From fd5d500009bc47d4f5b07c7d0cb2b88fdb337aa5 Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Thu, 23 Oct 2025 21:45:35 -0700
Subject: Do not divide by PI

---
 shaders/cook_torrance.frag | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/shaders/cook_torrance.frag b/shaders/cook_torrance.frag
index 1975491..1a4faf8 100644
--- a/shaders/cook_torrance.frag
+++ b/shaders/cook_torrance.frag
@@ -121,7 +121,11 @@ vec3 cook_torrance(
   vec3  F = fresnel_schlick(F0, HdotV);
   float G = geometry_smith(roughness, NdotL, NdotV);
   vec3 Kd = mix(vec3(1.0) - F, vec3(0.0), metallic);
-  vec3 diffuse = Kd*albedo*INV_PI;
+  // 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/
+  // vec3 diffuse = Kd * albedo * INV_PI;
+  vec3 diffuse = Kd * albedo * INV_PI;
   // Take a max to prevent division by 0 when either dot product is 0.
   vec3 specular = (D*F*G) / max(4.0 * NdotV * NdotL, 0.0001);
   return diffuse + specular;
@@ -135,7 +139,11 @@ vec3 cook_torrance_IBL(
   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 diffuse = Kd * albedo * INV_PI * irradiance;
+  // 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/
+  // vec3 diffuse = Kd * albedo * INV_PI * irradiance;
+  vec3 diffuse = Kd * albedo * irradiance;
   vec3 specular = prefiltered_env * (F * BRDF_env.x + BRDF_env.y);
   return occlusion * (diffuse + specular);
 }
-- 
cgit v1.2.3