Re: Gradient Color Fonts, COLR v1 Contributions to FreeType

[Dominik Röttsches]

Hi Werner and others,

I've made some progress with the COLR v1 extension implementation in
FreeType and would be happy to hear initial feedback on the general
approach and API style.
I am not requesting a full review yet, but I would be very happy if
you had some time to take an initial high-level look. (Caveat: The
code still has a set of TODOs and formatting needs tuning to follow
the FreeType conventions more precisely).

I've added a set of FT_Paint_* / FT_COLR_* structures to freetype.h
representing the primitives in the COLR v1 Gradients spec here:
https://github.com/googlefonts/colr-gradients-spec/blob/master/colr-gradients-spec.md

I've also added two new main entry points:

  FT_EXPORT ( FT_Bool )
  FT_Get_Color_Glyph_Layer_Gradients ( FT_Face           face,
                                       FT_UInt           base_glyph,
                                       FT_UInt           *aglyph_index,
                                       FT_COLR_Paint *   paint,
                                       FT_LayerIterator *iterator );

For retrieval of the v1 layers, returning an FT_COLR_Paint union with
solid, linear or radial gradient information depending on the union's
format identifier.

  FT_EXPORT ( FT_Bool )
  FT_Get_Colorline_Stops ( FT_Face               face,
                           FT_ColorStop *       color_stop,
                           FT_ColorStopIterator *iterator );

This methods work analogously to the layer retrieval functions for
COLR v0 and v1, but defines a new ColorStopIterator to retrieve color
stops of a gradient without requiring large client side allocations.

I'm attaching git format-patch style patch on top of ToT, and you can also use
https://github.com/drott/freetype2-colr/compare/master...colrV1ApiSquashed
if you like for providing feedback.

Thank you very much in advance for taking a look. I'll be
out-of-office for two weeks but I'll respond to any feedback as soon
as possible when getting back.

Dominik

On Tue, Jun 30, 2020 at 11:46 AM Dominik Röttsches <drott@chromium.org> wrote:
>
> Hi Werner,
>
> thanks for the encouragement.
>
> On Tue, Jun 30, 2020 at 10:40 AM Werner LEMBERG <wl@gnu.org> wrote:
>
> > > In a team effort with Roderick Sheeter & Cosimo Lupo & Dave Crossland
> > > (Google Fonts) and myself (Blink/Chrome)
> >
> > ... and Behdad Esfahbod ...
>
> Yes!
>
> > Thanks.  Are there fundamental differences to the googledocs version at
> >   
> > https://docs.google.com/document/d/1EPndWsdMK_M135FOIxQZ--dHpH727rXEBvLZqgIzZvs/edit?ts=5de68042#heading=h.no8g2ykeo8za
>
> I don't think there are major differences, we moved it to GitHub to
> allow public comments and issue tracking.
>
> Dominik

0001-Draft-COLR-Table-v1-Gradient-Extensions-API-Proposal.patch
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