[cairo] Fwd: Re: Enabling GTK drawing area alpha channel

Robert O'Callahan robert at ocallahan.org
Wed May 18 03:38:20 PDT 2011

On Wed, May 18, 2011 at 9:22 PM, Tobias Hoffmann <
lcairo-list at thax.hardliners.org> wrote:

> Manu wrote:
>> On 05/17/2011 01:35 PM, Tobias Hoffmann wrote:
>>>  Hint: Don't use the default visual for the widget:
>>>  GdkColormap *rgba = gdk_screen_get_rgba_colormap (screen);  // GTK3:
>>>  gdk_screen_get_rgba_visual(screen), etc...
>>>  if (rgba&&  gdk_screen_is_composited (screen)) {
>>>  gtk_widget_set_colormap(GTK_WIDGET(drawing_area), rgba);
>>>  }
>> Sorry, sill one issue: gdk_screen_get_rgba_colormap() is not implemented
>> in the win32 backend. The gdk visual depth is always reported as 24.
> AFAIU this has to do with limitations on GDI surfaces on windows, so
> there's no easy workaround.
> Maybe someone with better win32 knowledge can tell you (and me) more.

It is possible to draw RGBA windows with cairo-win32 on Windows Vista and
Windows 7. Firefox 4 does it. We don't use GTK on Windows, though.

"Now the Bereans were of more noble character than the Thessalonians, for
they received the message with great eagerness and examined the Scriptures
every day to see if what Paul said was true." [Acts 17:11]
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