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[paint] Document that colors are unpremultiplied 

And mention that gradient interpolation must happen
in premultiplied space.
This commit is contained in:
Matthias Clasen 2022-12-23 00:30:40 -05:00 committed by Behdad Esfahbod
parent b6e98cf758
commit 21f78c8774
1 changed files with 7 additions and 2 deletions
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9
src/hb-paint.h
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@ -210,7 +210,7 @@ typedef void (*hb_paint_pop_clip_func_t) (hb_paint_funcs_t *funcs,
* @funcs: paint functions object * @funcs: paint functions object
* @paint_data: The data accompanying the paint functions in hb_font_paint_glyph() * @paint_data: The data accompanying the paint functions in hb_font_paint_glyph()
* @is_foreground: whether the color is the foreground * @is_foreground: whether the color is the foreground
* @color: The color to use * @color: The color to use, unpremultiplied
* @user_data: User data pointer passed to hb_paint_funcs_set_color_func() * @user_data: User data pointer passed to hb_paint_funcs_set_color_func()
* *
* A virtual method for the #hb_paint_funcs_t to paint a * A virtual method for the #hb_paint_funcs_t to paint a
@ -291,13 +291,18 @@ typedef void (*hb_paint_image_func_t) (hb_paint_funcs_t *funcs,
* hb_color_stop_t: * hb_color_stop_t:
* @offset: the offset of the color stop * @offset: the offset of the color stop
* @is_foreground: whether the color is the foreground * @is_foreground: whether the color is the foreground
* @color: the color * @color: the color, unpremultiplied
* *
* Information about a color stop on a color line. * Information about a color stop on a color line.
* *
* Color lines typically have offsets ranging between 0 and 1, * Color lines typically have offsets ranging between 0 and 1,
* but that is not required. * but that is not required.
* *
* Note: despite @color being unpremultiplied here, interpolation in
* gradients shall happen in premultiplied space. See the OpenType spec
* [COLR](https://learn.microsoft.com/en-us/typography/opentype/spec/colr)
* section for details.
*
* Since: REPLACEME * Since: REPLACEME
*/ */
typedef struct { typedef struct {