face_t *font_face);

Gets the glyph rendering function of a user-font.

Parameters

font_face

A user font face

 

Returns

The render_glyph callback of font_face or NULL if none set or an error has occurred.

Since: 1.8


cairo_user_font_face_set_unicode_to_glyph_func ()

void
cairo_user_font_face_set_unicode_to_glyph_func
                               (cairo_font_face_t *font_face,
                                cairo_user_scaled_font_unicode_to_glyph_func_t unicode_to_glyph_func);

Sets the unicode-to-glyph conversion function of a user-font. See cairo_user_scaled_font_unicode_to_glyph_func_t for details of how the callback works.

The font-face should not be immutable or a CAIRO_STATUS_USER_FONT_IMMUTABLE error will occur. A user font-face is immutable as soon as a scaled-font is created from it.

Parameters

font_face

A user font face

 

unicode_to_glyph_func

The unicode_to_glyph callback, or NULL

 

Since: 1.8


cairo_user_font_face_get_unicode_to_glyph_func ()

cairo_user_scaled_font_unicode_to_glyph_func_t
cairo_user_font_face_get_unicode_to_glyph_func
                               (cairo_font_face_t *font_face);

Gets the unicode-to-glyph conversion function of a user-font.

Parameters

font_face

A user font face

 

Returns

The unicode_to_glyph callback of font_face or NULL if none set or an error occurred.

Since: 1.8


cairo_user_font_face_set_text_to_glyphs_func ()

void
cairo_user_font_face_set_text_to_glyphs_func
                               (cairo_font_face_t *font_face,
                                cairo_user_scaled_font_text_to_glyphs_func_t text_to_glyphs_func);

Sets th text-to-glyphs conversion function of a user-font. See cairo_user_scaled_font_text_to_glyphs_func_t for details of how the callback works.

The font-face should not be immutable or a CAIRO_STATUS_USER_FONT_IMMUTABLE error will occur. A user font-face is immutable as soon as a scaled-font is created from it.

Parameters

font_face

A user font face

 

text_to_glyphs_func

The text_to_glyphs callback, or NULL

 

Since: 1.8


cairo_user_font_face_get_text_to_glyphs_func ()

cairo_user_scaled_font_text_to_glyphs_func_t
cairo_user_font_face_get_text_to_glyphs_func
                               (cairo_font_face_t *font_face);

Gets the text-to-glyphs conversion function of a user-font.

Parameters

font_face

A user font face

 

Returns

The text_to_glyphs callback of font_face or NULL if none set or an error occurred.

Since: 1.8

Types and Values

CAIRO_HAS_USER_FONT

#define CAIRO_HAS_USER_FONT 1

Defined if the user font backend is available. This macro can be used to conditionally compile backend-specific code. The user font backend is always built in versions of cairo that support this feature (1.8 and later).

Since: 1.8

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