[Port] [2021.3] Fix HDRP sky rendering when Camera Relative Rendering is disabled

This PR fixes HDRP sky rendering when Camera Relative Rendering is disabled.

`ComputePixelCoordToWorldSpaceViewDirectionMatrix()` in `HDCamera.cs` contains:

```
...
if (useGenericMatrix)
{
    var viewSpaceRasterTransform = new Matrix4x4(
        new Vector4(2.0f * resolution.z, 0.0f, 0.0f, -1.0f),
        new Vector4(0.0f, -2.0f * resolution.w, 0.0f, 1.0f),
        new Vector4(0.0f, 0.0f, 1.0f, 0.0f),
        new Vector4(0.0f, 0.0f, 0.0f, 1.0f));

    var transformT = viewConstants.invViewProjMatrix.transpose * Matrix4x4.Scale(new Vector3(-1.0f, -1.0f, -1.0f));
    return viewSpaceRasterTransform * transformT;
}
...
```

Here the view matrix that was used to build `viewConstants.invViewProjMatrix` has translation component if Camera Relative Rendering is disabled when it shouldn't.

![scene_view](https://github.cds.internal.unity3d.com/unity/unity/assets/7006/95047d07-52f3-4d19-8698-0120ad86987f)
Scene View

![game_view_wrong](https://github.cds.internal.unity3d.com/unity/unity/assets/7006/2cb0b898-ddb2-4819-ae58-7900ce0c4221)
Game View, bugged

![game_view_fixed](https://github.cds.internal.unity3d.com/unity/unity/assets/7006/0a9b4699-9737-426f-a605-fd51a5ae72eb)
Game View, fixed

Author: svc-reach-platform-support

Packages/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/Camera/HDCamera.cs

@@ -1063,12 +1063,12 @@ internal void GetPixelCoordToViewDirWS(Vector4 resolution, float aspect, ref Mat
             {
                 for (int viewIndex = 0; viewIndex < viewCount; ++viewIndex)
                 {
-                    transforms[viewIndex] = ComputePixelCoordToWorldSpaceViewDirectionMatrix(m_XRViewConstants[viewIndex], resolution, aspect);
+                    transforms[viewIndex] = ComputePixelCoordToWorldSpaceViewDirectionMatrix(m_XRViewConstants[viewIndex], resolution, aspect, ShaderConfig.s_CameraRelativeRendering);
                 }
             }
             else
             {
-                transforms[0] = ComputePixelCoordToWorldSpaceViewDirectionMatrix(mainViewConstants, resolution, aspect);
+                transforms[0] = ComputePixelCoordToWorldSpaceViewDirectionMatrix(mainViewConstants, resolution, aspect, ShaderConfig.s_CameraRelativeRendering);
             }
         }
 
@@ -1618,7 +1618,7 @@ void UpdateViewConstants(ref ViewConstants viewConstants, Matrix4x4 projMatrix,
             viewConstants.viewProjectionNoCameraTrans = gpuVPNoTrans;
 
             var gpuProjAspect = HDUtils.ProjectionMatrixAspect(gpuProj);
-            viewConstants.pixelCoordToViewDirWS = ComputePixelCoordToWorldSpaceViewDirectionMatrix(viewConstants, screenSize, gpuProjAspect);
+            viewConstants.pixelCoordToViewDirWS = ComputePixelCoordToWorldSpaceViewDirectionMatrix(viewConstants, screenSize, gpuProjAspect, ShaderConfig.s_CameraRelativeRendering);
 
             if (updatePreviousFrameConstants)
             {
@@ -1859,8 +1859,9 @@ internal Matrix4x4 GetJitteredProjectionMatrix(Matrix4x4 origProj)
         ///
         /// It is different from the aspect ratio of <paramref name="resolution"/> for anamorphic projections.
         /// </param>
+        /// <param name="cameraRelativeRendering">If non-zero, then assume Camera Relative Rendering is enabled.</param>
         /// <returns></returns>
-        Matrix4x4 ComputePixelCoordToWorldSpaceViewDirectionMatrix(ViewConstants viewConstants, Vector4 resolution, float aspect = -1)
+        internal Matrix4x4 ComputePixelCoordToWorldSpaceViewDirectionMatrix(ViewConstants viewConstants, Vector4 resolution, float aspect = -1, int cameraRelativeRendering = 1)
         {
             // In XR mode, use a more generic matrix to account for asymmetry in the projection
             var useGenericMatrix = xr.enabled;
@@ -1879,7 +1880,20 @@ Matrix4x4 ComputePixelCoordToWorldSpaceViewDirectionMatrix(ViewConstants viewCon
                     new Vector4(0.0f, 0.0f, 1.0f, 0.0f),
                     new Vector4(0.0f, 0.0f, 0.0f, 1.0f));
 
-                var transformT = viewConstants.invViewProjMatrix.transpose * Matrix4x4.Scale(new Vector3(-1.0f, -1.0f, -1.0f));
+                Matrix4x4 transformT;
+                if (cameraRelativeRendering == 0)
+                {
+                    // In case we are not camera relative, the view matrix used to calculate viewConstants.invViewProjMatrix
+                    // contains translation component, so we need to remove it.
+                    var viewNoTrans = viewConstants.viewMatrix;
+                    viewNoTrans.SetColumn(3, new Vector4(0, 0, 0, 1));
+                    var invViewProj = (viewConstants.projMatrix * viewNoTrans).inverse;
+                    transformT = invViewProj.transpose * Matrix4x4.Scale(new Vector3(-1.0f, -1.0f, -1.0f));
+                }
+                else
+                {
+                    transformT = viewConstants.invViewProjMatrix.transpose * Matrix4x4.Scale(new Vector3(-1.0f, -1.0f, -1.0f));
+                }
                 return viewSpaceRasterTransform * transformT;
             }
 

Packages/com.unity.render-pipelines.high-definition/Tests/Editor/HDCameraTests.cs

@@ -0,0 +1,55 @@
+using NUnit.Framework;
+using UnityEngine.Experimental.Rendering;
+
+namespace UnityEngine.Rendering.HighDefinition.Tests
+{
+    class HDCameraTests
+    {
+        [SetUp]
+        public void Setup()
+        {
+            if (GraphicsSettings.currentRenderPipeline is not HDRenderPipelineAsset)
+                Assert.Ignore("This is an HDRP Tests, and the current pipeline is not HDRP.");
+        }
+
+        [Test]
+        public void TestCameraRelativeRendering()
+        {
+            GameObject cameraGameObject = new GameObject("Camera");
+            var camera = cameraGameObject.AddComponent<Camera>();
+            var hdCamera = new HDCamera(camera);
+
+            var positionSettings = new CameraPositionSettings();
+            positionSettings.position = new Vector3(100.0f, 300.0f, 500.0f);
+            positionSettings.rotation = Quaternion.Euler(62.34f, 185.53f, 323.563f);
+
+            var resolution = new Vector4(1920.0f, 1080.0f, 1.0f / 1920.0f, 1.0f / 1080.0f);
+            float aspect = resolution.x * resolution.w;
+
+            camera.worldToCameraMatrix = positionSettings.ComputeWorldToCameraMatrix();
+            camera.projectionMatrix = Matrix4x4.Perspective(75.0f, aspect, 0.1f, 1000.0f);
+
+            var view = camera.worldToCameraMatrix;
+            var proj = camera.projectionMatrix;
+
+            // Minimal setup for ComputePixelCoordToWorldSpaceViewDirectionMatrix().
+            var viewConstants = new HDCamera.ViewConstants();
+            viewConstants.viewMatrix = view;
+            viewConstants.projMatrix = proj;
+            viewConstants.invViewProjMatrix = (proj * view).inverse;
+
+            // hdCamera.xr must be initialized for ComputePixelCoordToWorldSpaceViewDirectionMatrix().
+            var hdrp = HDRenderPipeline.currentPipeline;
+            hdCamera.Update(hdCamera.frameSettings, hdrp, XRSystem.emptyPass, allocateHistoryBuffers: false);
+
+            var matrix0 = hdCamera.ComputePixelCoordToWorldSpaceViewDirectionMatrix(viewConstants, resolution, aspect, 0);
+            var matrix1 = hdCamera.ComputePixelCoordToWorldSpaceViewDirectionMatrix(viewConstants, resolution, aspect, 1);
+
+            // These matrices convert a clip space position to a world space view direction,
+            // therefore should be same regardless of Camera Relative Rendering.
+            Assert.AreEqual(matrix0, matrix1, $"{matrix0} != {matrix1}");
+
+            CoreUtils.Destroy(cameraGameObject);
+        }
+    }
+}