bug#56182: 28.1; Display of SVG file with transparent background is incorrect

[Alan Third]

On Sat, Jun 25, 2022 at 12:25:54PM +0300, Eli Zaretskii wrote:
> > Cc: 56182@debbugs.gnu.org
> > From: Lars Ingebrigtsen <larsi@gnus.org>
> > Date: Fri, 24 Jun 2022 11:23:01 +0200
> > 
> > Pascal Quesseveur <pquessev@gmail.com> writes:
> > 
> > > Frame background color is set to royalblue4. Display of splash.svg
> > > shows a background which looks dark orange.
> > >
> > > emacs -Q \
> > > --eval="(setq initial-frame-alist (list (cons 'background-color
> > > \"royalblue4\")))"
> > >
> > > (display-about-screen)
> > 
> > I'm unable to reproduce this problem on Debian/bookworm, so it sounds
> > like it might be Windows/svg library version dependent:
> 
> Alan, any chance you could look into this?
> 
> If you cannot reproduce it, can you tell which factors affect the
> background of SVG images in Emacs 28 in the above scenario?

I can't reproduce it.

An SVG can override the background colour, but splash.svg does not.

The background colour can be set directly using the :background image
spec.

If none of the above is true then the background is picked up from the
face defined in the "it" struct in xdisp.c where the image is defined.

If the face number is "-1" (which I suspect is only used for internal
usage in image.c, but you'll know better than me) then it uses the
default frame face.

Barring bugs, I don't think the version of librsvg should have any
effect on this as we set the background colour in the wrapper, which
as far as I'm aware will work on any SVG renderer.

I'm not sure if it's relevant, but in the screenshots provided by
Pascal some of the other face colours are different from what I see
here and what Lars has in his screenshot. It makes me wonder if
there's some other difference that's perhaps affecting this, although
I can't imagine what it could be.
-- 
Alan Third