[css-compositing] Not all hue/saturation/luminosity knockouts are possible

The current level of css-compositing specifies four blend modes that
replace one or two of the hue, saturation, and luminosity components
of the bottom color with those from the top color, but there are six
possible such modes; two are missing.

Mode       Hue    Saturation Luminosity
---------- ------ ---------- ----------
hue        top    bottom     bottom
saturation bottom top        bottom
luminosity bottom bottom     top

color      top    top        bottom
???        top    bottom     top
???        bottom top        top

I don't have a use case for replacing hue and luminosity, but I do for
saturation and luminosity: suppose you want to define a background
pattern which consists of variations on one hue, and reuse the same
pattern in more than one place with different base hues.  Not all such
variations can be achieved by varying only saturation or luminosity.
In particular, making a light color _appear_ darker often requires
both reducing its luminosity and _increasing_ its saturation.

I'd suggest 'shade' for the name of the mode that replaces saturation
and luminosity.  I don't have a name for the other one but I think it
should exist for completeness.

zw

Received on Sunday, 8 March 2015 18:06:49 UTC