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I am doing some gpgpu calculations with GL and want to read my results from the framebuffer. My framebuffer-texture is logically an 1D array, but I made it 2D to have a bigger area. Now I want to read from any arbitrary pixel in the framebuffer-texture with any given length.

That means all calculations are already done on GPU side and I only need to pass certain data to the cpu that could be aligned over the border of the texture.

Is this possible? If yes is it slower/faster than glReadPixels on the whole image and then cutting out what I need?

EDIT Of course I know about OpenCL/CUDA but they are not desired because I want my program to run out of the box on (almost) any platform.

Also I know that glReadPixels is very slow and one reason might be that it offers some functionality that I do not need (Operating in 2D). Therefore I asked for a more basic function that might be faster.

Nobody moving away from SE
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    "Now I want to read from any arbitrary pixel in the framebuffer-texture with any given length." -- what does that mean? Read from within a shader, or from within your application? One can be done with relatively simple math, or with a very fast buffer copy to a buffer texture (copy buffer api), assuming that's supported. The other can only be done with glReadPixels, which you will not get around (it _may_ be called differently in CUDA or OpenCL, but it's still more or less glReadPixels) – Damon Aug 03 '11 at 20:13

2 Answers2

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Reading the whole framebuffer with glReadPixels just to discard it all except for a few pixels/lines would be grossly inefficient. But glReadPixels lets you specify a rect within the framebuffer, so why not just restrict it to fetching the few rows of interest ? So you maybe end up fetching some extra data at the start and end of the first and last lines fetched, but I suspect the overhead of that is minimal compared with making multiple calls.

Possibly writing your data to the framebuffer in tiles and/or using Morton order might help structure it so a tighter bounding box can be be found and the extra data retrieved minimised.

timday
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You can use a pixel buffer object (PBO) to transfer pixel data from the framebuffer to the PBO, then use glMapBufferARB to read the data directly:

http://www.songho.ca/opengl/gl_pbo.html

MSN
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