Subject: bug#36315: 27.0.50; SVG transparency handling is inaccurate

From: YAMAMOTO Mitsuharu <[email-masked]>
Date: Mon, 24 Jun 2019 16:56:45 +0900

On Fri, 21 Jun 2019 05:26:53 +0900,
Pip Cet wrote:
> 
> [1  <text/plain; UTF-8 (7bit)>]
> Evaluate the following in emacs -Q:
> 
> (require 'svg)
> 
> (defun make-image (color)
>   (let ((svg (svg-create 100 100)))
>     (svg-rectangle svg 0 0 100 100 :fill color)
>     (svg-image svg)))
> 
> (set-frame-parameter (window-frame) 'background-color "black")
> 
> (insert (propertize " " 'display (make-image "#f00000")))
> 
> The expected result is a rectangle (on black background) of color
> #f00000. The actual result is a rectangle of color #ef0000. For black
> backgrounds, white is no longer representable.
> 
> This is related to bug #36304, but much easier to fix.
> 
> Patch attached.

An alternative way would be to use rsvg_handle_render_cairo, which is
recommended by librsvg, and let it blend with the background color.

Patch attached.  Note that this does not require --with-cairo.
Raising the required version of librsvg to 2.14 is not a problem, as
we are already using rsvg_handle_get_dimensions that requires that
version.  Is Windows librsvg DLL compiled with libcairo?

				     YAMAMOTO Mitsuharu
				[email-masked]