improve random behaviour; dropled size scaled by grid

shaders/random.glsl

@@ -0,0 +1,39 @@
+
+uint Hash(uint s);
+uint Hash(uvec2 s);
+uint Hash(uvec3 s);
+uint Hash(float s);
+uint HashCombine(uint seed, uint value);
+
+// Evolving Sub-Grid Turbulence for Smoke Animation
+// H. Schechter and R. Bridson
+uint Hash(uint s) {
+  s ^= 2747636419u;
+  s *= 2654435769u;
+  s ^= s >> 16u;
+  s *= 2654435769u;
+  s ^= s >> 16u;
+  s *= 2654435769u;
+  return s;
+}
+
+uint Hash(uvec2 s) {
+  return HashCombine(Hash(s.x), s.y);
+}
+
+uint Hash(uvec3 s) {
+  return HashCombine(Hash(s.xy), s.z);
+}
+
+uint HashCombine(uint seed, uint value) {
+  return seed ^ (Hash(value) + 0x9e3779b9u + (seed<<6u) + (seed>>2u));
+}
+
+uint Rand(inout uint seed) {
+  seed = Hash(seed);
+  return seed;
+}
+
+float RandF(inout uint seed) {
+  return float(Rand(seed)) * 2.3283064365386963e-10;
+}