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

[Alan Third <alan@idiocy.org>]

On Sat, Dec 07, 2024 at 04:49:41PM +0200, Eli Zaretskii wrote:
> > Date: Sat, 7 Dec 2024 13:13:58 +0100
> > From:  David Ponce via "Bug reports for GNU Emacs,
> >  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> > 
> > While working with images, I found what seems an issue to me with
> > `create-image' which unconditionally set the :scale image property to
> > 'default' when not specified, ignoring the value of the option
> > `image-scaling-factor'.
> > 
> > Here is an illustration:
> > 
> > (let ((image-scaling-factor 1.0))
> >     (image-size
> >      (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg")))
> >      t))
> > => (63 . 63)
> > 
> > (let ((image-scaling-factor 2.0))
> >     (image-size
> >      (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg")))
> >      t))
> > => (63 . 63)
> > 
> > (image-size
> >    (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg" :scale 1)))
> >    t)
> > => (48 . 48)
> > 
> > (image-size
> >    (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg" :scale 2)))
> >    t)
> > => (96 . 96)
> > 
> > You can replace `image-size' with `insert-image' and observe the same.
> > 
> > Here is a simple patch which fix the issue for me:
> > 
> > diff --git a/lisp/image.el b/lisp/image.el
> > index ce97eeb3ca1..2c1e865c336 100644
> > --- a/lisp/image.el
> > +++ b/lisp/image.el
> > @@ -536,7 +536,9 @@ create-image
> >                            file-or-data)
> >                      (and (not (plist-get props :scale))
> >                           ;; Add default scaling.
> > -                        (list :scale 'default))
> > +                        (list :scale (if (numberp image-scaling-factor)
> > +                                         image-scaling-factor
> > +                                       'default)))
> >   	           props)))
> >         ;; Add default smoothing.
> >         (unless (plist-member props :transform-smoothing)
> 
> AFAIU, this is supposed to be taken care of in image.c.
> 
> Alan, any ideas why this doesn't seem to work?

It's because the image spec doesn't change so the image is pulled from
the cache each time.

Flushing the image between calls to image-size fixes it:

    (image-flush (find-image '((:file "icons/hicolor/scalable/apps/emacs.svg"))))

I'm not sure what the solution is here. My feeling is that
image-scaling-factor isn't intended as something you set for each
image as you load it, it's a set-and-forget setting, so perhaps we
just need to document that in order for it to take effect the image
cache needs to be flushed.

Alternatively we make the image cache aware of it.

Perhaps we can flush the cache automatically when it changes? That
might give unexpected results too.
-- 
Alan Third