[HDRP] Fix Transmission not being affected by shadow from Volumetric Clouds.

While developping arctic demo, we noticed that the transmission in our icy moutains was still glowing eventhough they were in the volumetric clouds shadows, this was a huge discrepancy in lighting which is now eaisly fixed by this PR thanks to @adrien-tocqueville snippet.

This wasn't the case for cloud layer because it's using the cookie of the directionnal light. 


![c5784cf2073607debfbe678b852b052e](https://media.github.cds.internal.unity3d.com/user/1764/files/7819d363-b832-4a2c-856b-e9d0d4599b8e)

Fix hard earth shadow
![image](https://media.github.cds.internal.unity3d.com/user/2154/files/5adb6416-6dff-4ad4-9afc-bd154048475d)
![image](https://media.github.cds.internal.unity3d.com/user/2154/files/64573645-fe94-48e1-bfa2-de8aa97af159)

Author: remi-chapelain

Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/LightLoop/LightLoop.hlsl

@@ -232,12 +232,6 @@ void LightLoop( float3 V, PositionInputs posInput, PreLightData preLightData, BS
                     context.shadowValue = GetDirectionalShadowAttenuation(context.shadowContext,
                                                                           posInput.positionSS, posInput.positionWS, GetNormalForShadowBias(bsdfData),
                                                                           light.shadowIndex, L);
-
-                    #ifndef LIGHT_EVALUATION_NO_CLOUDS_SHADOWS
-                    // Apply the volumetric cloud shadow if relevant
-                    if (_VolumetricCloudsShadowOriginToggle.w == 1.0)
-                        context.shadowValue *= EvaluateVolumetricCloudsShadows(light, posInput.positionWS);
-                    #endif
                 }
             }
         }

Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/SurfaceShading.hlsl

@@ -82,12 +82,24 @@ DirectLighting ShadeSurface_Directional(LightLoopContext lightLoopContext,
         float4 lightColor = EvaluateLight_Directional(lightLoopContext, posInput, light);
         lightColor.rgb *= lightColor.a; // Composite
 
+        float cloudShadow = 1.0f;
+#ifndef LIGHT_EVALUATION_NO_CLOUDS_SHADOWS
+        // Apply the volumetric cloud shadow if relevant
+        // We evaluate them here instead of inside EvaluateLight_Directional to be able to apply it on objects
+        // with transmission and still benefit from shadow dimmer and colored shadows
+        if (light.shadowIndex >= 0 && _VolumetricCloudsShadowOriginToggle.w == 1.0)
+        {
+            cloudShadow = EvaluateVolumetricCloudsShadows(light, posInput.positionWS);
+            lightLoopContext.shadowValue *= cloudShadow;
+        }
+#endif
+
 #ifdef MATERIAL_INCLUDE_TRANSMISSION
         if (ShouldEvaluateThickObjectTransmission(V, L, preLightData, bsdfData, 0))
         {
             // Transmission through thick objects does not support shadowing
             // from directional lights. It will use the 'baked' transmittance value.
-            lightColor *= _DirectionalTransmissionMultiplier;
+            lightColor *= _DirectionalTransmissionMultiplier * cloudShadow;
         }
         else
 #endif

Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/VolumetricClouds/HDRenderPipeline.VolumetricCloudsShadows.cs

@@ -45,15 +45,11 @@ void ReleaseVolumetricCloudsShadows()
 
         bool HasVolumetricCloudsShadows(HDCamera hdCamera, in VolumetricClouds settings)
         {
-            return (HasVolumetricClouds(hdCamera, in settings)
-                && GetMainLight() != null
-                && settings.shadows.value);
-        }
-
-        bool HasVolumetricCloudsShadows(HDCamera hdCamera)
-        {
-            VolumetricClouds settings = hdCamera.volumeStack.GetComponent<VolumetricClouds>();
-            return HasVolumetricCloudsShadows(hdCamera, settings);
+            return HasVolumetricClouds(hdCamera, settings) &&
+                settings.shadows.value &&
+                m_CurrentSunLight != null &&
+                m_CurrentSunLight.shadows != LightShadows.None &&
+                m_CurrentSunLightAdditionalLightData.shadowDimmer != 0.0f;
         }
 
         void EvaluateShadowRegionData(HDCamera hdCamera, CommandBuffer cmd)
@@ -63,15 +59,15 @@ void EvaluateShadowRegionData(HDCamera hdCamera, CommandBuffer cmd)
             VolumetricClouds settings = hdCamera.volumeStack.GetComponent<VolumetricClouds>();
 
             // Grab the light and make sure it is valid
-            Light targetLight = GetMainLight();
-            if (!HasVolumetricClouds(hdCamera, in settings) || targetLight == null || !settings.shadows.value)
+            if (!HasVolumetricCloudsShadows(hdCamera, in settings))
             {
                 // Bind the invalid volumetric clouds shadow texture
                 cmd.SetGlobalTexture(HDShaderIDs._VolumetricCloudsShadowsTexture, Texture2D.blackTexture);
                 return;
             }
 
             // Grab the volume profile of the volumetric clouds
+            Light targetLight = GetMainLight();
             Matrix4x4 wsToLSMat = targetLight.transform.worldToLocalMatrix;
             Matrix4x4 lsToWSMat = targetLight.transform.localToWorldMatrix;
 

Packages/com.unity.render-pipelines.high-definition/Runtime/Lighting/VolumetricClouds/VolumetricCloudsUtilities.hlsl

@@ -153,8 +153,8 @@ EnvironmentLighting EvaluateEnvironmentLighting(CloudRay ray, float3 entryEvalua
 
     #ifdef PHYSICALLY_BASED_SUN
     // evaluate the attenuation at both points (entrance and exit of the cloud layer)
-    lighting.sunColor0 *= EvaluateSunColorAttenuation(entryEvaluationPointPS, lighting.sunDirection);
-    lighting.sunColor1 *= EvaluateSunColorAttenuation(exitEvaluationPointPS, lighting.sunDirection);
+    lighting.sunColor0 *= EvaluateSunColorAttenuation(entryEvaluationPointPS, lighting.sunDirection, true);
+    lighting.sunColor1 *= EvaluateSunColorAttenuation(exitEvaluationPointPS, lighting.sunDirection, false);
     #endif
 
     // Evaluate cos of the theta angle between the view and light vectors
@@ -405,7 +405,7 @@ float3 EvaluateSunLuminance(float3 positionWS, float3 sunDirection, float3 sunCo
     float3 luminance = float3(0.0, 0.0, 0.0);
 
     // If we early out, this means we've hit the earth itself
-    if (ExitCloudVolume(ConvertToPS (positionWS), sunDirection, _HighestCloudAltitude, totalLightDistance))
+    if (ExitCloudVolume(ConvertToPS(positionWS), sunDirection, _HighestCloudAltitude, totalLightDistance))
     {
         // Because of the very limited numebr of light steps and the potential humongous distance to cover, we decide to potnetially cover less and make it more useful
         totalLightDistance = clamp(totalLightDistance, 0, _NumLightSteps * LIGHT_STEP_MAXIMAL_SIZE);

Packages/com.unity.render-pipelines.high-definition/Runtime/Sky/CloudSystem/CloudLayer/CloudLayerCommon.hlsl

@@ -117,10 +117,11 @@ float4 GetCloudLayerColor(float3 dir, int index)
         cloud = SampleCloudMap(dir, index);
 
     float3 lightColor = _SunLightColor(index);
-    float3 ambient = SampleSH9(_AmbientProbeBuffer, float3(0, -1, 0)) * _AmbientDimmer(index);
+    float3 ambient = max(SampleSH9(_AmbientProbeBuffer, float3(0, -1, 0)), 0) * _AmbientDimmer(index);
 
     #ifdef PHYSICALLY_BASED_SUN
-    lightColor *= EvaluateSunColorAttenuation(position + float3(0,_PlanetaryRadius,0), _SunDirection);
+    float3 positionPS = position + float3(0,_PlanetaryRadius,0);
+    lightColor *= EvaluateSunColorAttenuation(positionPS, _SunDirection, true);
     #endif
 
     return float4(cloud.x * lightColor + ambient * cloud.y, cloud.y) * _Opacity;

Packages/com.unity.render-pipelines.high-definition/Runtime/Sky/PhysicallyBasedSky/PhysicallyBasedSkyCommon.hlsl

@@ -494,7 +494,7 @@ float3 EvaluateSunColorAttenuation(float cosTheta, float r)
 }
 
 // This function evaluates the sun color attenuation from the physically based sky
-float3 EvaluateSunColorAttenuation(float3 positionPS, float3 sunDirection)
+float3 EvaluateSunColorAttenuation(float3 positionPS, float3 sunDirection, bool estimatePenumbra = false)
 {
     float r        = length(positionPS);
     float cosTheta = dot(positionPS, sunDirection) * rcp(r); // Normalize
@@ -507,7 +507,9 @@ float3 EvaluateSunColorAttenuation(float3 positionPS, float3 sunDirection)
     {
         float3 oDepth = ComputeAtmosphericOpticalDepth(r, cosTheta, true);
         float3 opacity = 1 - TransmittanceFromOpticalDepth(oDepth);
-        return 1 - (Desaturate(opacity, _AlphaSaturation) * _AlphaMultiplier);
+        float penumbra = saturate((cosTheta - cosHoriz) / 0.0019f); // very scientific value
+        float3 attenuation = 1 - (Desaturate(opacity, _AlphaSaturation) * _AlphaMultiplier);
+        return estimatePenumbra ? attenuation * penumbra : attenuation;
     }
     else
     {