rand() function in Compute Shader?

[Hoda Naghibijouybari]

Hello,

 

Is there any way to use rand() function or implement it in compute shader in WebGL?

Best,

Hoda

[Jan Scheurer]

Afaik the API is not responsible for the actual implementation of the rand functions but the GPU vendors are and none of them ever did it so you're most likely left to roll your own or use any of the existing open source GLSL pseudo random implementations.

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[Evgeny Demidov]

Simple (and old :) linear congruential generator with int32 data is used in 2D Ising model at  https://www.ibiblio.org/e-notes/Perc/ising.htm (WebGL2 is used for int32 textures). But there are better modern algorithms see https://en.wikipedia.org/wiki/Xorshift. E.g. Algorithm "xor128" from p. 5 of Marsaglia, "Xorshift RNGs" (see wiki). In JavaScript it is

let s = 0, N = 1000000, N1 = 1/N, m = 1/2147483647
let state = new Uint32Array([1,2,3,4])

    for (let y = 0; y < N; y++){
      let t = xorshift128()*m
      s += t
    }
    console.log(s/N)

function xorshift128(){
 let s, t = state[3];
 state[3] = state[2];
 state[2] = state[1];
 state[1] = s = state[0];
 t ^= t << 11;
 t ^= t >> 8;
 return state[0] = t ^ s ^ (s >> 19);
}

Evgeny

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