cairo-user-font: User Fonts
Description
Font support with font data provided by the userDetailed 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.User Functions
cairo_user_scaled_font_init_func_t(scaled.font, cr, 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.
Since 1.8
scaled.font- [
CairoScaledFont] the scaled-font being created cr- [
Cairo] a cairo context, in font space extents- [
CairoFontExtents] font extents to fill in, in font space
Returns: [CairoStatus] CAIRO_STATUS_SUCCESS upon success, or
CAIRO_STATUS_USER_FONT_ERROR or any other error status on error. cairo_user_scaled_font_render_glyph_func_t(scaled.font, glyph, cr, 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 cairoMask instead of cairoPaint. 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.
Since 1.8
scaled.font- [
CairoScaledFont] user scaled-font glyph- [unsignedlong] glyph code to render
cr- [
Cairo] cairo context to draw to, in font space extents- [
CairoTextExtents] glyph extents to fill in, in font space
Returns: [CairoStatus] CAIRO_STATUS_SUCCESS upon success, or
CAIRO_STATUS_USER_FONT_ERROR or any other error status on error. cairo_user_scaled_font_text_to_glyphs_func_t(scaled.font, utf8, utf8.len, glyphs, num.glyphs, clusters, num.clusters, cluster.flags)-
cairo_user_scaled_font_text_to_glyphs_func_t is the type of function which
is called to convert input text to a list of glyphs. This is used by the
cairoShowText 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 list is too short for
the conversion (or for convenience), a new glyph list may be allocated
using cairoGlyphAllocate() 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 list using cairoGlyphFree().
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 list allocation works is similar to the glyph list. That is,
if clusters initially points to a non-NULL value, that list may be used
as a cluster buffer, and num.clusters points to the number of cluster
entries available there. If the provided cluster list is too short for
the conversion (or for convenience), a new cluster list may be allocated
using cairoTextClusterAllocate() and placed in clusters. 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 list using cairoTextClusterFree(). The callback is optional. If num.glyphs is negative upon
the callback returning, 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.
Since 1.8
scaled.font- [
CairoScaledFont] the scaled-font being created utf8- [char] a string of text encoded in UTF-8
utf8.len- [integer] length of
utf8 in bytes glyphs- [
CairoGlyph] pointer to list of glyphs to fill, in font space num.glyphs- [integer] pointer to number of glyphs
clusters- [
CairoTextCluster] pointer to list of cluster mapping information to fill, or NULL num.clusters- [integer] pointer to number of clusters
cluster.flags- [
CairoTextClusterFlags] pointer to location to store cluster flags corresponding to the
output clusters
Returns: [CairoStatus] CAIRO_STATUS_SUCCESS upon success, or
CAIRO_STATUS_USER_FONT_ERROR or any other error status on error. cairo_user_scaled_font_unicode_to_glyph_func_t(scaled.font, unicode, 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 cairoShowText 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, 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.
Since 1.8
scaled.font- [
CairoScaledFont] the scaled-font being created unicode- [unsignedlong] input unicode character code-point
glyph.index- [long] output glyph index
Returns: [CairoStatus] CAIRO_STATUS_SUCCESS upon success, or
CAIRO_STATUS_USER_FONT_ERROR or any other error status on error.