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Re: About the :distant-foreground face attribute

From: Chong Yidong
Subject: Re: About the :distant-foreground face attribute
Date: Wed, 08 Jan 2014 05:57:41 +0800
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux)

Jan Djärv <address@hidden> writes:

> Too lazy to read documentation?
> "`:distant-foreground'
>      Alternative foreground color, a string.  This is like `:foreground'
>      but the color is only used as a foreground when the background
>      color is near to the foreground that would have been used.  This
>      is useful for example when marking text (i.e. the region face).
>      If the text has a foreground that is visible with the region face,
>      that foreground is used.  If the foreground is near the region
>      face background, `:distant-foreground' is used instead so the text
>      is readable.

I read that, but the explanation was pretty darn cryptic.  But OK: from
your message, I gather that this feature was introduced to deal with the
problem which setting the `region' face from system (GTK or NS)

But is there any other even vaguely envisioned usage?  Because, as
explained before, there are better ways to deal with this problem than
something so intrusive as defining a new face attribute.

>> First of all, the name :distant-foreground is not intuitive.  What does
>> "distant" mean in this context?
> The same as in any other context, far removed from something else,
> i.e. the background.

Things that are "distant" are in the BACKGROUND, so "distant foreground"
sounds tautological.  The fact that there isn't a good name for this
face attribute is an indicator that it's not well-conceived.

The feature does not fit well with the design of the rest of the
face-handling code.  We already have a mechanism for checking to see
when to use a particular face: the DISPLAY element in a face spec.  The
:distant-foreground face attribute, by its very existence, is redundant
with what the DISPLAY element was meant to do.  This adds extra
complexity to the design, for no good reason.

If you need to check for a background, the right thing would be to
extend the DISPLAY feature to do what you need.  For example, a DISPLAY
element of `(background dark)' is used to test for a dark background.
Maybe what you want is to be able to specify a color in place of `dark',
which would mean a background close to that color.

>> Also, this feature has one ugly consequence.  Previously, the `default'
>> face must have all its face attributes specified, but now its
>> :distant-foreground face is unspecified.
> Default face does not need to have its font specified, so this is not
> new.

M-: (face-attribute 'default :font) RET
  => #<font-object "-unknown-DejaVu Sans 

M-: (face-attribute 'default :distant-foreground) RET
  => unspecified

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