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.
  [COLRv2] Proposal: Reusing layers with the "self GID" #370

• PaintTemplates: To optimize fonts that apply different paints (gradients, etc) to the same glyph shapes.
  [COLRv2] Paint templates #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.
  Support compositing-2, esp plus-lighter #364

• PaintBitmap referencing glyphs in sbix/CBDT; possibly an inline version PaintBitmapInline. Highly requested.
  Reusing bitmaps #272

• Mesh gradients.
  Mesh gradients #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.
  Blurs and filters #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.
  Variable Components for COLRv1 #277

• Stroke paints:
  stroke: discussion about inclusion in COLRv1 #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:
  CPAL variation #356

• Color-spaces. Might also need new primitives in COLR to specify blending space. Discussed in:
  CPAL variation #356

• Referencing foreground color from CPAL:
  CPAL can't reference foreground color #354

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

[behdad]

Should we start thinking about COLRv2?