More clanker adjustments to glitter

1 files changed, 37 insertions, 8 deletions

diff --git a/glitter.cginc b/glitter.cginc
index 512be96..5475ec7 100644
--- a/glitter.cginc
+++ b/glitter.cginc
@@ -35,6 +35,12 @@
 // Remaps [0, UINT_MAX] to [0, 1]
 #define UINT_TO_UNIT (1.0 / 4294967296.0)
 
+#define GLITTER_AMOUNT_MAX 64.0
+#define GLITTER_REFERENCE_AMOUNT 0.5
+#define GLITTER_REFERENCE_N 8.0e6
+#define GLITTER_POPULATION_SCALE \
+  (GLITTER_AMOUNT_MAX / GLITTER_REFERENCE_AMOUNT)
+
 // Lambert azimuthal equal area projection
 float2 lambert(float3 v) {
   return v.xy / sqrt(1 + v.z);
@@ -210,7 +216,7 @@ float3 disk_to_ndf_ggx(float2 v_disk, float alpha) {
 
 // Algorithm 1 from Kemppinen et. al.
 float D_Kemppinen(float3 h, float alpha, float glint_alpha, int angular_cells,
-    float2 uv, float2x2 uv_J, float N, float filter_size,
+    float2 uv, float2x2 uv_J, float N, float amount, float filter_size,
     out float3 micro_normal) {
   float res = sqrt(N);
   float2 x_s = uv;
@@ -219,6 +225,21 @@ float D_Kemppinen(float3 h, float alpha, float glint_alpha, int angular_cells,
   float d = x_a_and_d.z;
   int angular_sample_count = clamp(angular_cells, 1, 4);
 
+  // The paper normalizes both Gaussian kernels and the point population, so
+  // narrower kernels and smaller populations have taller individual peaks.
+  // That is desirable for an energy-preserving NDF, but makes roughness and
+  // density alter the apparent size of a glint.
+  // Preserve the peaks at the original defaults instead: roughness controls
+  // angular width, filter_size controls spatial width, and amount controls only
+  // how many flakes are active.
+  float angular_peak_scale = pow(glint_alpha / 0.01, 2.0);
+  float spatial_peak_scale = pow(filter_size / 0.7, 2.0);
+  // Scaling the maximum population and each point's weight together keeps the
+  // reference amount's population, peak, and average energy unchanged.
+  float profile_scale = GLITTER_POPULATION_SCALE
+    * angular_peak_scale * spatial_peak_scale;
+  float amount_fraction = saturate(amount / GLITTER_AMOUNT_MAX);
+
   // Both the spatial and angular neighborhoods require at least a 2x2 grid.
   float max_lod = floor(log2(res)) - 1.0;
   float lambda = clamp(QueryLod(res * uv_J, filter_size), 1.0,
@@ -293,9 +314,12 @@ float D_Kemppinen(float3 h, float alpha, float glint_alpha, int angular_cells,
 
         float r = Rand1D(i_s_neighbor, i_a_neighbor, l, 4u);
         float roulette = smoothstep(max(.0, r-.1), min(1.0, r+.1), w_lambda);
+        float amount_r = Rand1D(i_s_neighbor, i_a_neighbor, l, 8u);
+        float active = smoothstep(max(0.0, amount_r - 0.02),
+            min(1.0, amount_r + 0.02), amount_fraction);
 
-        float w = roulette * normal(sigma_a, x_a - g_a)
-          * normal(sigma_s, x_s - g_s) / N;
+        float w = active * roulette * normal(sigma_a, x_a - g_a)
+          * normal(sigma_s, x_s - g_s) * profile_scale / N;
         D_filter += w;
         if (w > best_weight) {
           best_weight = w;
@@ -303,8 +327,9 @@ float D_Kemppinen(float3 h, float alpha, float glint_alpha, int angular_cells,
         }
       }
     }
-    D_filter += w_lambda * compensation(x_a, sigma_a, res_a, i_a,
-        angular_step, angular_sample_count);
+    D_filter += amount_fraction * w_lambda * profile_scale
+      * compensation(x_a, sigma_a, res_a, i_a, angular_step,
+          angular_sample_count);
   }
 
   micro_normal = normalize(disk_to_ndf_ggx(best_g_a, alpha));
@@ -327,13 +352,16 @@ LightGlitter GetGlitterLighting(
     float3 normal, float3 V, float3 direct_H, float3 indirect_dir) {
   LightGlitter g;
   float2x2 uv_J = uv_ellipsoid(transpose(float2x2(ddx(uv), ddy(uv))));
-  float N = 8.0e5f * pow(10.0f, glitter_amount * 6.0f - 2.0f);
+  // Keep the procedural population fixed. `glitter_amount` independently
+  // controls the fraction of that population which is active in D_Kemppinen.
+  float N = GLITTER_REFERENCE_N * GLITTER_POPULATION_SCALE;
 
   // Direct
   float3 direct_H_tangent = mul(direct_H, transpose(tbn));
   float3 direct_micro_normal;  // unused
   g.direct_D = D_Kemppinen(direct_H_tangent, roughness, glitter_roughness,
-      glitter_angular_cells, uv, uv_J, N, glitter_filter_size,
+      glitter_angular_cells, uv, uv_J, N, glitter_amount,
+      glitter_filter_size,
       direct_micro_normal);
 
   // Indirect
@@ -341,7 +369,8 @@ LightGlitter GetGlitterLighting(
   float3 indirect_H_tangent = mul(indirect_H, transpose(tbn));
   float3 indirect_micro_normal;  // unused, but required by D_Kemppinen
   g.indirect_D = D_Kemppinen(indirect_H_tangent, roughness, glitter_roughness,
-      glitter_angular_cells, uv, uv_J, N, glitter_filter_size,
+      glitter_angular_cells, uv, uv_J, N, glitter_amount,
+      glitter_filter_size,
       indirect_micro_normal);
   g.indirect_NoL = max(1e-4, dot(normal, indirect_dir));
   g.indirect_LoH = max(1e-4, dot(indirect_dir, indirect_H));