Variable array index not possible in webgl shaders?

Question

As the title says, I can't do vector_array[foo] (assuming foo is in-range and integer) in webgl vertex shaders, correct?

Are textures the best alternative, or is there a workaround or some better way to mimick a lookup table?

Answer 1

http://www.khronos.org/registry/webgl/specs/latest/#DYNAMIC_INDEXING_OF_ARRAYS "WebGL only allows dynamic indexing with constant expressions, loop indices or a combination. The only exception is for uniform access in vertex shaders, which can be indexed using any expression."

Did you try it? If it didn't work, there are a couple options.

If you have a small number of values, if-else could work ok. AFAIK the uniform values are going to be loaded into registers anyhow, so doing a dozen cycles of math on them isn't going to make your shader much slower.

For a large number of values, textures are your best bet.

Answer 2

I haven't tested it, but I don't get any compilation error from the following

//index as a float
attribute lowp float vColorIndex;
//the array
uniform vec4 Colors[16];

//type cast the float in an int
int index = int(vColorIndex);
//use index
vec4 col = Colors[index];