Fix for the preview rendering issue (no default sky ambient)

In the initial rendering of the preview in the inspector window, the ambient color of the default sky may not be ready (black ambient color), due to the lag of GPU readback of the SH coefficients. The ambient color is not updated until the preview is updated by some triggers like hovering the cursor over the inspector.
This PR fixes the issue by using the hard-coded value of the default sky's SH.

Author: Masayoshi Miyamoto

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

@@ -1773,7 +1773,7 @@ internal void UpdateCurrentSky(SkyManager skyManager)
                 visualSky.cloudSettings = null;
                 visualSky.volumetricClouds = null;
                 lightingSky = visualSky;
-                skyAmbientMode = SkyAmbientMode.Dynamic;
+                skyAmbientMode = SkyAmbientMode.Static;
             }
             else
 #endif

Packages/com.unity.render-pipelines.high-definition/Runtime/Sky/SkyManager.cs

@@ -188,6 +188,11 @@ class SkyManager
 #if UNITY_EDITOR
         // For Preview windows we want to have a 'fixed' sky, so we can display chrome metal and have always the same look
         HDRISky m_DefaultPreviewSky;
+
+        // Hard-coded SH for DefaultHDRISky.exr
+        // This is a temporary solution for the preview rendering issue when SH is not ready.
+        // A proper fix is needed when we want to expose the control of sky for preview.
+        SphericalHarmonicsL2 m_DefaultPreviewSkyAmbientProbe = new SphericalHarmonicsL2();
 #endif
 
         // Shared resources for sky rendering.
@@ -456,6 +461,36 @@ public void Build(HDRenderPipelineAsset hdAsset, HDRenderPipeline renderPipeline
 
 #if UNITY_EDITOR
             UnityEditor.Lightmapping.bakeStarted += OnBakeStarted;
+
+            {
+                m_DefaultPreviewSkyAmbientProbe[0, 0] = 0.1279895f;
+                m_DefaultPreviewSkyAmbientProbe[0, 1] = -0.01244975f;
+                m_DefaultPreviewSkyAmbientProbe[0, 2] = 0.002333597f;
+                m_DefaultPreviewSkyAmbientProbe[0, 3] = -0.01013585f;
+                m_DefaultPreviewSkyAmbientProbe[0, 4] = -0.006032045f;
+                m_DefaultPreviewSkyAmbientProbe[0, 5] = 0.0005331814f;
+                m_DefaultPreviewSkyAmbientProbe[0, 6] = 0.002311948f;
+                m_DefaultPreviewSkyAmbientProbe[0, 7] = -0.001873836f;
+                m_DefaultPreviewSkyAmbientProbe[0, 8] = 0.0231871f;
+                m_DefaultPreviewSkyAmbientProbe[1, 0] = 0.1585829f;
+                m_DefaultPreviewSkyAmbientProbe[1, 1] = 0.01596837f;
+                m_DefaultPreviewSkyAmbientProbe[1, 2] = 0.003311858f;
+                m_DefaultPreviewSkyAmbientProbe[1, 3] = -0.01475812f;
+                m_DefaultPreviewSkyAmbientProbe[1, 4] = -0.009350514f;
+                m_DefaultPreviewSkyAmbientProbe[1, 5] = 0.000841937f;
+                m_DefaultPreviewSkyAmbientProbe[1, 6] = 0.003378667f;
+                m_DefaultPreviewSkyAmbientProbe[1, 7] = -0.002562553f;
+                m_DefaultPreviewSkyAmbientProbe[1, 8] = 0.03318842f;
+                m_DefaultPreviewSkyAmbientProbe[2, 0] = 0.209883f;
+                m_DefaultPreviewSkyAmbientProbe[2, 1] = 0.06525062f;
+                m_DefaultPreviewSkyAmbientProbe[2, 2] = 0.004639104f;
+                m_DefaultPreviewSkyAmbientProbe[2, 3] = -0.02339679f;
+                m_DefaultPreviewSkyAmbientProbe[2, 4] = -0.01619671f;
+                m_DefaultPreviewSkyAmbientProbe[2, 5] = 0.001453806f;
+                m_DefaultPreviewSkyAmbientProbe[2, 6] = 0.003758613f;
+                m_DefaultPreviewSkyAmbientProbe[2, 7] = -0.003646188f;
+                m_DefaultPreviewSkyAmbientProbe[2, 8] = 0.04316145f;
+            }
 #endif
         }
 
@@ -608,6 +643,13 @@ internal SphericalHarmonicsL2 GetAmbientProbe(HDCamera hdCamera)
                 return m_BlackAmbientProbe;
             }
 
+#if UNITY_EDITOR
+            if (HDUtils.IsRegularPreviewCamera(hdCamera.camera))
+            {
+                return m_DefaultPreviewSkyAmbientProbe;
+            }
+#endif
+
             return GetAmbientProbe(GetLightingSky(hdCamera));
         }