ssfd bugfixes

1 files changed, 12 insertions, 8 deletions

diff --git a/ssfd.cginc b/ssfd.cginc
index c3e1c89..df45d81 100644
--- a/ssfd.cginc
+++ b/ssfd.cginc
@@ -32,19 +32,24 @@ float ssfd(float2 uv, float scale, float max_fwidth, float2 uv_offset, texture3D
   // Factor is 16.
   // log_2(factor) is 4.
   // Divide original by 16.
-  float fw_factor = uv_fw / max_fwidth;
-  float fractal_level = log2(fw_factor) / log2(bayer_res);
+  float fw_factor = max(uv_fw / max_fwidth, 1e-6);
+  // Fractal transitions need to happen in octaves to match the Bayer
+  // self-similarity used by the reference implementation.
+  float fractal_level = log2(fw_factor);
   float fractal_level_floor = floor(fractal_level);
   float fractal_remainder = fractal_level - fractal_level_floor;
 
-  uv *= pow(bayer_res, -fractal_level_floor);
-  uv += uv_offset * pow(bayer_res, -fractal_level_floor);
+  float uv_scale = exp2(-fractal_level_floor);
+  uv *= uv_scale;
+  uv += uv_offset * uv_scale;
 
   float n_layers = depth;
-  float not_used_lo = 1/(n_layers*2);
-  float not_used_hi = 1 - not_used_lo;
+  float min_sub_layer = max(1.0, 0.25 * n_layers);
+  float max_sub_layer = n_layers;
 
-  float uvw = (not_used_hi - not_used_lo) * (1 - fractal_remainder) + not_used_lo;
+  // Only the top 3/4 of the layer stack is fractally self-similar.
+  float sub_layer = lerp(min_sub_layer, max_sub_layer, 1 - fractal_remainder);
+  float uvw = (sub_layer - 0.5) / n_layers;
 
   float3 uv_3d = float3(uv, uvw);
 
@@ -68,4 +73,3 @@ void apply_ssfd(v2f i, float2 uv, LightData l, float3 normal, inout float3 albed
 }
 
 #endif  // __SSFD_INC
-