Glitter: tweak hash; looks less artifacty in game

1 files changed, 28 insertions, 10 deletions

diff --git a/glitter.cginc b/glitter.cginc
index a9bbf8b..baf6200 100644
--- a/glitter.cginc
+++ b/glitter.cginc
@@ -13,7 +13,9 @@
  * I have made changes to this code. They are:
  *   1. Syntax changes required to translate GLSL to HLSL.
  *   2. Stylistic preferences, like using "1" or "1.0" instead of "1.".
- *   3. The `GetGlitterLighting` function, which populates data required for
+ *   3. Replaced the original glitter RNG with a stronger integer-domain
+ *      permuted congruential generator (PCG) hash.
+ *   4. The `GetGlitterLighting` function, which populates data required for
  *      indirect glitter. The original paper only discusses analytic lighting.
  *
  * @article{KPT:2025:Glinty,
@@ -96,7 +98,7 @@ float normal(float2x2 cov, float2 x) {
     / (sqrt(determinant(cov)) * 2.0 * PI);
 }
 
-uint2 shuffle(uint2 v) {
+uint2 pcg2d(uint2 v) {
   v = v * 1664525u + 1013904223u;
   v.x += v.y * 1664525u;
   v.y += v.x * 1664525u;
@@ -109,17 +111,33 @@ uint2 shuffle(uint2 v) {
   return v;
 }
 
-float2 rand(uint2 v) {
-  return float2(shuffle(v)) * UINT_TO_UNIT;
+uint4 pcg4d(uint4 v) {
+  v = v * 1664525u + 1013904223u;
+  v.x += v.y * v.w;
+  v.y += v.z * v.x;
+  v.z += v.x * v.y;
+  v.w += v.y * v.z;
+
+  v = v ^ (v >> 16u);
+
+  v.x += v.y * v.w;
+  v.y += v.z * v.x;
+  v.z += v.x * v.y;
+  v.w += v.y * v.z;
+  return v;
+}
+
+float2 rand(uint4 v) {
+  return float2(pcg4d(v).xy) * UINT_TO_UNIT;
 }
 
 float2 Rand2D(float2 x, float2 y, float l, uint i) {
-  uint2 ux = asuint(x);
-  uint2 uy = asuint(y);
-  uint  ul = asuint(l);
-  // This is broken, but looks cool.
-  //return hash22_fast(asfloat((ux>>16|ux<<16) ^ uy ^ ul ^ (i*0x124u)));
-  return rand((ux>>16|ux<<16) ^ uy ^ ul ^ (i*0x124u));
+  // `x` and `y` are cell-center coordinates (n + 0.5). Hash the integer cell IDs
+  // directly so nearby cells do not alias through IEEE-754 bit patterns.
+  uint4 seed = uint4(uint2(x), uint2(y));
+  uint2 salt = pcg2d(uint2(asuint(l), i));
+  seed ^= uint4(salt.x, salt.y, salt.y ^ 0x9e3779b9u, salt.x ^ 0x85ebca6bu);
+  return rand(seed);
 }
 
 float Rand1D(float2 x, float2 y, float l, uint i) {