[MPEG-OTSPEC] COLRv1 to gray/alpha question (& color-blindness question)

Hin-Tak Leung htl10 at users.sourceforge.net
Wed Jul 19 22:08:00 CEST 2023

 On Wednesday, 19 July 2023 at 15:27:20 BST, Alexei Podtelezhnikov <apodtele at gmail.com> wrote:
 > Hin-Tak,

> This is probably both a spec question & a technical question. What is the recommendation for COLRv1 when the rendering target media is not capable of color?

> Are you asking about RGB to gray conversion? There are multiple specs with slightly different formulas and barely noticeable differences, with and without gamma correction. I’m is all pretty arbitrary without settled consensus.  
> Gray = 0.3R + 0.6G + 0.1B
> is good enough.
I am talking about 32-bit RGBA to 8-bit gray conversion, and I gave two examples/directions - throwing away the RGB part (and use the alpha channel as a mask on the current foreground colour), or throw away the A part, and doing 24-bit colour to gray conversion like you suggested (or even just gray = (r+g+b)/3).
There is probably also a complication in the 2nd case, for layered glyphs - do you throw away the alpha channel first and stack the resulting solid colours/gray on top of one another, or do the full 32-bit rendering, then throw away the alpha channel and collapse the 24-bit?
Werner also raised an interesting point - there is provision for palette on dark background vs light background I think, but is there a way of indicating some palettes are colour-blind-friendly? In the western world, it can be as bad as 1 in 30 male Caucasians being colour-blind.  
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