bug#74725: 31.0.50; image-scaling-factor is ignored by create-image

[Eli Zaretskii <eliz@gnu.org>]

> From: Po Lu <luangruo@yahoo.com>
> Cc: David Ponce <da_vid@orange.fr>,  alan@idiocy.org,  74725@debbugs.gnu.org
> Date: Sun, 08 Dec 2024 08:01:39 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> author	Po Lu <luangruo@yahoo.com>	2024-06-03 16:34:51 +0800
> >> committer	Po Lu <luangruo@yahoo.com>	2024-06-03 16:36:29 +0800
> >> commit	56376585134d627f96c71b7b063ec51548d3ad3f (patch)
> >> 
> >> Which replaced
> >> 
> >> -                        (list :scale
> >> -                              (image-compute-scaling-factor
> >> -                               image-scaling-factor)))
> >> 
> >> By this
> >> 
> >> +                        (list :scale 'default))
> >> 
> >> In create-image.
> >> 
> >> With the side effect that the image spec don't change when the scaling
> >> factor changes, so the same cached image in always used.
> >
> > Po Lu, what were the reasons for that particular part of the commit?
> 
> The scale applied by image-scaling-factor is liable to differ by
> display

How so?  Please elaborate.

> and computing the default scale in Lisp would result in images
> being displayed with an incorrect scale in the presence of multiple
> displays.

How does the above changeset solve this problem, then?

> Image caches must be flushed when image-scaling-factor is modified,
> unless it is set to `auto' and a display's scale changes, because
> image.c has no means of detecting variable modifications and so only the
> latter event can be automatically detected.

Please describe the issue in more detail, as I don't think I follow
what you are saying here.  If we need to detect changes in variables,
we can use the add-variable-watcher technique, similar to what we do
in frame.el with variables that need to force redisplay (but maybe I
don't understand the problem you are describing).

In any case, I don't think changes in image-scaling-factor are
supposed to be immediately reflected on display, if that's what you
have in mind.  This is not the documented effect of this variable.