[HDRP][Path Tracing] Added alpha channel to path traced results

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

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed some render texture leaks.
 - Fixed regression in Wizard that not fix runtime ressource anymore (case 1287627)
 - Fixed error in Depth Of Field near radius blur calculation (case 1306228).
+- Fixed path tracing alpha channel support (case 1304187).
 
 ### Changed
 - Change the source value for the ray tracing frame index iterator from m_FrameCount to the camera frame count (case 1301356).

com.unity.render-pipelines.high-definition/Documentation~/Alpha-Output.md

@@ -14,6 +14,8 @@ Rendering Buffer Format | Post-processing Buffer Format | Alpha Output
 **R16G16B16A16** | **R11G11B10** | Alpha channel without post-processing (AlphaCopy)
 **R16G16B16A16** | **R16G16B16A16** | Alpha channel with post-processing
 
+Note that alpha output is also supported in [Path Tracing](Ray-Tracing-Path-Tracing.md).
+
 ## DoF and Alpha Output
 Another case which might require post-processing of the alpha channel is for scenes that use Depth Of Field. In this case, if the alpha is not processed, compositing will result in a sharp cut-off of an object that should appear blurred. This is better is illustrated in the images below:
 

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/Accumulation/Shaders/Accumulation.compute

@@ -51,13 +51,15 @@ void KMain(uint3 dispatchThreadId : SV_DispatchThreadID)
     UNITY_XR_ASSIGN_VIEW_INDEX(dispatchThreadId.z);
     uint2 currentPixelCoord = dispatchThreadId.xy;
 
-    float exposureMultiplier = (_AccumulationNeedsExposure != 0) ? GetCurrentExposureMultiplier() : 1.0;
+    float4 exposureMultiplier;
+    exposureMultiplier.xyz = (_AccumulationNeedsExposure != 0) ? GetCurrentExposureMultiplier() : 1.0;
+    exposureMultiplier.w = 1.0;
 
     // Have we reached max sampling?
     uint sampleCount = _AccumulationFrameIndex;
     if (sampleCount >= _AccumulationNumSamples)
     {
-        _CameraColorTextureRW[COORD_TEXTURE2D_X(currentPixelCoord)] = float4(_AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)].xyz * exposureMultiplier, 1.0);
+        _CameraColorTextureRW[COORD_TEXTURE2D_X(currentPixelCoord)] = _AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)] * exposureMultiplier;
     }
     else
     {
@@ -68,16 +70,16 @@ void KMain(uint3 dispatchThreadId : SV_DispatchThreadID)
 #endif
 
         if (sampleCount++)
-            color.xyz = (_AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)].xyz * _AccumulationWeights.y + _AccumulationWeights.x * color.xyz) * _AccumulationWeights.z;
+            color = (_AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)] * _AccumulationWeights.y + _AccumulationWeights.x * color) * _AccumulationWeights.z;
 
-        _AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)] = float4(color.xyz, 1.0);
+        _AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)] = color;
 
         // Apply exposure modifier
         color *= exposureMultiplier;
 
         // Add a little convergence cue to our result
         AddConvergenceCue(currentPixelCoord, sampleCount, color.xyz);
 
-        _CameraColorTextureRW[COORD_TEXTURE2D_X(currentPixelCoord)] = float4(color.xyz, 1.0);
+        _CameraColorTextureRW[COORD_TEXTURE2D_X(currentPixelCoord)] = color;
     }
 }

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/PathTracing/Shaders/PathTracingIntersection.hlsl

@@ -6,12 +6,15 @@
 // Structure that defines the current state of the intersection, for path tracing
 struct PathIntersection
 {
+    // t as in: O + t*D = H (i.e. distance between O and H, if D is normalized)
     float t;
     // Resulting value (often color) of the ray
     float3 value;
+    // Resulting alpha (camera rays only)
+    float alpha;
     // Cone representation of the ray
     RayCone cone;
-    // The remaining available depth for the current Ray
+    // The remaining available depth for the current ray
     uint remainingDepth;
     // Pixel coordinate from which the initial ray was launched
     uint2 pixelCoord;

com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/PathTracing/Shaders/PathTracingMain.raytrace

@@ -25,7 +25,7 @@
 // Input(s)
 float4x4 _PixelCoordToViewDirWS;
 int      _RaytracingCameraSkyEnabled;
-float3   _RaytracingCameraClearColor;
+float4   _RaytracingCameraClearColor;
 
 // DoF related parameters
 float4   _PathTracedDoFConstants; // x: aperture radius, y: focus distance, zw: unused
@@ -36,9 +36,9 @@ RWTexture2D<float4> _RadianceTexture;
 [shader("miss")]
 void MissCamera(inout PathIntersection pathIntersection : SV_RayPayload)
 {
-    // If the _RaytracingCameraClearColor content is positive, we override the camera sky display with a bg color
-    pathIntersection.value = _EnvLightSkyEnabled && _RaytracingCameraSkyEnabled ?
-        SampleSkyTexture(WorldRayDirection(), 0.0, 0).xyz : _RaytracingCameraClearColor * GetInverseCurrentExposureMultiplier();
+    bool skyEnabled = _EnvLightSkyEnabled && _RaytracingCameraSkyEnabled;
+    pathIntersection.value = skyEnabled ? SampleSkyTexture(WorldRayDirection(), 0.0, 0).xyz : _RaytracingCameraClearColor.xyz * GetInverseCurrentExposureMultiplier();
+    pathIntersection.alpha = skyEnabled ? 1.0 : _RaytracingCameraClearColor.w;
 
     ApplyFogAttenuation(WorldRayOrigin(), WorldRayDirection(), pathIntersection.value);
 
@@ -137,6 +137,7 @@ void RayGen()
     // Create and init the PathIntersection structure for this
     PathIntersection pathIntersection;
     pathIntersection.value = 1.0;
+    pathIntersection.alpha = 1.0;
     pathIntersection.remainingDepth = _RaytracingMaxRecursion;
     pathIntersection.pixelCoord = currentPixelCoord;
     pathIntersection.maxRoughness = 0.0;
@@ -148,7 +149,7 @@ void RayGen()
     // Evaluate the ray intersection
     TraceRay(_RaytracingAccelerationStructure, RAY_FLAG_CULL_BACK_FACING_TRIANGLES, RAYTRACINGRENDERERFLAG_PATH_TRACING, 0, 1, 0, rayDescriptor, pathIntersection);
 
-    _RadianceTexture[currentPixelCoord] = float4(pathIntersection.value, 1.0);
+    _RadianceTexture[currentPixelCoord] = float4(pathIntersection.value, pathIntersection.alpha);
 }
 
 // This should never be called, return magenta just in case