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[docs] Add a section about rendering

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Matthias Clasen 2022-12-31 10:14:37 -05:00 committed by Behdad Esfahbod
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commit 956ccb11a8
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docs/usermanual-fonts-and-faces.xml
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@ -473,4 +473,46 @@
</para> </para>
</section> </section>
<section>
<title>Glyphs and rendering</title>
<para>
The main purpose of HarfBuzz is shaping, which creates a list of positioned
glyphs as output. The remaining task for text layout is to convert this list
into rendered output. While HarfBuzz does not handle rasterization of glyphs
per se, it does have APIs that provide access to the font data that is needed
to perform this task.
</para>
<para>
Traditionally, the shapes of glyphs in scalable fonts are provided as quadratic
or cubic Beziér curves defining outlines to be filled. To obtain the outlines
for a glyph, call <function>hb_font_draw_glyph()</function> and pass a
<type>hb_draw_funcs_t</type> struct. The callbacks in that struct will be called
for each segment of the outline. Note that this API provides access to outlines
as they are defined in the font, without applying hinting to fit the curves
to the pixel grid.
</para>
<para>
Fonts may provide pre-rendered images for glyphs instead of or in addition to
outlines. This is most common for fonts that contain colored glyphs, such as
Emoji. To access these images, use <function>hb_ot_color_reference_png()</function>
or <function>hb_ot_color_reference_svg()</function>.
</para>
<para>
Another way in which fonts provide colored glyphs is via paint graphs that
combine glyph outlines with gradients and allow for transformations and
compositing. In its simplest form, this can be presented as a series of
layers that are rendered on top of each other, each with its own color.
HarfBuzz has the <function>hb_ot_color_glyph_get_layers()</function> to
access glyph data in this form.
</para>
<para>
In the general case, you have to use <function>hb_font_paint_glyph()</function>
and pass a <type>hb_paint_funcs_t</type> struct with callbacks to obtain paint
graphs for glyphs that have them. The <function>hb_font_paint_glyph()</function>
API can handle outline and image glyphs as well, so it provides a unified API for
access to glyph rendering information.
</para>
</section>
</chapter> </chapter>