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Rendering to the alpha channel

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Hello everyone. I have a somewhat complex problem. I am working on an application which displays an texture on a quad (stretched to full screen 800x600 ortho mode) and I want to programmatic generate the alpha channel. So I did a trick, I rendered my alpha image by drawing lots of blured blobs, next I used glCopyTexImage2D to "grab" the image, i cleared the scene and using some blending tricks did the rest. All was well until I tried running this on an ATI card, OUCH, oh so very slow. I did a little reading and discovered that glCopyTexImage2D works ok with nVidia but very slow with ATI. Now I have to find some solution that works with ATI, preferably without having to rewrite everything. Ideas ? Thanks ! Shachar.

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Which ATi/NVIDIA cards and how much of a performance difference are you seeing?

You could try using 'glCopyTexSubImage2D' but I doubt it'll improve performance in your case.

Alternatively you can use P-Buffers or Frame Buffer Objects to directly render to a texture (avoids copying the texture back to system memory).

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