bug#74055: 31.0.50; color-lighten-name not lightening black

[Eli Zaretskii <eliz@gnu.org>]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Mon, 28 Oct 2024 17:57:49 +0100
> Cc: Gerd Möllmann <gerd.moellmann@gmail.com>,
>  Julien Danjou <julien@danjou.info>,
>  74055@debbugs.gnu.org,
>  Noah Friedman <noah@splode.com>
> 
> 28 okt. 2024 kl. 13.39 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > Our notion of "lighten color" seems to be to increase the color's
> > luminance by P percent.  Since the black color's luminance is zero,
> > increasing that by 50% still yields zero.
> > 
> > By contrast, the page you point to seems to interpret "lighten" to
> > mean that P is the percentage of the full scale, not of the original
> > color's luminance.
> > 
> > This goes back to commit 656c2dd66e, which was supposed to fix
> > bug#54514.  But maybe Noah's interpretation of "lighten" was
> > incorrect, and we should revert that change?  OTOH, if we do revert
> > it, then Noah's example will disagree with the above page.
> 
> That change may have been made in haste. For example, it didn't touch the corresponding saturate and desaturate functions which use similar mechanics, so there is now an inconsistency in that respect.
> 
> But which interpretation is better isn't obvious. It doesn't have much to do with colour theory per se. As luminance is already a percentage of sorts, it's not at all clear what it means by increasing it by a certain percentage. Personally I wouldn't use either function because of how ill-defined they are.

Maybe there are widely-accepted de-facto standards for that?

Or maybe we should support more than one interpretation, by way of
some user option?