OpenGL Gaussian Kernel on 3D texture

Question

I would like to perform a blur on a 3D texture in openGL. Since it is separable I should be able to do it in 3 passes. My question is, what would be the best way to cope with it?

I currently have the 3D texture and fill it using imageStore. Should I create other 2 copies of the texture for the blur or is there a way to do it while using a single texture?

I am already using glCompute to compute the mip map of the 3D texture, but in this case I read from the texture at level 0 and write to the one at the next level so there is no conflict, while in this case I would need some copy.

Answer 1

In short it can't be done in 3 passes, because is not a 2D image. Even if kernel is separable. You have to blur each image slice separately, wich is 2 passes for image (if you are using a 256x256x256 texture then you have 512 passes just for blurring along U and V coordinates). The you still have to blur along T and U (or T and V: indifferent) coordinates wich is another 512 passes. You can gain performance by using bilinear filter and read values between texels to save some constant processing cost. The 3D blur will be very costly.

Performance tip: maybe you don't need to blur the whole texture but only a part of it? (the visible part?)

The problem wich a such high number of passes , is the number of interactions between GPU and CPU: drawcalls and FBO setup wich are both slow operations that hang the CPU (probably a different API with low CPU overhead would be faster)

Try to not separate the kernel:

If you have a small kernel (I guess up to 5^3, only profiling will show the max kernel size) probably the fastest way is to NOT separate the kernel (that's it, you save a lot of drawcalls and FBO binding and leverage everything to GPU fillrate and bandwith).

Spread work over time:

Does not matter if your kernel is separated or not. Instead of computing a Gaussian Blur every frame, you could just compute it every second (maybe with a bigger kernel). Then you use as source of "continuos blurring data" the interpolation of the previouse blur and the next blur (wich is a 2x 3D Texture samples each frame, wich is much cheaper than continuosly blurring).