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Re: [PATCH] Re: About the :distant-foreground face attribute

From: Daniel Colascione
Subject: Re: [PATCH] Re: About the :distant-foreground face attribute
Date: Tue, 14 Jan 2014 14:38:49 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

Thanks. This is productive.

On 01/14/2014 11:32 AM, Drew Adams wrote:
Emacs should act that way by default, as it always has.  Anyone
who wants automatic foreground twiddling for a given face should
ask for that explicitly.  Whether that twiddling is to accommodate
font-locking or for some other reason.

I don't have a strong opinion about the default, although I lean
toward the behavior #3 (3b specifically) below since it looks
blingy and might help attract new users.

Bof.  Today's sparkling blingy is tomorrow's annoying.

Fair enough, but it looks like we're going to go with some variant of #3 (according to Stefan's latest message), so we might as well choose the least objectionable #3 variant.

And note that the "problem", of text highlighted by the selection
(region) having foreground and background the same, is the same
problem you will anyway encounter for Isearch.  In your example,
even when searching you can run into the same problem.

Will you be proposing that Isearch highlighting too let font-lock
highlighting show through?  There is a reason we give Isearch
highlighting such a high priority: it should clearly _highlight_
text - even text that might be font-locked.

If I had my druthers, we'd turn on :contrast-function for the default face. It's not generally possible for packages written by arbitrary third parties to coordinate color choices in the presence of arbitrary themes. Most packages can base colors on existing font-lock faces, but font-lock has a limited built-in palette and plenty of packages define entirely new faces with arbitrary colors. Automatic adjustment can make these faces readable no matter the circumstances of their use.

I'm trying to be conservative by applying automatic contrast adjustment only to region right now, since it's this face, being based on system colors, that has the highest risk of being unreadable.

A face being equal to its default setting does not imply
that the user gives Emacs license to change it.

I disagree. If our user leaves a face at the default setting, she's
giving us *explicit license* to use whatever heuristics we think
work best.

And how does she specify to you that the default colors are exactly
what she happens to prefer?  How does she produce the same effect as
if the default colors were different and she customized them to be
what the default colors actually are?  How does she tell you
"Hands off!  This is really what I want!"?

She shouldn't have to, because if she chooses reasonable colors, the automatic adjustment will never go into effect. If she really wants to customize the region face so that it is illegible, she can uncheck :contrast-function.

If a user never expressed a preference with respect to foreground
and background colors, there's no contract to violate.

And how does she express that preference, if it happens to coincide
with the default colors?

See above.

What you are saying is like saying that you think you have a license
to change the value of option `default-frame-alist' automatically,
if the current value is nil, because that's also the default value.

Well, yes, we do have such a license. By this logic, we can't change any default ever.

Right now, it depends on the order in which the faces are merged.
The last face that specifies a :contrast-function gets to control the
contrast behavior.

Users are going to have trouble with that complication.  Too much
wondering about what happened and why.  Of course, I could be wrong.

I don't think it's any more confusing than other aspects of the face model --- or CSS, for that matter. Sure, the model is complex, but it's powerful.

Right now, region is the only face that specifies a
contrast-function, and I can't think of another good use case at
the moment.

Maybe that in itself should tell you something...

See the discussion of generality above. Personally, I'd like to apply automatic adjustment (or other filters) more broadly, but for now, it makes sense to be conservative, which means applying the adjustment only to region.

But it still makes life more complicated for Lisp code that
wants to get or set the actual appearance of the face.  Whereas
before code needed only to get or set attribute :foreground,
now it will need to also check for a non-nil :contrast-function
and apply that.

I don't understand why lisp code would need to know the
post-adjustment colors used for display. Can you explain why
we'd want to know?

Lisp code that checks or changes a given color attribute is trying
to check or affect the actual appearance of the face (again, not
considering merges with other faces etc.).

Doing this breaks the one-to-one relationship between the face's
attributes and its appearance.

But there's only a one-to-one relationship if the face is fully specified --- this patch doesn't change that.

That means that the Lisp code cannot
just examine or set `foreground' and `background' colors.  It will
need to be changed to also invoke the :contrast-function.  Or if
that takes effect automatically, it will need to be aware that that
can happen, and perhaps even accommodate it to cancel it out
(depending on the purpose of the Lisp code).

Consider code that does something based on distance of the current
foreground appearance from some given value - the same kind of thing
you are doing with a given :contrast-function, for instance.  It
will no longer be sufficient to just measure the distance to the
current `foreground' attribute value.  Now, it will need to invoke
the current :contrast-function too, to accurately estimate the

Also, today, any lisp code that wants to mimic the redisplay face
combination logic needs to take into account text properties,
frame-local variables, overlays, display attributes, and so on. It
would be a big job, and I'm not aware of anyone who's done it.

I specifically spoke of the color attributes for an individual face,
and separated this from combining faces and other display properties.

Consider a Lisp function that helps you customize the foreground
and background colors of an individual face.  What it checks and what
it produces should reflect the face appearance (again, in isolation).

Consider a function that lets you modify the foreground of a given
face incrementally, showing you the effect, WYSIWYG-style, in your
buffer of C code or whatever.  Maybe it increments hue or saturation
or the blue component.  With your feature, what you see by its
changing attribute `foreground' will presumably "jump" when the
:contrast-function decides that the new value would be too close to
the background.

Yes, that's what would happen, but an editor of this sort --- and I'm not aware of any that currently exist --- can just explicitly turn off the adjustment, either at the face level or through an overlay in the preview area. We should optimize emacs as a text editor, not a precise color picker or a floor wax. :-)

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