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spalter

OpenGL Is this legal OpenGL?

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spalter    122
hi there, I have a multi-pass (GLSL) fragment shader which needs to access the result of the previous pass. For that, I render into a FBO render target, and use the resulting texture as input texture at the same time. This works fine, but I wonder whether it is legal OpenGL? If not, are there other (fast) ways to do this? Alpha-blending is not an option because of the fixed-pipeline limitations.. thanks martin

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taby    1265
I haven't found anything in the docs so far which state you can't use the same object to read and write. Just don't do any reading on the 2nd pass. :)

If it worked, you would obviously save half the RAM. Nice thought.

I'd love to see if it works. I can't see why not, since it appears that GLSL has no concept of what it's rendering to. I'm guessing that the shader would compile, without complaining that one of the samplers has the same base memory address as the render target.

If not, why not try a double-buffer?

How large could this texture possibly be? Say, 1920x1080x4B is 8294400B, or 15.8203125MB for both buffers. 63.28125MB if using fp16 values.

I would consider that affordable on a card that has 256MB+.


This doc uses a double-buffer system, because it is single-pass:
It is referred to by nvidia's Mark Harris as "RTT" in section "38.3 Implementation" of his paper "Fast Fluid Simulation on the GPU".

http://download.developer.nvidia.com/developer/SDK/Individual_Samples/DEMOS/OpenGL/src/gpgpu_fluid/docs/GPU_Gems_Fluids_Chapter.pdf

[Edited by - taby on April 24, 2006 10:55:45 AM]

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spalter    122
hmm..unfortunately i do need to read the texture in the 2nd pass. guess it's illegal then. the shader does indeed compile, and the rendering seems to be ok (although there seem to be some filtering artifacts, but that may also be a shader problem)...looks like i need to go with double buffering..

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ldb    138
If you are reading/writing to the same texture, you will get strange results. For my application (a seperable 2d convolution... seperable gaussian filter), it appeared like strange lines that could easily have been confused with a filter-bug.

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