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Wanted: Performance Tips for Reading Back Render Target Data

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A graphics algorithm I am using requires that I render scene depth information to a render target and then read it back to the CPU. That's all great, but it drops my framerate dramatically. In fact, my framerate goes from 2000+ to around 100. This is understandable since the CPU must flush the graphics commands, the GPU must respond to it (possibly having to render the scene), and then the CPU must wait for the texture data. My question is: What tricks can I use to speed this up? I've currently tried reducing the dimensions of the render target by half (currently 1024x768), and that boosts the framerate to about 300, but I haven't determined if my graphics algorithm will still work with a reduced render target and it's not an optimization I'm able to make right now. So ... experts ... any advice?

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Can't you do this through a shader? Without really leavning the shader, avoiding roundtripping between CPU and GPU or DirectX and Shaders...

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Quote:
Original post by leidegren
Can't you do this through a shader? Without really leavning the shader, avoiding roundtripping between CPU and GPU or DirectX and Shaders...
Unfortunately, no. The algorithm I am using requires depth values to generate new geometry.

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One thing that might speed it up a bit is to double buffer it - while the GPU is drawing to one buffer you read from the other one with the CPU.

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Quote:
Original post by think_different
2000+ to around 100


... careful, remember that FPS isn't linear ;).

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