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

[Richard Copley <rcopley@gmail.com>]

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On 21 March 2018 at 12:47, Eli Zaretskii <eliz@gnu.org> wrote:

> > 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.


I don't see it. You're talking about this?

          ((>= (apply '+ (color-values bg-color frame))
               ;; Just looking at the screen, colors whose
               ;; values add up to .6 of the white total
               ;; still look dark to me.
               (* (apply '+ (color-values "white" frame)) .6))

That is a luminance comparison against 60%, except that instead of
computing luminance in any of the usual ways, it simply adds up the R, G
and B components.

Also note that color-distance is based on a
> metric that we didn't invent.  Not sure whether it makes that
> "standard".
>

It's this particular way of using color-distance that we appear to have
invented. If we didn't then we took it from somewhere else, and someone
(Tom?) should know where.

And we didn't invent 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.


Fair enough.


>  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.
>

I don't believe the concept of a sphere centred on black is a useful one in
this context, or that allowing the user to vary its radius makes it any
better.

Thanks.

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