Re: region-based face-remapping

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 15 Jan 2024 14:55:34 -0500
> From:  Stefan Monnier via "Emacs development discussions." <emacs-devel@gnu.org>
> 
> There are some exceptions, most importantly the size attribute which can
> be a float to increase/reduce the base size.  I think such a third option
> should be available for many other attributes, so we can have faces
> which make the background color more red, or which increases/reduces
> contrast, ...

How do you express these "more red" or "more/less contrast" using just
the existing face attributes?  The :height example doesn't help
because it just defines special values for an existing attribute, so
the analogy would be to define a value for :foreground that would make
it "more red".  How to do that?  And the same with "more or less
contrast with the background color".

> With such a functionality, your request would be satisfied with a simple
> overlay with a single `face` property.

But only if the effect is to transform all the faces in the region in
the same way.  My understanding is that the request is not necessarily
limited to that.