GNU bug report logs - #77841
Some svg image's black color is replaced by emacs's theme foreground color

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From: Alan Third <alan <at> idiocy.org>
To: Ihor Radchenko <yantar92 <at> posteo.net>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 77841 <at> debbugs.gnu.org, Billy Lei <LeiYiXia29 <at> outlook.com>
Subject: bug#77841: Some svg image's black color is replaced by emacs's theme foreground color
Date: Sun, 20 Apr 2025 13:56:37 +0100

On Sun, Apr 20, 2025 at 12:19:02PM +0000, Ihor Radchenko wrote:
> Alan Third <alan <at> idiocy.org> writes:
> 
> >> This image was generated by dvisvgm.
> >> It indeed explicitly draws a rectangle over the page, effectively
> >> inverting background and foreground (Emacs has no reason to apply
> >> background color to the rect object fill).
> >
> > I don't understand what you mean, because it doesn't apply the
> > background colour to the rect. It shows as white on red. Red is the
> > foreground colour: the surrounding text is red. This is exactly the
> > same as the first case where the rect is filled with black because
> > black is the foreground colour.
> 
> You are right, but you may be missing the context I am coming from.
>
<snip explanation>
>
> However, dvisvgm does _not_ use this method simply because it is not
> fully portable - some browsers and svg viewers ignore fill
> attribute. So, drawing background rectangle is often recommended for
> svg. It is also apparently a common practice.

Having reviewed our code we actually set the background the same way,
by drawing a background coloured rectangle then inserting the SVG over
it.

I did a little digging and it looks as though explicitly setting the
foreground colour to black is often considered bad practice, which I
guess is why we see this a lot, although that practice is
inconsistently applied. Even that German flag earlier in the thread
didn't set the fill colour, but did set the stroke colour to black.

> > If you believe Emacs should behave differently then I don't see any
> > reason why we can't change it, as long as it still allows the use of
> > SVGs inline as widgets or icons, etc. without forcing the programmer
> > to explicitly handle changes to colours when faces change, or the
> > mouse hovers over the line, etc.
> 
> I am not sure if Emacs should behave differently in this scenario or
> not. It may be a judgment call and may also be non-trivial to implement.

To be frank it probably should, but I don't know exactly *how* it
should behave.

-- 
Alan Third

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