bug#40845: SVG rendering issues

[Pip Cet <pipcet@gmail.com>]

On Sat, Apr 25, 2020 at 5:46 PM Alan Third <alan@idiocy.org> wrote:
> On Sat, Apr 25, 2020 at 01:24:13PM -0400, Clément Pit-Claudel wrote:
> > Yes, of course: if an image has a foreground color, it should be
> > respected. But it's not uncommon for SVG images to not include a
> > foreground color, as shown in the repro. In that case, the image
> > should use the current foreground color, I think. (of course, a
> > :foreground keyword, if any, should take precedence; that is issue
> > 4).

For reference, the relevant chunk of librsvg source code is this:

// https://www.w3.org/TR/SVG/color.html#ColorProperty
make_property!(
    ComputedValues,
    Color,
    // The SVG spec allows the user agent to choose its own default
for the "color" property.
    // We don't allow passing in an initial CSS in the public API, so
we'll start with black.
    //
    // See https://bugzilla.gnome.org/show_bug.cgi?id=764808 for a
case where this would
    // be useful - rendering equations with currentColor, so they take
on the color of the
    // surrounding text.
    default: cssparser::RGBA::new(0, 0, 0, 0xff),
    inherits_automatically: true,
    newtype_parse: cssparser::RGBA,
);

> Lars fixed the foreground issue for eww by wrapping svg files in
> another svg. See svg--wrap-svg in shr.el (bug#37159).
>
> I think this is the only practical way to handle svg files with no
> foreground colour set. To do this when loading _any_ svg we’d probably
> have to move it into create-image or image.c.

I think it's a neat hack, but it's just one of the more obvious ways
of working around an annoying limitation of librsvg (Ideally, we'd use
a library without such a limitation by default and fall back to
modifying/wrapping SVGs when using a limited library.)

For applications which change glyphs' foreground colors a lot, it
might be preferrable to coax rsvg into providing an alpha-only map for
the foreground plus an RGBA map for the background and blend them
together, and that approach would work for other image types.