From 67d2e2836fb3d7aee4ddd3cea273a9f85f82725a Mon Sep 17 00:00:00 2001
From: Jonathan Stone <jstone@lucasfilm.com>
Date: Thu, 8 Aug 2024 13:46:56 -0700
Subject: [PATCH] Fix Fresnel computation in GLSL transmission (#1962)

This changelist fixes an error in the Fresnel computation for GLSL transmission, improving the visual parity of transmissive materials across shading languages.

With this fix in place, the graph definition of LamaGeneralizedSchlick no longer requires a manual inversion of its F0 and F90 inputs for transmission, as this inversion is handled correctly in GLSL as well as OSL and MDL.
---
 .../bxdf/lama/lama_generalized_schlick.mtlx   | 12 +----
 .../genglsl/lib/mx_environment_fis.glsl       |  4 +-
 .../genglsl/mx_generalized_schlick_bsdf.glsl  |  2 +-
 .../pbrlib/bsdf/generalized_schlick.mtlx      | 54 +++++++------------
 4 files changed, 24 insertions(+), 48 deletions(-)

diff --git a/libraries/bxdf/lama/lama_generalized_schlick.mtlx b/libraries/bxdf/lama/lama_generalized_schlick.mtlx
index fd7c2ade5d..887273de45 100644
--- a/libraries/bxdf/lama/lama_generalized_schlick.mtlx
+++ b/libraries/bxdf/lama/lama_generalized_schlick.mtlx
@@ -63,14 +63,6 @@
       <input name="in2" type="color3" nodename="ior_multiply" />
       <input name="which" type="integer" interfacename="fresnelMode" />
     </switch>
-    <subtract name="transmission_color0" type="color3">
-      <input name="in1" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="in2" type="color3" nodename="fresnel_mode_switch" />
-    </subtract>
-    <subtract name="transmission_color90" type="color3">
-      <input name="in1" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="in2" type="color3" interfacename="reflectivity90" />
-    </subtract>
 
     <!-- Exponent -->
     <divide name="exponent" type="float">
@@ -132,8 +124,8 @@
     <!-- Transmission -->
     <generalized_schlick_bsdf name="transmission_bsdf" type="BSDF">
       <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" nodename="transmission_color0" />
-      <input name="color90" type="color3" nodename="transmission_color90" />
+      <input name="color0" type="color3" nodename="fresnel_mode_switch" />
+      <input name="color90" type="color3" interfacename="reflectivity90" />
       <input name="exponent" type="float" nodename="exponent" />
       <input name="roughness" type="vector2" nodename="roughness_anisotropic_squared_clamped" />
       <input name="normal" type="vector3" interfacename="normal" />
diff --git a/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl b/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl
index 0b28f3645f..1d61576650 100644
--- a/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl
+++ b/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl
@@ -43,8 +43,8 @@ vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distributio
         // Compute the geometric term.
         float G = mx_ggx_smith_G2(NdotL, NdotV, avgAlpha);
 
-        // Compute the combined FG term, which is inverted for refraction.
-        vec3 FG = fd.refraction ? vec3(1.0) - (F * G) : F * G;
+        // Compute the combined FG term, which simplifies to inverted Fresnel for refraction.
+        vec3 FG = fd.refraction ? vec3(1.0) - F : F * G;
 
         // Add the radiance contribution of this sample.
         // From https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf
diff --git a/libraries/pbrlib/genglsl/mx_generalized_schlick_bsdf.glsl b/libraries/pbrlib/genglsl/mx_generalized_schlick_bsdf.glsl
index 764412b90d..f4598dd0d0 100644
--- a/libraries/pbrlib/genglsl/mx_generalized_schlick_bsdf.glsl
+++ b/libraries/pbrlib/genglsl/mx_generalized_schlick_bsdf.glsl
@@ -66,7 +66,7 @@ void mx_generalized_schlick_bsdf_transmission(vec3 V, float weight, vec3 color0,
     {
         float avgF0 = dot(safeColor0, vec3(1.0 / 3.0));
         fd.ior = vec3(mx_f0_to_ior(avgF0));
-        bsdf.response = mx_surface_transmission(N, V, X, safeAlpha, distribution, fd, safeColor0) * weight;
+        bsdf.response = mx_surface_transmission(N, V, X, safeAlpha, distribution, fd, vec3(1.0)) * weight;
     }
 }
 
diff --git a/resources/Materials/TestSuite/pbrlib/bsdf/generalized_schlick.mtlx b/resources/Materials/TestSuite/pbrlib/bsdf/generalized_schlick.mtlx
index 05eb071558..bc31308a47 100644
--- a/resources/Materials/TestSuite/pbrlib/bsdf/generalized_schlick.mtlx
+++ b/resources/Materials/TestSuite/pbrlib/bsdf/generalized_schlick.mtlx
@@ -2,11 +2,8 @@
 <materialx version="1.39">
   <nodegraph name="schlick_bsdf">
     <generalized_schlick_bsdf name="schlick_R" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="color82" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="exponent" type="float" value="5.0" />
+      <input name="color0" type="color3" value="0.8, 0.8, 0.8" />
+      <input name="roughness" type="vector2" value="0.02, 0.02" />
       <input name="scatter_mode" type="string" value="R" />
     </generalized_schlick_bsdf>
     <surface name="surface_R" type="surfaceshader">
@@ -15,11 +12,8 @@
     <output name="R_out" type="surfaceshader" nodename="surface_R" />
 
     <generalized_schlick_bsdf name="schlick_T" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="color82" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="exponent" type="float" value="5.0" />
+      <input name="color0" type="color3" value="0.2, 0.2, 0.2" />
+      <input name="roughness" type="vector2" value="0.02, 0.02" />
       <input name="scatter_mode" type="string" value="T" />
     </generalized_schlick_bsdf>
     <surface name="surface_T" type="surfaceshader">
@@ -28,11 +22,8 @@
     <output name="T_out" type="surfaceshader" nodename="surface_T" />
 
     <generalized_schlick_bsdf name="schlick_RT" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="color82" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="exponent" type="float" value="5.0" />
+      <input name="color0" type="color3" value="0.2, 0.2, 0.2" />
+      <input name="roughness" type="vector2" value="0.02, 0.02" />
       <input name="scatter_mode" type="string" value="RT" />
     </generalized_schlick_bsdf>
     <surface name="surface_RT" type="surfaceshader">
@@ -50,11 +41,9 @@
     <output name="layer_RT_out" type="surfaceshader" nodename="surface_layer_RT" />
 
     <generalized_schlick_bsdf name="schlick_R2" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.5, 0.0, 0.0" />
-      <input name="color82" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color90" type="color3" value="0.0, 1.0, 0.0" />
-      <input name="exponent" type="float" value="5.0" />
+      <input name="color0" type="color3" value="0.8, 0.2, 0.2" />
+      <input name="color90" type="color3" value="0.2, 0.8, 0.2" />
+      <input name="roughness" type="vector2" value="0.02, 0.02" />
       <input name="scatter_mode" type="string" value="R" />
     </generalized_schlick_bsdf>
     <surface name="surface_R2" type="surfaceshader">
@@ -63,11 +52,9 @@
     <output name="R2_out" type="surfaceshader" nodename="surface_R2" />
 
     <generalized_schlick_bsdf name="schlick_T2" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.5, 0.0, 0.0" />
-      <input name="color82" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color90" type="color3" value="0.0, 1.0, 0.0" />
-      <input name="exponent" type="float" value="5.0" />
+      <input name="color0" type="color3" value="0.8, 0.2, 0.2" />
+      <input name="color90" type="color3" value="0.2, 0.8, 0.2" />
+      <input name="roughness" type="vector2" value="0.02, 0.02" />
       <input name="scatter_mode" type="string" value="T" />
     </generalized_schlick_bsdf>
     <surface name="surface_T2" type="surfaceshader">
@@ -76,11 +63,9 @@
     <output name="T2_out" type="surfaceshader" nodename="surface_T2" />
 
     <generalized_schlick_bsdf name="schlick_RT2" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.5, 0.0, 0.0" />
-      <input name="color82" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color90" type="color3" value="0.0, 1.0, 0.0" />
-      <input name="exponent" type="float" value="5.0" />
+      <input name="color0" type="color3" value="0.8, 0.2, 0.2" />
+      <input name="color90" type="color3" value="0.2, 0.8, 0.2" />
+      <input name="roughness" type="vector2" value="0.02, 0.02" />
       <input name="scatter_mode" type="string" value="RT" />
     </generalized_schlick_bsdf>
     <surface name="surface_RT2" type="surfaceshader">
@@ -98,11 +83,10 @@
     <output name="layer_RT2_out" type="surfaceshader" nodename="surface_layer_RT2" />
 
     <generalized_schlick_bsdf name="schlick_R3" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color82" type="color3" value="0.0, 0.0, 1.0" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="exponent" type="float" value="5.0" />
+      <input name="color0" type="color3" value="0.8, 0.8, 0.8" />
+      <input name="color82" type="color3" value="0.2, 0.2, 0.8" />
+      <input name="color90" type="color3" value="0.8, 0.8, 0.8" />
+      <input name="roughness" type="vector2" value="0.02, 0.02" />
       <input name="scatter_mode" type="string" value="R" />
     </generalized_schlick_bsdf>
     <surface name="surface_R3" type="surfaceshader">