Question

I need some quick advice.

I would like to simulate a cellular automata (from A Simple, Efficient Method
for Realistic Animation of Clouds) on the GPU. However, I am limited to OpenGL ES 2.0 shaders (in WebGL) which does not support any bitwise operations.

Since every cell in this cellular automata represents a boolean value, storing 1 bit per cell would have been the ideal. So what is the most efficient way of representing this data in OpenGL’s texture formats? Are there any tricks or should I just stick with a straight-forward RGBA texture?

EDIT: Here’s my thoughts so far…

At the moment I’m thinking of going with either plain GL_RGBA8, GL_RGBA4 or GL_RGB5_A1:

• Possibly I could pick GL_RGBA8, and try to extract the original bits using floating point ops. E.g. x*255.0 gives an approximate integer value. However, extracting the individual bits is a bit of a pain (i.e. dividing by 2 and rounding a couple times). Also I’m wary of precision problems.

• If I pick GL_RGBA4, I could store 1.0 or 0.0 per component, but then I could probably also try the same trick as before with GL_RGBA8. In this case, it’s only x*15.0. Not sure if it would be faster or not seeing as there should be fewer ops to extract the bits but less information per texture read.

• Using GL_RGB5_A1 I could try and see if I can pack my cells together with some additional information like a color per voxel where the alpha channel stores the 1 bit cell state.

Answer 1

Create a second texture and use it as a lookup table. In each 256×256 block of the texture you can represent one boolean operation where the inputs are represented by the row/column and the output is the texture value. Actually in each RGBA texture you can represent four boolean operations per 256×256 region. Beware texture compression and MIP maps, though!