http://bugs.freedesktop.org/show_bug.cgi?id=19910
Summary: Can't blend with the alpha from an RGB texture updated
via texture-from-pixmap
Product: Mesa
Version: unspecified
Platform: x86 (IA32)
OS/Version: Linux (All)
Status: NEW
Severity: normal
Priority: medium
Component: Drivers/DRI/i915
AssignedTo: dri-devel@lists.sourceforge.net
ReportedBy: n...@linux.intel.com
Created an attachment (id=22487)
--> (http://bugs.freedesktop.org/attachment.cgi?id=22487)
Test case
In metacity-clutter (Mutter) we use multi-texturing to render shaped windows.
The shape of the window is copied into a GL_ALPHA texture which is blended with
the window's texture using multi-texturing such that the areas outside the
window's shape end up with zero alpha and aren't displayed.
The texture for the window should usually be GL_RGB format and is updated using
the texture from pixmap extension. The texture environment is set up so that
layer 0 (the window texture) is left at the default and layer 1 uses
GL_COMBINE. The RGB is set to GL_REPLACE with the values from the previous
texture (the window texture). The alpha is set to GL_MODULATE with the mask
layer and the previous layer.
As far as I understand from the man pages for glTexEnv, the alpha from an RGB
texture when used for blending will be constant 1 so it should work. It works
fine on my i965 laptop with Mesa 7.2 (from Ubuntu Intrepid) when
LIBGL_ALWAYS_INDIRECT enabled and also on an Nvidia box.
However, when testing on an Aspire One (i945) with the Moblin alpha release
image, the blending does not work correctly. The shaped windows appear
completely transparent as if maybe the non-existent alpha value from the window
texture is being used.
We currently workaround this in Mutter by using a different blending mode when
the window has a 24 bit depth. In this case we ignore the alpha from the window
texture and modulate directly with GL_PRIMARY_COLOR.
The attached test case demonstrates this without using Clutter. It creates a
small white window with a black circle outline drawn it and then creates
another window which clones the small window using texture from pixmap. The
texture is blended with a 2x2 GL_ALPHA texture which is full opacity except the
bottom left pixel. So the window should appear as a black scaled up circle on a
white background except for the bottom left corner which should show the blue
underneath.
Under Moblin, it shows a completely blue window instead. You can enable the
workaround by setting the WORKAROUND environment variable.
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