How expensive is fetching a texture in GLSL?

Question

I've heard fetching a texture is quite expensive operation. But how much? like ten-times of arithmetic multiplication.

For instance, there are 3D-Look-up table technique to process an image which requires 3d texture fetch once:

http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter24.html

Even if the conversion can be achieved with only some matrix and vector product in shader, can I expect that 3D-LUT is still useful in terms of performance?

3D-LUT vs matrix/vector product is just an example. What I want to know is general way to estimate the overhead by texture fetch before measuring the exact running time. Or, is there something like 'cheat-sheet' for GLSL overhead?

Answer 1

It always depends. One concern with texture fetches is memory bandwidth and - quite related to that - cache efficiency. The caching is optimized for the case where neighboring fragments access neighboring texels. I did some benchmarks, and in such a scenario, I did not even notice any difference between sampling with nearest or sampling with bilinear filter and directly using texelFetch.

In another scenario where I used a 3D texture as color lookup, the cache efficiency became a major concern. The image content actually influences the performance significantly. My benchmark scenario was post-processing a 1920x1080 frame, and with a quite cache-friendly content (screenshot of ms office), I measured about 0.35ms for the operation, while with an image containing random noise, processing time went up to ~4ms (with bilinear filtering) or ~2ms (nearest/texelFetch).

Of course, this depends very much on your specific scenario and the hardware, so the only advice I can give is: benchmark/profile it.