Problem displaying image with alpha channel in a transparent window

[Yann Droneaud <ydroneaud mandriva com>]

Hi,

I'm trying to write a program to display a picture with full
transparency in its own window (without border), using Composite
extension (ARGB colormap and visual).

I've written a couple of programs doing this using Xlib alone or with
libXrender.

Using GTK+ to do the same seems easy since programs like cairo-clock [1]
are able to draw window with non rectangular shape and full
transparency.

But after many hours, and many version of my GTK+ demonstration program,
I'm still wandering how to display a GdkPixBuf with alpha layer in a
GtkWindow without any background or artefact.

So here are my little demonstration programs

- test-gdkpixbuf.c : 

  This program does the following:
  - looks up the RGBA colormap with gdk_screen_get_rgba_colormap()
  - installs the colormap with gtk_widget_set_default_colormap()
  - creates a GdkPixbuf with gdk_pixbuf_new_from_file()
  - creates a GtkWindow
  - sets the widget as GTK_APP_PAINTABLE with  
    gtk_widget_set_app_paintable()
  - disables double buffering with gtk_widget_set_double_buffered()
  - in the realize signal handler, it removes any background pixmap 
    using gdk_window_set_back_pixmap()
  - in the expose event handler, it draws the GdkPixBuf using
    gdk_draw_pixbuf()

  At first, it seems to work, but in fact, as soon as you move the 
  picture using CTRL+Button 1, you will realize the problem: the 
  window's background is a snapshot of the root window (see post from 
  Matt Hoosier [2]), and is fixed for the window's lifetime.
  At least, the content of the picture is well composited with the   
  background, but it's not the full transparency I'm looking for.

  See:
  - test-gdkpixbuf.png

- test-gdkpixbuf-bg.patch:
  
  Apply the patch on top of test-gdkpixbuf.c to create 
  test-gdkpixbuf-background.c
  This patch set a pixmap as a background, and draw nothing on the 
  window. Only the realize signal and expose event handlers are 
  modified:
  - realize signal handler creates a new pixmap with gdk_pixmap_new(),
    writes the content of the GdkPixBuf with gdk_draw_pixbuf() and then
    install the background with gdk_window_set_back_pixmap().
  - expose event handler only call to gdk_window_clear()

  With this version, the background is really transparent, but many 
  artefacts are visibles: the background is corrupted.
  On some configuration, I'm even able to see some other pixmap content 
  as background, strange !

  See:
  - test-gdkpixbuf-background.png

Note: those were tested with Gtk+ 2.10 and Gtk+ 2.12.

So I'm stuck. Either I'm always getting a unwanted background (filled
with solid color or a root window snapshot) or I'm getting a corrupted
image.

I've tried to: 
- use a GtkImage widget made from the GdkPixBuf
- use a GtkDrawingArea in the main window and draw into it
- use gdk_draw_rgb_32_image() instead of gdk_draw_pixbuf()
- paint with gdk_cairo_set_source_pixbuf()
I'm always getting an image composited with the window background.

Could someone provide me a link, a tips, or something to help me.
Thanks.

[1] http://macslow.thepimp.net/?page_id=23
[2] http://mail.gnome.org/archives/gtk-list/2006-October/msg00246.html

-- 
Yann Droneaud <ydroneaud mandriva com>