Subject: Re: bug#15900: 24.3.50; foreground-color-at-point returns wrong results

From: Michael Heerdegen <[email-masked]>
Date: Fri, 15 Nov 2013 23:18:11 +0100

Hello,

CCing Drew cause the Change Log mentions him with regard to
`foreground-color-at-point'.

Eli Zaretskii <[email-masked]> writes:

> As you later discovered, even (face-at-point nil t) doesn't do the
> job.
>
> Which doesn't surprise me a bit: this kind of things cannot be done
> reliably from Lisp, even at a price of the kind of obfuscated code
> that face-at-point and foreground-color-at-point use.  It is much
> simpler to write a C primitive that simulates the display, then
> returns the resulting face at a given character position, a simple and
> straightforward task on the C level, something the display engine does
> all the time.

That sounds good.  Can we just do that?

> > P.S. Some background: I'm working on an addition to stripe-buffer.el
> > that changes the foreground color continuously, instead of changing the
> > background.  This is for better readability.
> > 
> > I want to keep the foreground colors already present, so that e.g. links
> > in w3m are still recognizable.  Paragraphs in italic can be colored
> > OTOH.
> > 
> > So, what I need is a reliable `foreground-color-at-point'.  Tips and
> > alternatives welcome.
>
> Why can't you detect that a portion of text is covered by specific
> properties (e.g., one of a list of known properties), and leave those
> portions alone?

What do you mean with "properties" - text and overlay properties?  If
faces are among them, I still must figure out if one of these faces
changes the foreground.  If not, i.e., if a face e.g. just underlines, I
do want to color the text nevertheless.  Probably I didn't understand
what you meant. This expression works ok for me:

--8<---------------cut here---------------start------------->8---
(cl-some
 (lambda (face) (not (memq (face-foreground face nil t)
		      `(nil
			,(face-attribute 'default :foreground)
			"unspecified-fg" "unspecified-bg"))))
 (cl-mapcan (lambda (face-or-list) (if (facep face-or-list)
				  (list face-or-list)
				face-or-list))
	    (list
	     (get-text-property (point) 'face)
	     (get-text-property (point) 'font-lock-face))))
--8<---------------cut here---------------end--------------->8---

Overlays are not covered by this - which is not even necessary, because
overlays have priorities that can be controlled.

Thanks,

Michael.