[Qt-interest] GLSL and two semi-transparent textures

[Josiah Bryan]

Josiah Bryan wrote:
> Just one problem - When I draw two textures, one over top of the other 
> (think a PiP), and the top (pip) has an alpha channel (say, the Qt cube 
> logo from the pbuffers examples), the qglClearColor shows thru in the 
> transparent areas of the top texture, NOT the bottom texture!
>
> Screen shot of the problem here:
> http://corp.productiveconcepts.com/dropbox/TextureSnapshot.jpg
Minor update as to what I've tried since sending the first email, still 
no luck:

- I've tried changing the clear color call from:
qglClearColor(Qt::blue);
to
glClearColor(0.0,0.0,1.0,0.0);
(Same result)

-I've tried EVERY combination of glBlendFunc calls from 
qt/src/opengl/qpaintengine_opengl.cpp:
QOpenGLPaintEngine::updateCompositionMode(QPainter::CompositionMode) - 
no luck at all getting it to combine correctly. No combination of 
glBlendFunc calls EVER gets the rear texture to show thru the 
transparent areas of the front texture.

- I've tried setting a different glBlendFunc for the front texture than 
the rear texture, grasping as straws such as here:
http://hacksoflife.blogspot.com/2010/02/alpha-blending-back-to-front-front-to.html

- I've tried setting a 0.75 opacity using a call like: 
glColor4f(opacity,opacity,opacity,opacity) - no change in transparency 
to either texture (likely due to the GLSL usage.) - This does work when 
not using GLSL, but I need the GLSL to do the YUV conversion on the GPU 
for formats that use YUV instead of RGB32 and friends.

- I've tried several different PNGs - thinking the original png image 
had a bad alpha channel or something. Nope. All work as expected.

- I've tried setting a hard coded alpha value (something like 0.5) in 
the GLSL fragment shader 'qt_glsl_argbShaderProgram' (like 63 approx in 
the original linked sources) - just blends with the clear color still, 
not with the rear texture.

Does any of this spark any ideas in anyones mind on how to actually 
blend the front and back textures properly while still using GLSL?

Is it because I'm using GLSL instead of ArbFp shaders? Just a random 
thought - qpaintengine_opengl.cpp seems to use ArbFp shaders intead of 
GLSL - just wondering. Can anybody at Trolltech comment on that with 
regards to this problem?

Thanks for your help.
Regards,
Josiah Bryan