[HDRP] Fix rendering issues for the first frame (#2836)

* Fix first frame exposure, depth pyramid / AO

* Update changelog

* Comment

* Typo

* Add missing RenderGraphBuilder.ReadTexture call

* Move ComputePackedMipChainInfo at the beginning of ExecuteWithRenderGraph

Co-authored-by: sebastienlagarde <sebastien@unity3d.com>

Author: pmavridis

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

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Fixed issue with faulty shadow transition when view is close to an object under some aspect ratio conditions
 - Fixed NullPointerException when baking probes from the lighting window (case 1289680)
 - Fixed volumetric fog with XR single-pass rendering.
+- Fixed issues with first frame rendering when RenderGraph is used (auto exposure, AO)
 
 ### Changed
 - Removed the material pass probe volumes evaluation mode.

com.unity.render-pipelines.high-definition/Runtime/PostProcessing/PostProcessSystem.RenderGraph.cs

@@ -317,9 +317,7 @@ TextureHandle DynamicExposurePass(RenderGraph renderGraph, HDCamera hdCamera, Te
                     {
                         passData.source = builder.ReadTexture(source);
                         passData.parameters = PrepareApplyExposureParameters(hdCamera);
-                        RTHandle prevExp;
-                        GrabExposureHistoryTextures(hdCamera, out prevExp, out _);
-                        passData.prevExposure = builder.ReadTexture(renderGraph.ImportTexture(prevExp));
+                        passData.prevExposure = builder.ReadTexture(renderGraph.ImportTexture(GetPreviousExposureTexture(hdCamera)));
 
                         TextureHandle dest = GetPostprocessOutputHandle(renderGraph, "Apply Exposure Destination");
                         passData.destination = builder.WriteTexture(dest);;

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

@@ -34,6 +34,9 @@ void ExecuteWithRenderGraph(RenderRequest           renderRequest,
 
             m_RenderGraph.Begin(renderGraphParams);
 
+            // We need to initalize the MipChainInfo here, so it will be available to any render graph pass that wants to use it during setup
+            m_DepthBufferMipChainInfo.ComputePackedMipChainInfo(new Vector2Int(hdCamera.actualWidth, hdCamera.actualHeight));
+
 #if UNITY_EDITOR
             var showGizmos = camera.cameraType == CameraType.Game
                 || camera.cameraType == CameraType.SceneView;