Re: Buffer-local faces

[Stefan Monnier <monnier@iro.umontreal.ca>]

> It's a much less obvious operation (what does subtracting red from black
> yield?  [Ok, maybe cyan :-]) and I can't offhand think of any obvious uses
> for it....

As I suggested, we might want to add a notion of color composition, so we
could have a face that make the foreground darker, or redder, or ...
But supposedly withing RGB this doesn't work so well, so maybe it would be
better to do it on CYK colors.  In any case this is for post-release.

> A _different_ feature that might work is a way to mark a face attribute as
> `non overridable'.  For instance if a face attribute could be a list like
> (fixed VALUE), where VALUE is a normal value for that attribute, face
> merging would always use the `base' value for that attribute instead of the
> `normally overriding' value.

A related question is how to make a face that uses explicitly the default
font.  E.g. let's say my header-line face uses helvetica font and I want to
put text in the header-line that uses the same font as the default font.
Right now, the best I could come up with is to extract the font of `default'
and pass it to `myface', but that's done "statically".

I'm not sure what's a useful generalization, maybe a "filtered inheritance"
where you can specify which attributes are inherited from a given parent.


        Stefan