Initial support for EON diffuse

libraries/bxdf/open_pbr_surface.mtlx

@@ -201,6 +201,7 @@
       <input name="color" type="color3" nodename="base_color_nonnegative" />
       <input name="roughness" type="float" interfacename="base_diffuse_roughness" />
       <input name="normal" type="vector3" interfacename="geometry_normal" />
+      <input name="energy_compensation" type="boolean" value="true" />
     </oren_nayar_diffuse_bsdf>
     <convert name="subsurface_selector" type="float">
       <input name="in" type="boolean" interfacename="geometry_thin_walled" />

libraries/pbrlib/genglsl/lib/mx_microfacet_diffuse.glsl

@@ -1,7 +1,7 @@
 #include "mx_microfacet.glsl"
 
-// Based on the implementation of Oren-Nayar diffuse in Open Shading Language.
-// https://github.com/AcademySoftwareFoundation/OpenShadingLanguage/blob/main/src/testrender/shading.cpp
+// Qualitative Oren-Nayar diffuse with improvements from Fujii:
+// https://mimosa-pudica.net/improved-oren-nayar.html
 float mx_oren_nayar_diffuse(float NdotV, float NdotL, float LdotV, float roughness)
 {
     float s = LdotV - NdotL * NdotV;
@@ -79,6 +79,33 @@ float mx_oren_nayar_diffuse_dir_albedo(float NdotV, float roughness)
     return clamp(dirAlbedo, 0.0, 1.0);
 }
 
+float mx_oren_nayar_diffuse_avg_albedo(float NdotV, float roughness)
+{
+    float sigma2 = mx_square(roughness);
+    float A = 1.0 - 0.5 * (sigma2 / (sigma2 + 0.33));
+    float B = 0.45 * sigma2 / (sigma2 + 0.09);
+    return A + (2.0 / 3.0 - 64.0 / (45.0 * M_PI)) * B;
+}
+
+// Energy compensation for Oren-Nayar diffuse from the OpenPBR project at openpbr.org.
+vec3 mx_oren_nayar_diffuse_energy_compensation(float NdotV, float NdotL, float roughness, vec3 color)
+{
+    // Compute directional and average albedos.
+    float dirAlbedoV = mx_oren_nayar_diffuse_dir_albedo(NdotV, roughness);
+    float dirAlbedoL = mx_oren_nayar_diffuse_dir_albedo(NdotL, roughness);
+    float avgAlbedo = mx_oren_nayar_diffuse_avg_albedo(NdotV, roughness);
+
+    // Compute the multi-scatter color term.
+    vec3 rhoMultiScatter = mx_square(color) * avgAlbedo /
+                           (vec3(1.0) - color * max(0.0, 1.0 - avgAlbedo));
+
+    // Return the final energy compensation.
+    return rhoMultiScatter *
+           max(M_FLOAT_EPS, 1.0 - dirAlbedoV) *
+           max(M_FLOAT_EPS, 1.0 - dirAlbedoL) /
+           max(M_FLOAT_EPS, 1.0 - avgAlbedo);
+}
+
 // https://media.disneyanimation.com/uploads/production/publication_asset/48/asset/s2012_pbs_disney_brdf_notes_v3.pdf
 // Section 5.3
 float mx_burley_diffuse(float NdotV, float NdotL, float LdotH, float roughness)

libraries/pbrlib/genglsl/mx_oren_nayar_diffuse_bsdf.glsl

@@ -1,6 +1,6 @@
 #include "lib/mx_microfacet_diffuse.glsl"
 
-void mx_oren_nayar_diffuse_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlusion, float weight, vec3 color, float roughness, vec3 normal, inout BSDF bsdf)
+void mx_oren_nayar_diffuse_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlusion, float weight, vec3 color, float roughness, vec3 normal, bool energy_compensation, inout BSDF bsdf)
 {
     bsdf.throughput = vec3(0.0);
 
@@ -15,14 +15,15 @@ void mx_oren_nayar_diffuse_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlusi
     float NdotL = clamp(dot(normal, L), M_FLOAT_EPS, 1.0);
     float LdotV = clamp(dot(L, V), M_FLOAT_EPS, 1.0);
 
-    bsdf.response = color * occlusion * weight * NdotL * M_PI_INV;
-    if (roughness > 0.0)
-    {
-        bsdf.response *= mx_oren_nayar_diffuse(NdotV, NdotL, LdotV, roughness);
-    }
+    vec3 compColor = energy_compensation ?
+                     mx_oren_nayar_diffuse_energy_compensation(NdotV, NdotL, roughness, color) :
+                     vec3(0.0);
+
+    bsdf.response = color * mx_oren_nayar_diffuse(NdotV, NdotL, LdotV, roughness) + compColor;
+    bsdf.response *= occlusion * weight * NdotL * M_PI_INV;
 }
 
-void mx_oren_nayar_diffuse_bsdf_indirect(vec3 V, float weight, vec3 color, float roughness, vec3 normal, inout BSDF bsdf)
+void mx_oren_nayar_diffuse_bsdf_indirect(vec3 V, float weight, vec3 color, float roughness, vec3 normal, bool energy_compensation, inout BSDF bsdf)
 {
     bsdf.throughput = vec3(0.0);
 
@@ -35,7 +36,11 @@ void mx_oren_nayar_diffuse_bsdf_indirect(vec3 V, float weight, vec3 color, float
 
     float NdotV = clamp(dot(normal, V), M_FLOAT_EPS, 1.0);
 
-    vec3 Li = mx_environment_irradiance(normal) *
-              mx_oren_nayar_diffuse_dir_albedo(NdotV, roughness);
-    bsdf.response = Li * color * weight;
+    vec3 compColor = energy_compensation ?
+                     mx_oren_nayar_diffuse_energy_compensation(NdotV, 1.0, roughness, color) :
+                     vec3(0.0);
+
+    vec3 Li = mx_environment_irradiance(normal);
+    bsdf.response = color * mx_oren_nayar_diffuse_dir_albedo(NdotV, roughness) + compColor;
+    bsdf.response *= Li * weight;
 }

libraries/pbrlib/pbrlib_defs.mtlx

@@ -28,6 +28,7 @@
     <input name="color" type="color3" value="0.18, 0.18, 0.18" />
     <input name="roughness" type="float" value="0.0" />
     <input name="normal" type="vector3" defaultgeomprop="Nworld" />
+    <input name="energy_compensation" type="boolean" value="false" uniform="true" />
     <output name="out" type="BSDF" />
   </nodedef>
 

resources/Materials/TestSuite/pbrlib/bsdf/oren_nayar_diffuse.mtlx

@@ -5,6 +5,7 @@
     <oren_nayar_diffuse_bsdf name="diffuse1" type="BSDF">
       <input name="color" type="color3" value="0.6, 0.6, 0.6" />
       <input name="roughness" type="float" value="0.0" />
+      <input name="energy_compensation" type="boolean" value="true" />
     </oren_nayar_diffuse_bsdf>
     <surface name="surface1" type="surfaceshader">
       <input name="bsdf" type="BSDF" nodename="diffuse1" />
@@ -14,6 +15,7 @@
     <oren_nayar_diffuse_bsdf name="diffuse2" type="BSDF">
       <input name="color" type="color3" value="0.6, 0.6, 0.6" />
       <input name="roughness" type="float" value="0.25" />
+      <input name="energy_compensation" type="boolean" value="true" />
     </oren_nayar_diffuse_bsdf>
     <surface name="surface2" type="surfaceshader">
       <input name="bsdf" type="BSDF" nodename="diffuse2" />
@@ -23,6 +25,7 @@
     <oren_nayar_diffuse_bsdf name="diffuse3" type="BSDF">
       <input name="color" type="color3" value="0.6, 0.6, 0.6" />
       <input name="roughness" type="float" value="0.5" />
+      <input name="energy_compensation" type="boolean" value="true" />
     </oren_nayar_diffuse_bsdf>
     <surface name="surface3" type="surfaceshader">
       <input name="bsdf" type="BSDF" nodename="diffuse3" />
@@ -32,6 +35,7 @@
     <oren_nayar_diffuse_bsdf name="diffuse4" type="BSDF">
       <input name="color" type="color3" value="0.6, 0.6, 0.6" />
       <input name="roughness" type="float" value="0.75" />
+      <input name="energy_compensation" type="boolean" value="true" />
     </oren_nayar_diffuse_bsdf>
     <surface name="surface4" type="surfaceshader">
       <input name="bsdf" type="BSDF" nodename="diffuse4" />
@@ -41,6 +45,7 @@
     <oren_nayar_diffuse_bsdf name="diffuse5" type="BSDF">
       <input name="color" type="color3" value="0.6, 0.6, 0.6" />
       <input name="roughness" type="float" value="1.0" />
+      <input name="energy_compensation" type="boolean" value="true" />
     </oren_nayar_diffuse_bsdf>
     <surface name="surface5" type="surfaceshader">
       <input name="bsdf" type="BSDF" nodename="diffuse5" />