bug#36315: 27.0.50; SVG transparency handling is inaccurate

[YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>]

On Thu, 27 Jun 2019 23:16:07 +0900,
Eli Zaretskii wrote:
> 
> > Date: Thu, 27 Jun 2019 12:33:46 +0900
> > From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
> > Cc: pipcet@gmail.com,
> > 	36315@debbugs.gnu.org
> > 
> > > > What kind of operations do you think librsvg should provide us with,
> > > > instead of letting us use cairo?
> > > 
> > > Those for which you called the Cairo functions directly.
> > 
> > Which one, concretely?  Or you mean something in other parts?
> 
> All of the Cairo functions you called:
> 
>    cairo_create
>    cairo_destroy
>    cairo_image_surface_create
>    cairo_image_surface_get_data
>    cairo_image_surface_get_stride
>    cairo_paint
>    cairo_set_source_rgb
>    cairo_surface_destroy
>    cairo_surface_flush
>    cairo_surface_status

Why do you think so?  Librsvg does not provide us with any further
abstractions over the cairo data structures.  Also, the first
paragraph of README.md in the source code says:

  This is librsvg - A small library to render Scalable Vector Graphics
  (SVG), associated with the GNOME Project.  It renders SVG files to
  Cairo surfaces.  Cairo is the 2D, antialiased drawing library that
  GNOME uses to draw things to the screen or to generate output for
  printing.

In librsvg, only 2 functions deal with the cairo data structures.
Librsvg is, at least except its very early versions, clearly designed
so it is used together with cairo.

> > > I tried with the second patch.  I tried now again with the above one:
> > > still no rectangle.
> > 
> > Seems like a problem in DLL loading.  Please try the attached one.
> 
> This works, but I don't think I see any difference in the color of the
> rectangle wrt what I see in Emacs 26, i.e. without the patch.

Usually the sense of sight by human cannot see the difference between
the colors #ef0000 and #f00000.  You would need some color picker to
tell the difference.

				     YAMAMOTO Mitsuharu
				mituharu@math.s.chiba-u.ac.jp