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com.unity.render-pipelines.high-definition/HDRP/Lighting/LightEvaluation.hlsl

 // Note: When doing transmission we always have only one shadow sample to do: Either front or back. We use NdotL to know on which side we are
 void EvaluateLight_Directional(LightLoopContext lightLoopContext, PositionInputs posInput,
                                DirectionalLightData lightData, BakeLightingData bakeLightingData,
-                               float3 N, float3 L, bool isThinModeTransmission,
+                               float3 N, float3 L,
                                out float3 color, out float attenuation)
 {
     float3 positionWS = posInput.positionWS;
     shadow = shadowMask = (lightData.shadowMaskSelector.x >= 0.0) ? dot(bakeLightingData.bakeShadowMask, lightData.shadowMaskSelector) : 1.0;
 #endif
 
-    float NdotL = dot(N, L);
-
-    UNITY_BRANCH if (lightData.shadowIndex >= 0 && (isThinModeTransmission || NdotL >= 0.0))
+    UNITY_BRANCH if (lightData.shadowIndex >= 0 && (dot(N, L) >= 0.0))
-        shadow = GetDirectionalShadowAttenuation(lightLoopContext.shadowContext, positionWS, NdotL > 0.0 ? N : -N, lightData.shadowIndex, L, posInput.positionSS);
+        shadow = GetDirectionalShadowAttenuation(lightLoopContext.shadowContext, positionWS, N, lightData.shadowIndex, L, posInput.positionSS);
 #endif
 
 #ifdef SHADOWS_SHADOWMASK
 
         // Volumetric don't use screenspace shadow. Transparent neither (as we don't have the depth information)
 #if (SHADERPASS != SHADERPASS_VOLUMETRIC_LIGHTING && !defined(_SURFACE_TYPE_TRANSPARENT))
-        // Don't apply shadow contact on transmismission
-        shadow = isThinModeTransmission ? shadow : min(shadow, GetContactShadow(lightLoopContext, lightData.contactShadowIndex));
+        shadow = min(shadow, GetContactShadow(lightLoopContext, lightData.contactShadowIndex));
 #endif
     }
 
 // Note: When doing transmission we always have only one shadow sample to do: Either front or back. We use NdotL to know on which side we are
 void EvaluateLight_Punctual(LightLoopContext lightLoopContext, PositionInputs posInput,
                             LightData lightData, BakeLightingData bakeLightingData,
-                            float3 N, float3 L, float3 lightToSample, float4 distances, bool isThinModeTransmission,
+                            float3 N, float3 L, float3 lightToSample, float4 distances,
                             out float3 color, out float attenuation)
 {
     float3 positionWS    = posInput.positionWS;
     shadow = shadowMask = (lightData.shadowMaskSelector.x >= 0.0) ? dot(bakeLightingData.bakeShadowMask, lightData.shadowMaskSelector) : 1.0;
 #endif
 
-    float NdotL = dot(N, L);
-
-    // We test NdotL > 0.0 to not sample the shadow map if it is not required.
-    // In case of thin mode we always need to perform the fetch as we reuse it for back and front lighting
-    UNITY_BRANCH if (lightData.shadowIndex >= 0 && (isThinModeTransmission || NdotL >= 0.0))
+    // We test NdotL >= 0.0 to not sample the shadow map if it is not required.
+    // Note that volumetric use N of 0 so it still work
+    UNITY_BRANCH if (lightData.shadowIndex >= 0 && (dot(N, L) >= 0.0))
-        shadow = GetPunctualShadowAttenuation(lightLoopContext.shadowContext, positionWS, NdotL > 0.0 ? N : -N, lightData.shadowIndex, L, distances.x, posInput.positionSS);
+        shadow = GetPunctualShadowAttenuation(lightLoopContext.shadowContext, positionWS, N, lightData.shadowIndex, L, distances.x, posInput.positionSS);
 
 #ifdef SHADOWS_SHADOWMASK
         // Note: Legacy Unity have two shadow mask mode. ShadowMask (ShadowMask contain static objects shadow and ShadowMap contain only dynamic objects shadow, final result is the minimun of both value)
 
         // Volumetric don't use screenspace shadow. Transparent neither (as we don't have the depth information)
 #if (SHADERPASS != SHADERPASS_VOLUMETRIC_LIGHTING && !defined(_SURFACE_TYPE_TRANSPARENT))
-        // Don't apply shadow contact on transmismission
-        shadow = isThinModeTransmission ? shadow : min(shadow, GetContactShadow(lightLoopContext, lightData.contactShadowIndex));
+        shadow = min(shadow, GetContactShadow(lightLoopContext, lightData.contactShadowIndex));
 #endif
     }
 

com.unity.render-pipelines.high-definition/HDRP/Material/Lit/Lit.hlsl

 
     float3 color;
     float attenuation;
-    float attenuationNoContactShadows;
-    EvaluateLight_Directional(lightLoopContext, posInput, lightData, bakeLightingData, N, L, HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS), color, attenuation);
+
+    // When using thin transmission mode we don't fetch shadow map for back face, we reuse front face shadow
+    // However we flip the normal for the bias (and the NdotL test) and disable contact shadow
+    if (HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS) && NdotL < 0)
+    {
+        //  Disable shadow contact in case of transmission and backface shadow
+        N = -N;
+        lightData.contactShadowIndex = -1; // This is only modify for the scope of this function
+    }
+
+    EvaluateLight_Directional(lightLoopContext, posInput, lightData, bakeLightingData, N, L, color, attenuation);
 
     float intensity = max(0, attenuation * NdotL); // Warning: attenuation can be greater than 1 due to the inverse square attenuation (when position is close to light)
 
 
     float3 color;
     float attenuation;
-    float attenuationNoContactShadows;
+
+    // When using thin transmission mode we don't fetch shadow map for back face, we reuse front face shadow
+    // However we flip the normal for the bias (and the NdotL test) and disable contact shadow
+    if (HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS) && NdotL < 0)
+    {
+        //  Disable shadow contact in case of transmission and backface shadow
+        N = -N;
+        lightData.contactShadowIndex = -1; // This is only modify for the scope of this function
+    }
-                           lightToSample, distances, HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS),
-                            color, attenuation);
+                           lightToSample, distances, color, attenuation);
 
     float intensity = max(0, attenuation * NdotL); // Warning: attenuation can be greater than 1 due to the inverse square attenuation (when position is close to light)
 

com.unity.render-pipelines.high-definition/HDRP/Material/StackLit/StackLit.hlsl

     // color and attenuation are outputted  by EvaluateLight:
     float3 color;
     float attenuation;
-    float attenuationNoContactShadows;
+
+    // When using thin transmission mode we don't fetch shadow map for back face, we reuse front face shadow
+    // However we flip the normal for the bias (and the NdotL test) and disable contact shadow
+    if (HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS) && NdotL < 0)
+    {
+        //  Disable shadow contact in case of transmission and backface shadow
+        N = -N;
+        lightData.contactShadowIndex = -1; // This is only modify for the scope of this function
+    }
 
     // For shadow attenuation (ie receiver bias), always use the geometric normal:
     EvaluateLight_Directional(lightLoopContext, posInput, lightData, bakeLightingData, bsdfData.geomNormalWS, L, HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS), color, attenuation);
 
     float3 color;
     float attenuation;
-    float attenuationNoContactShadows;
+
+    // When using thin transmission mode we don't fetch shadow map for back face, we reuse front face shadow
+    // However we flip the normal for the bias (and the NdotL test) and disable contact shadow
+    if (HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS) && NdotL < 0)
+    {
+        //  Disable shadow contact in case of transmission and backface shadow
+        N = -N;
+        lightData.contactShadowIndex = -1; // This is only modify for the scope of this function
+    }
-                           lightToSample, distances, HasFeatureFlag(bsdfData.materialFeatures, MATERIAL_FEATURE_FLAGS_TRANSMISSION_MODE_THIN_THICKNESS),
-                            color, attenuation);
+                           lightToSample, distances, color, attenuation);
 
 
     float intensity = max(0, attenuation); // Warning: attenuation can be greater than 1 due to the inverse square attenuation (when position is close to light)
         // Note: we do not modify the distance to the light, or the light angle for the back face.
         // This is a performance-saving optimization which makes sense as long as the thickness is small.
         // We use diffuse lighting for accumulation since it is going to be blurred during the SSS pass.
-        lighting.diffuse += EvaluateTransmission(bsdfData, transmittance, NdotL, NdotV, LdotV, attenuationNoContactShadows * lightData.diffuseScale);
+        lighting.diffuse += EvaluateTransmission(bsdfData, transmittance, NdotL, NdotV, LdotV, attenuation * lightData.diffuseScale);
     }
 
     // Save ALU by applying light and cookie colors only once.