mattdesl · June 20, 2025 09:51 · Jul 1, 2024 · Jul 1, 2024
diff --git a/random.wgsl b/random.wgsl
@@ -0,0 +1,47 @@
+// each pixel is assigned 4 random u32 values per frame
+@group(0) @binding(1)
+var<storage, read> rnd_state: array<u32>;
+
+// the current state within this pixel
+var<private> local_rnd_state:vec4u;
+
+fn random_u32(state:ptr<private,vec4u>) -> u32 {
+  var st:vec4u = *state;
+  /* Algorithm "xor128" from p. 5 of Marsaglia, "Xorshift RNGs" */
+  // Load the state from the storage buffer
+  var t: u32 = st.w;
+  var s: u32 = st.x;
+  t ^= t << 11;
+  t ^= t >> 8;
+  var x:u32 = t ^ s ^ (s >> 19);
+  *state = vec4u(
+    x, s, st.y, st.z
+  );
+  return x;
+}
+
+fn random() -> f32 {
+  return f32(random_u32(&local_rnd_state)) / 0x100000000;
+}
+
+fn my_main () {
+  var idx:u32 = /* index of this pixel into your rnd_state buffer */;
+
+  // each pixel is assigned its own set of 4 random u32 for xorshift
+  // then this is stored in a local private space, so that each time
+  // random() is called, state will be shifted forward with xorshift
+  local_rnd_state = vec4u(
+    rnd_state[idx * 4 + 0],
+    rnd_state[idx * 4 + 1],
+    rnd_state[idx * 4 + 2],
+    rnd_state[idx * 4 + 3]
+  );
+
+  // now you are ready to call random() as many times as you like!
+  out_color = vec4(
+    random(),
+    random(),
+    random(),
+    1.0
+  );
+}