[HDRP][Path Tracing] Environment (sky) importance sampling (#6472)

* WIP...

* ...

* Refactor.

* WIP...

* Added marginal texture.

* Data generation (almost) complete.

* First working version.

* Further polish.

* ...

* ...

* Added dichotomic search.

* Fixed visibility in light evaluation.

* Removed unused code (incl. PDF storage).

* Updated changelog.

* Fixed Jacobian.

* Removed commented out code.

* Fixed volume attenuation on env light eval.

* Renamed variables only used in path tracing.

* Only activate sky importance sampling for HDRIs.

* Updated ref images that use HDRIs.

* Fixed formatting.

* Cosmetic.

* Update HDRP resources asset.

* Fixed sky auto-exposure mismatch.

* Updated changelog.

* Fixed camera clear color.

* Fixed unlit transparent.

* Minor clean up.

* Changed intensity clamping method.

* Added sky importance sampling parameter.

* Reordered params.

* Improved robustness with null PDFs.

* ...

* Updated doc with section on sky sampling.

* Cosmetic.

Author: eturquin

TestProjects/HDRP_DXR_Tests/Assets/ReferenceImages/Linear/WindowsEditor/Direct3D12/None/1000_RaytracingQualityKeyword_PathTracer_Default.png

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Added FSR sharpness override to camera and pipeline asset.
 - Added an option on the lit shader to perform Planar and Triplanar mapping in Object Space.
 - Added a button in the Probe Volume Baking window to open the Probe Volume debug panel.
+- Added importance sampling of the sky in path tracing (aka environment sampling).
 
 ### Fixed
 - Fixed some XR devices: Pulling camera world space position from mainViewConstants instead of transform.

TestProjects/HDRP_DXR_Tests/Assets/ReferenceImages/Linear/WindowsEditor/Direct3D12/None/5001_PathTracing.png

@@ -33,12 +33,13 @@ Path tracing uses the [Volume](Volumes.md) framework, so to enable this feature,
 
 ## Properties
 
-| Property              | Description                                                  |
-| --------------------- | ------------------------------------------------------------ |
-| **Maximum Samples**   | Set the number of frames to accumulate for the final image. There is a progress bar at the bottom of the Scene view which indicates the current accumulation with respect to this value. |
-| **Minimum Depth**     | Set the minimum number of light bounces in each path.        |
-| **Maximum Depth**     | Set the maximum number of light bounces in each path. You can not set this to be lower than Minimum Depth.<br /> **Note**: You can set this and Minimum Depth to 1 if you only want to direct lighting. You can set them both to 2 if you only want to visualize indirect lighting (which is only visible on the second bounce). |
-| **Maximum Intensity** | Set a value to clamp the intensity of the light value each bounce returns. This avoids very bright, isolated pixels in the final result.<br />**Note**: This property makes the final image dimmer, so if the result looks dark, increase the value of this property. |
+| Property                    | Description                                                  |
+| --------------------------- | ------------------------------------------------------------ |
+| **Maximum Samples**         | Set the number of frames to accumulate for the final image. There is a progress bar at the bottom of the Scene view which indicates the current accumulation with respect to this value. |
+| **Minimum Depth**           | Set the minimum number of light bounces in each path.        |
+| **Maximum Depth**           | Set the maximum number of light bounces in each path. You can not set this to be lower than Minimum Depth.<br /> **Note**: You can set this and Minimum Depth to 1 if you only want to direct lighting. You can set them both to 2 if you only want to visualize indirect lighting (which is only visible on the second bounce). |
+| **Maximum Intensity**       | Set a value to clamp the intensity of the light value each bounce returns. This avoids very bright, isolated pixels in the final result.<br />**Note**: This property can make the final image dimmer, so if the result looks dark, increase the value of this property. |
+| **Sky Importance Sampling** | Set the sky sampling mode. Importance sampling favors the brightest directions, which is beneficial when using a sky model with high contrast and very intense spots (like a sun, or street lights). On the other hand, it can be slightly detrimental when using a smooth, uniform sky. It is active by default for HDRI skies only, but can also be turned On and Off, regardless of the type of sky in use. |
 
 ![](Images/RayTracingPathTracing4.png)
 

TestProjects/HDRP_DXR_Tests/Assets/ReferenceImages/Linear/WindowsEditor/Direct3D12/None/5003_PathTracing_transparency.png

@@ -17,6 +17,7 @@ class PathTracingEditor : VolumeComponentEditor
         SerializedDataParameter m_MinDepth;
         SerializedDataParameter m_MaxDepth;
         SerializedDataParameter m_MaxIntensity;
+        SerializedDataParameter m_SkyImportanceSampling;
 
         public override void OnEnable()
         {
@@ -28,6 +29,7 @@ public override void OnEnable()
             m_MinDepth = Unpack(o.Find(x => x.minimumDepth));
             m_MaxDepth = Unpack(o.Find(x => x.maximumDepth));
             m_MaxIntensity = Unpack(o.Find(x => x.maximumIntensity));
+            m_SkyImportanceSampling = Unpack(o.Find(x => x.skyImportanceSampling));
         }
 
         public override void OnInspectorGUI()
@@ -54,6 +56,7 @@ public override void OnInspectorGUI()
                         PropertyField(m_MinDepth);
                         PropertyField(m_MaxDepth);
                         PropertyField(m_MaxIntensity);
+                        PropertyField(m_SkyImportanceSampling);
                     }
 
                     // Make sure MaxDepth is always greater or equal than MinDepth