COLRv2 laundry list

[behdad]

Let's try to converge on which features we want in COLRv2.

Based on the open issues, here's a starting point:

• PaintGlyphSelf / PaintGlyphDelta: To optimize fonts that apply the same paints (gradients, etc) to many glyphs. https://github.com/googlefonts/colr-gradients-spec/issues/370

• PaintTemplates: To optimize fonts that apply different paints (gradients, etc) to the same glyph shapes. https://github.com/googlefonts/colr-gradients-spec/issues/371

• Two more compositing modes: plus-lighter and plus-darker from the Compositing 2 W3C spec. Might need implementation in graphics libraries, but should be straightforward. https://github.com/googlefonts/colr-gradients-spec/issues/364

• PaintBitmap referencing glyphs in sbix/CBDT; possibly an inline version PaintBitmapInline. Highly requested. https://github.com/googlefonts/colr-gradients-spec/issues/272

• Mesh gradients. https://github.com/googlefonts/colr-gradients-spec/issues/312

• Blurs and filters. By far the hardest implementation-wise as some graphics libraries might not have support for them (eg. cairo). But highly requested. https://github.com/googlefonts/colr-gradients-spec/issues/270

• Variable-components. Probably after similar feature lands in glyf table. Not clear to me what the separate implementation here would provide that can't be achieved through the glyf table counterpart. To be studied. Might be more useful when we add variability to CPAL. https://github.com/googlefonts/colr-gradients-spec/issues/277

• Stroke paints: https://github.com/googlefonts/colr-gradients-spec/issues/276

• 24bit versions of PaintGlyph, PaintColrGlyph (and the proposed PaintGlyphDelta and others). These are to align with the beyond-64k effort in boring-expansion.

CPAL improvements:

• Variability: https://github.com/googlefonts/colr-gradients-spec/issues/356

• Color-spaces. Might also need new primitives in COLR to specify blending space. Discussed in: https://github.com/googlefonts/colr-gradients-spec/issues/356

• Referencing foreground color from CPAL: https://github.com/googlefonts/colr-gradients-spec/issues/354

• Derived colors from base colors. Requires color-spaces to be useful.

[behdad]

Should we start thinking about COLRv2?