I have a rendering step which I would like to perform on a dynamically-generated texture.
The algorithm can operate on rows independently in parallel. For each row, the algorithm will visit each pixel in left-to-right order and modify it in situ (no distinct output buffer is needed, if that helps). Each pass uses state variables which must be reset at the beginning of each row and persist as we traverse the columns.
Can I set up OpenGL shaders, or OpenCL, or whatever, to do this? Please provide a minimal example with code.
Yes, you can do it. No, you don’t need 4.X hardware for that, you need fragment shaders (with flow control), framebuffer objects and floating point texture support.
You need to encode your data into 2D texture.
Store “state variable” in 1st pixel for each row, and encode the rest of the data into the rest of the pixels. It goes without saying that it is recommended to use floating point texture format.
Use two framebuffers, and render them onto each other in a loop using fragment shader that updates “state variable” at the first column, and performs whatever operation you need on another column, which is “current”. To reduce amount of wasted resources you can limit rendering to columns you want to process. NVidia OpenGL SDK examples had “game of life”, “GDGPU fluid”, “GPU partciles” demos that work in similar fashion – by encoding data into texture and then using shaders to update it.
However, because you can do it, it doesn’t mean you should do it and it doesn’t mean that it is guaranteed to be fast. Some GPUs might have a very high memory texture memory read speed, but relatively slow computation speed (and vice versa) and not all GPUs have many conveyors for processing things in parallel.
Also, depending on your app, CUDA or OpenCL might be more suitable.