GLSL Diffraction Works!

Finally, the GLSL implementation of the diffraction project is working. Here is a sample image rendered using OpenGL:


The speedup seen is enormous. Lets see how far we have come in terms of efficiency:

Implementation	Number of Rays	Time in seconds
Matlab	2^24	700
CUDA Scatter	2^32	30
CUDA Gather	2^32	10
CUDA Gather+randomize	2^32	30
GLSL gather	2^32	1.5

The reason for such a speedup is mostly because of the number of texture lookups that are performed in the shader. The CUDA application does this using global memory, and that is a lot slower than texture lookups (because textures use the texture caches present in hardware).

An interesting issue that pops up here is that each shader kernel should complete in a very short duration of time (around 5 to 10 seconds). I have not found any reference to this online, but I have seen this messing with the results for 2^36 rays. It seems to be similar to the CUDA limit of around 5 to 10 seconds per kernel execution. One solution is to come up with shorter kernels, and another option is to run the application on a GPU that is not connected to an X-server. Since running GLSL on such a (offline) GPU is out of question, I guess I am back to smaller kernels, or implementing the whole thing in CUDA using texture memory.

Note: For people who want to try out large kernels in GLSL/CUDA, a warning: running such kernels will make your system unresponsive for the entire duration, and may as well cause display corruption. I have lost count of the system restarts I have had to do in such cases... You will know that your kernel is taking too much time, if you get garbled results like this one here: