From f25912c87291bd84a9d1a430c3c35b3604d4bd56 Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Fri, 24 Oct 2025 14:49:06 -0700
Subject: Add references

---
 shaders/cook_torrance.frag | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/shaders/cook_torrance.frag b/shaders/cook_torrance.frag
index 1a4faf8..74a89e2 100644
--- a/shaders/cook_torrance.frag
+++ b/shaders/cook_torrance.frag
@@ -1,3 +1,6 @@
+/*
+[1] BRDF reference: https://graphicrants.blogspot.com/2013/08/specular-brdf-reference.html
+*/
 precision highp float;
 
 uniform vec4  BaseColorFactor;
@@ -86,8 +89,10 @@ vec3 get_ws_normal(vec3 normalWs, vec3 normalMapSample) {
 }
 #endif // HAS_TANGENTS || HAS_TEXCOORDS
 
+// Normal distribution function (NDF).
+// Eq (4) in reference [1].
 float trowbridge_reitz_GGX(float roughness, float NdotH) {
-  float a = roughness * roughness;
+  float a = roughness * roughness; // "alpha roughness" in the reference.
   float a2 = a * a;
   float d = NdotH * NdotH * (a2 - 1.0) + 1.0;
   return a2 / (PI * d * d);
@@ -97,6 +102,8 @@ float geometry_schlick_GGX(float k, float NdotV) {
   return NdotV / (NdotV * (1.0 - k) + k);
 }
 
+// Geometry function.
+// See "Smith" under "Geometric Shadowing" in reference [1].
 float geometry_smith(float roughness, float NdotL, float NdotV) {
   float k = roughness * roughness / 2.0; // IBL
   return geometry_schlick_GGX(k, NdotV) * geometry_schlick_GGX(k, NdotL);
@@ -138,7 +145,7 @@ vec3 cook_torrance_IBL(
     vec3 irradiance, vec3 prefiltered_env, vec2 BRDF_env) {
   vec3 F0 = mix(vec3(0.04), albedo, metallic);
   vec3  F = fresnel_schlick_roughness(F0, NdotV, roughness);
-  vec3 Kd = mix(vec3(1.0) - F, vec3(0.0), metallic);
+  vec3 Kd = (vec3(1.0) - F) * (1.0 - metallic);
   // A non-HDR environment map essentially has the 1/pi baked in as it does not
   // use physical units. See:
   // https://seblagarde.wordpress.com/2012/01/08/pi-or-not-to-pi-in-game-lighting-equation/
-- 
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