[css-gcpm][css-color] device-cmyk() interaction with RGB-space colors.

Hi,

In CSS Color Level 3, all values of the <color> type (eventually) 
resolve to a (R, G, B, A) value.

When multiple colors interact with each other in an interpolation 
(transition, animation, or gradient) or with alpha compositing 
(transparency), they do so in RGBA space.


The addition of device-cmyk() changes that: not all colors are in the 
same space anymore. The spec needs to define how interpolation and alpha 
compositing works.

Note that probably none of these cases are useful, we just need to pick 
something so that they’re well-defined.

Possible suggestions:

* Gradients whose color stops are not all in the same color space could 
be invalid.

* I don’t expect any media to support both CMYK colors and 
transition/animations. But in case it happens, interpolating colors in 
different color spaces could switch at 50%, like interpolation of other 
discrete types.

* Alpha compositing could be done in device CMYK space, after all RGB 
colors have been converted per a device-specific color profile.


There may be further interactions with the Filter Effects and 
Compositing and Blending specs that need to be specified.

-- 
Simon Sapin