RE: [External] : Re: Foreground color opacity

[Drew Adams]

> > > Why not cover the "inactive" portion with an overlay which has the
> > > 'face' (or font-lock-face) property that specifies only the opacity?
> > > That is IMO simpler and doesn't need to run any functions after
> > > fontifications.
> > > (Caveat: I didn't actually try this approach, so maybe I'm missing
> > > something.)
> >
> > That would be the perfect solution, but as far as I can tell we don't
> > have an opacity face attribute, do we?
> 
> Too bad.  (The alpha parameter is what I had in mind , but I see now
> that it's only supported for the entire frame, not for a single face.)
> 
> Anyway, it sounds inelegant that we need to jump through such hoops to
> achieve such a simple effect.  It could even be a problem, if some
> Lisp program expects to find the font-lock faces in a buffer, but
> instead sees your special faces which replaced them.  E.g., don't we
> have features that detect strings and comments in code by looking for
> their respective font-lock faces?

I agree that an alpha face attribute would be a good thing.
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FWIW, in my library `hl-spotlight.el' I shade/highlight the
background over several lines around the cursor - which is
somewhat similar to the feature Filippo described, at least
wrt dimming/highlighting a zone of text.

I use an overlay, similar to what Eli suggested.  Because in
my case the zone is just some contiguous lines I leverage
`hl-line-mode', substituting a different face for the line
background dimming/highlighting.

https://www.emacswiki.org/emacs/HighlightCurrentLine#HlSpotlight
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`hl-spotlight.el' just uses a face for the overlay, leaving
it up to a user to customize that as needed.

Library `isearch-prop.el' provides similar dimming/lightening
of a zone of text (not necessarily lines).  But it lets you
choose whether to use a particular background color or to dim
or lighten the background by a given factor.  In the latter
case it uses `color-saturate-name' or `color-lighten-name'
from standard library `color.el' to do the dimming/lightening.

https://www.emacswiki.org/emacs/download/isearch-prop.el
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HTH.