Re: CSS contrast (#30295) (was Re: Heads-up: Emacs 26.1 RC1)

[Eli Zaretskii <eliz@gnu.org>]

> From: Richard Copley <rcopley@gmail.com>
> Date: Wed, 21 Mar 2018 11:34:47 +0000
> Cc: John Wiegley <johnw@gnu.org>, Tom Tromey <tom@tromey.com>, 
> 	Emacs Development <emacs-devel@gnu.org>
> 
> Is the current criterion (use white text on backgrounds inside a sphere of a given radius centred on black, in
> the color-distance metric space) a standard way of doing things?

We do something very similar in frame-set-background-mode and in
tty-color-approximate.  Also note that color-distance is based on a
metric that we didn't invent.  Not sure whether it makes that
"standard".

> I'm not saying that the luminance-less-than-50% calculation is a standard, either, but I've seen it used before,
> which is more than I can say for the color-inside-a-sphere calculation.
> I'm also not saying that color-distance is any more or less "invented" than luminance.

I'm not dismissing these arguments, I'm just saying that it's too late
to raise them for what will become Emacs 26.1.  We can make those
changes on master, if people agree with your reasoning, or we could
use the contrast criterion there, or something else.  But for the
release branch, the only change I can think of which will allow you to
get what you want in a safe way is the one I proposed: introduce a
customizable threshold.