[HDRP][Path Tracing] Fixed issues with path-traced volumetrics (light anim dirtiness, truncated volume shafts) #2771

Author: eturquin

com.unity.render-pipelines.core/ShaderLibrary/GeometricTools.hlsl

@@ -29,15 +29,16 @@ float3x3 RotationFromAxisAngle(float3 A, float sinAngle, float cosAngle)
 
 // Solves the quadratic equation of the form: a*t^2 + b*t + c = 0.
 // Returns 'false' if there are no real roots, 'true' otherwise.
-// Numerically stable.
-// Ref: Numerical Recipes in C++ (3rd Edition)
+// Ensures that roots.x <= roots.y.
 bool SolveQuadraticEquation(float a, float b, float c, out float2 roots)
 {
-    float d = b * b - 4 * a * c;
-    float q = -0.5 * (b + CopySign(sqrt(d), b));
-    roots   = float2(c / q, q / a);
+    float det = Sq(b) - 4.0 * a * c;
 
-    return (d >= 0);
+    float sqrtDet = sqrt(det);
+    roots.x = (-b - sign(a) * sqrtDet) / (2.0 * a);
+    roots.y = (-b + sign(a) * sqrtDet) / (2.0 * a);
+
+    return (det >= 0.0);
 }
 
 //-----------------------------------------------------------------------------
@@ -159,8 +160,8 @@ bool IntersectRayCone(float3 rayOrigin,  float3 rayDirection,
     // Check whether we have at least 1 root.
     bool hit = SolveQuadraticEquation(a, 2 * b, c, roots);
 
-    tEntr = min(roots.x, roots.y);
-    tExit = max(roots.x, roots.y);
+    tEntr = roots.x;
+    tExit = roots.y;
     float3 pEntr = o + tEntr * d;
     float3 pExit = o + tExit * d;
 

com.unity.render-pipelines.high-definition/CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 - Fixed issue where some ShaderGraph generated shaders were not SRP compatible because of UnityPerMaterial cbuffer layout mismatches (case 1292501)
 - Fixed computation of geometric normal in path tracing (case 1293029).
+- Fixed issues with path-traced volumetric scattering (cases 1295222, 1295234).
 
 ### Changed
 - Now reflection probes cannot have SSAO, SSGI, SSR, ray tracing effects or volumetric reprojection.

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/PathTracing/PathTracing.cs

@@ -198,6 +198,13 @@ private void CheckDirtiness(HDCamera hdCamera)
                 return;
             }
 
+            // Check light or geometry transforms dirtiness
+            if (m_TransformDirty)
+            {
+                m_TransformDirty = false;
+                ResetPathTracing();
+            }
+
             // Check lights dirtiness
             if (m_CacheLightCount != m_RayTracingLights.lightCount)
             {
@@ -208,10 +215,9 @@ private void CheckDirtiness(HDCamera hdCamera)
 
             // Check geometry dirtiness
             ulong accelSize = m_CurrentRAS.GetSize();
-            if (accelSize != m_CacheAccelSize || m_TransformDirty)
+            if (accelSize != m_CacheAccelSize)
             {
                 m_CacheAccelSize = accelSize;
-                m_TransformDirty = false;
                 ResetPathTracing();
             }
         }

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/Raytracing/HDRaytracingManager.cs

@@ -341,8 +341,10 @@ AccelerationStructureStatus AddInstanceToRAS(Renderer currentRenderer,
             if (instanceFlag == 0) return AccelerationStructureStatus.Added;
 
             // Add it to the acceleration structure
-            m_TransformDirty |= currentRenderer.transform.hasChanged;
             m_CurrentRAS.AddInstance(currentRenderer, subMeshMask: subMeshFlagArray, subMeshTransparencyFlags: subMeshCutoffArray, enableTriangleCulling: singleSided, mask: instanceFlag);
+
+            // Indicates that a transform has changed in our scene (mesh or light)
+            m_TransformDirty |= currentRenderer.transform.hasChanged;
             currentRenderer.transform.hasChanged = false;
 
             // return the status
@@ -387,6 +389,10 @@ internal void BuildRayTracingAccelerationStructure(HDCamera hdCamera)
                     m_RayTracedShadowsRequired |= (hdLight.useRayTracedShadows && screenSpaceShadowsSupported);
                     m_RayTracedContactShadowsRequired |= (hdLight.useContactShadow.@override && hdLight.rayTraceContactShadow);
 
+                    // Indicates that a transform has changed in our scene (mesh or light)
+                    m_TransformDirty |= hdLight.transform.hasChanged;
+                    hdLight.transform.hasChanged = false;
+
                     switch (hdLight.type)
                     {
                         case HDLightType.Directional: