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

On Sun, Mar 8, 2015 at 11:06 AM, Zack Weinberg <zackw@panix.com> wrote:

> 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.
>

I think this sounds reasonable.
One snag is that this will prove difficult to implement as the graphic
libraries that browsers depend on don't have support for this.
This means that IE, Firefox and Safari will have to get OS level changes to
implement it.