[Bug 324854] Re: UXA: artifacts with compiz

[Peter Clifton]

The problem is that UXA's texture from pixmap doesn't do anything
special to override the alpha channel to unity for 24 bit colour depth
pixmaps on the X server. (The old TFP implementations did).

Having discussed this a bit with the Intel guys, it seems the consensus
is that the required information should be tracked in the FBConfig of
the GLXPixmap created to represent the X pixmap being mapped as a
texture.

Unfortunately, the Intel (mesa) driver doesn't currently expose any
24bit colour depth visuals / FBconfigs, so we end up mapping a 32 bit
depth texture, with undefined alpha channel state.

Since GL / Compiz is using textures with pre-multiplied alpha, if the
alpha channel is zero (for example), the arithmetic assumption is that
the R,G and B components have already been multiplied by this number -
giving a transparent window. The background contents are then summed in,
whcih will very likely max out the R, G and B channels - hence the white
images you're seeing rather than transaprency.

This effect is only seen whilst animations are attempting to adjust the
opacity of the window.. in other cases, Compiz switches off the blending
functions, and thus the alpha channel of the texture doesn't affect the
display.

-- 
UXA: artifacts with compiz
https://bugs.launchpad.net/bugs/324854
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