Metallic materials should have an F0 equal to their base color in Usd…

…PreviewSurface. Fixes #1174 (Internal change: 2090921)
PixarAnimationStudios · Aug 17, 2020 · f11ab36 · f11ab36
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commit f11ab36
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diff --git a/pxr/usdImaging/plugin/usdShaders/shaders/previewSurface.glslfx b/pxr/usdImaging/plugin/usdShaders/shaders/previewSurface.glslfx
@@ -214,7 +214,7 @@ evaluateLight(
         if (!useSpecularWorkflow) {
             float R = (1.0 - ior) / (1.0 + ior);
             vec3 specColor = mix(vec3(1.0), diffuseColor, metallic);
-            F0  = R * R * specColor;
+            F0 = mix(R * R * specColor, specColor, metallic);
             F90 = specColor;
 
             // For metallic workflows, pure metals have no diffuse
@@ -297,7 +297,7 @@ evaluateIndirectLighting(
     if (!useSpecularWorkflow) {
         float R = (1.0 - ior) / (1.0 + ior);
         vec3 specColor = mix(vec3(1.0), diffuseColor, metallic);
-        F0  = R * R * specColor;
+        F0 = mix(R * R * specColor, specColor, metallic);
         F90 = specColor;
 
         // For metallic workflows, pure metals have no diffuse

diff --git a/pxr/usdImaging/plugin/usdShaders/shaders/shaderDefs.usda b/pxr/usdImaging/plugin/usdShaders/shaders/shaderDefs.usda
@@ -54,11 +54,11 @@ def Shader "UsdPreviewSurface" (
     float inputs:metallic = 0.0 (
         doc = """Used only in the metalness workflow.
             1 for metallic surfaces and 0 for non-metallic.
-            - If metallic is 1, then F0 (reflectivity at 0 degree incidence)
-            will be derived from ior ( (1-ior)/(1+ior) )^2, then multiplied by Albedo;
-            while edge F90 reflectivity will simply be the Albedo.
-            (As an option, you can put ior to 0 such that F0 comes equal to F90 and thus the Albedo).
-            - If metallic is 0, then Albedo is ignored; F0 is derived from ior and F90 is white.
+            - If metallic is 1, then both F0 (reflectivity at 0 degree 
+            incidence) and edge F90 reflectivity will simply be the Albedo.
+            - If metallic is 0, then Albedo is ignored in the calculation of F0
+            and F90; F0 is derived from ior via ( (1-ior)/(1+ior) )^2 and F90 is
+            white. 
             In between, we interpolate."""
     )
 

diff --git a/third_party/renderman-23/shaders/UsdPreviewSurfaceParameters.osl b/third_party/renderman-23/shaders/UsdPreviewSurfaceParameters.osl
@@ -103,7 +103,7 @@ shader UsdPreviewSurfaceParameters
         float metal = clamp(metallic, 0.0, 1.0); 
         color spec = mix(color(1.0,1.0,1.0), diffuseColor, metal);
         float r = (1.0 - ior) / (1.0 + ior);
-        specularFaceColorOut = r * r * spec;
+        specularFaceColorOut = mix(r * r * spec, spec, metal);
         specularEdgeColorOut = spec;
 
         diffuseGainOut *= 1.0 - metal;