[HDRP][Path Tracing] Fixed auto-exposure mistmatch between sky background and rendered objects (#6525)

* Fixed sky auto-exposure mismatch.

* Updated changelog.

* Fixed camera clear color.

Co-authored-by: sebastienlagarde <[email protected]>

Author: eturquin

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

@@ -62,6 +62,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed FOV change when enabling physical camera.
 - Fixed spot light shadows near plane
 - Fixed unsupported material properties show when rendering pass is Low Resolution.
+- Fixed auto-exposure mismatch between sky background and scene objects in path tracing (case 1385131).
 
 ## [13.1.2] - 2021-11-05
 

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

@@ -383,9 +383,15 @@ void RenderSky(RenderGraph renderGraph, HDCamera hdCamera, TextureHandle skyBuff
                 passData.skyManager = m_SkyManager;
 
                 builder.SetRenderFunc(
-                    (RenderSkyPassData data, RenderGraphContext context) =>
+                    (RenderSkyPassData data, RenderGraphContext ctx) =>
                     {
-                        data.skyManager.RenderSky(data.hdCamera, data.sunLight, data.colorBuffer, data.depthTexture, data.debugDisplaySettings, context.cmd);
+                        // Override the exposure texture, as we need a neutral value for this render
+                        ctx.cmd.SetGlobalTexture(HDShaderIDs._ExposureTexture, m_EmptyExposureTexture);
+
+                        data.skyManager.RenderSky(data.hdCamera, data.sunLight, data.colorBuffer, data.depthTexture, data.debugDisplaySettings, ctx.cmd);
+
+                        // Restore the regular exposure texture
+                        ctx.cmd.SetGlobalTexture(HDShaderIDs._ExposureTexture, GetExposureTexture(hdCamera));
                     });
             }
         }

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

@@ -45,8 +45,8 @@ void MissCamera(inout PathIntersection pathIntersection : SV_RayPayload)
     }
 
     bool skyEnabled = _EnvLightSkyEnabled && _RaytracingCameraSkyEnabled;
-    float4 missColor = skyEnabled ? _SkyCameraTexture[COORD_TEXTURE2D_X(pathIntersection.pixelCoord)] : _RaytracingCameraClearColor;
-    pathIntersection.value = missColor.rgb * GetInverseCurrentExposureMultiplier();
+    float4 missColor = skyEnabled ? _SkyCameraTexture[COORD_TEXTURE2D_X(pathIntersection.pixelCoord)] : _RaytracingCameraClearColor * GetInverseCurrentExposureMultiplier();
+    pathIntersection.value = missColor.rgb;
     pathIntersection.alpha = missColor.a;
 
     ApplyFogAttenuation(WorldRayOrigin(), WorldRayDirection(), pathIntersection.value, pathIntersection.alpha);