Add a stub for the volume voxelization pass

ScriptableRenderPipeline/Core/CoreRP/GeometryUtils.cs

     [GenerateHLSL]
     public struct OrientedBBox
     {
-        // 3 x float4 = 48 bytes
+        // 3 x float4 = 48 bytes.
+        // TODO: pack the axes into 16-bit UNORM per channel, and consider a quaternionic representation.
         public Vector3 center;
         public float   extentX;
         public Vector3 right;

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/HDAssetFactory.cs

                 newAsset.deferredComputeShader = Load<ComputeShader>(HDRenderPipelinePath + "Lighting/LightLoop/Deferred.compute");
 
                 newAsset.deferredDirectionalShadowComputeShader = Load<ComputeShader>(HDRenderPipelinePath + "Lighting/DeferredDirectionalShadow.compute");
+                newAsset.volumeVoxelizationCS = Load<ComputeShader>(HDRenderPipelinePath + "Lighting/Volumetrics/Resources/VolumeVoxelization.compute");
                 newAsset.volumetricLightingCS = Load<ComputeShader>(HDRenderPipelinePath + "Lighting/Volumetrics/Resources/VolumetricLighting.compute");
 
                 newAsset.subsurfaceScatteringCS = Load<ComputeShader>(HDRenderPipelinePath + "Material/SubsurfaceScattering/SubsurfaceScattering.compute");

ScriptableRenderPipeline/HDRenderPipeline/HDRP/HDRenderPipeline.cs

                         continue;
                     }
 
-                    // Frustum cull density volumes on CPU.
+                    // Frustum cull density volumes on the CPU. Can be performed as soon as the camera is set up.
+
+                    // Perform the voxelization step which fills the density 3D texture.
+                    // Requires the clustered lighting data structure to be built.
+                    m_VolumetricLightingModule.VoxelizeDensityVolumes(densityVolumes);
 
                     // Note: Legacy Unity behave like this for ShadowMask
                     // When you select ShadowMask in Lighting panel it recompile shaders on the fly with the SHADOW_MASK keyword.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/VolumetricLighting.cs

 
     public VolumetricLightingPreset preset { get { return (VolumetricLightingPreset)Math.Min(ShaderConfig.s_VolumetricLightingPreset, (int)VolumetricLightingPreset.Count); } }
 
+    ComputeShader m_VolumeVoxelizationCS = null;
     ComputeShader m_VolumetricLightingCS = null;
 
     List<VBuffer>                 m_VBuffers                = null;
 
     float       m_VBufferNearPlane = 0.5f;  // Distance in meters; dynamic modifications not handled by reprojection
     float       m_VBufferFarPlane  = 64.0f; // Distance in meters; dynamic modifications not handled by reprojection
-    const float k_LogScale         = 0.5f;
+    const float k_LogScale         = 0.5f;  // Tweak constant, controls the logarithmic depth distribution
+        m_VolumeVoxelizationCS          = asset.renderPipelineResources.volumeVoxelizationCS;
         m_VolumetricLightingCS          = asset.renderPipelineResources.volumetricLightingCS;
         m_VBuffers                      = new List<VBuffer>();
         m_VisibleVolumeBounds           = new List<OrientedBBox>();
     {
         if (preset == VolumetricLightingPreset.Off) return;
 
+        m_VolumeVoxelizationCS = null;
         m_VolumetricLightingCS = null;
 
         for (int i = 0, n = m_VBuffers.Count; i < n; i++)
         // Collect all visible finite volume data, and upload it to the GPU.
         HomogeneousDensityVolume[] volumes = Object.FindObjectsOfType(typeof(HomogeneousDensityVolume)) as HomogeneousDensityVolume[];
 
-        foreach (HomogeneousDensityVolume volume in volumes)
+        for (int i = 0; i < Math.Min(volumes.Length, k_MaxVisibleVolumeCount); i++)
+            HomogeneousDensityVolume volume = volumes[i];
+
             // Only test active finite volumes.
             if (volume.enabled && volume.parameters.IsLocalVolume())
             {
         densityVolumes.properties = m_VisibleVolumeProperties;
 
         return densityVolumes;
+    }
+
+    public void VoxelizeDensityVolumes(DensityVolumeList densityVolumes)
+    {
     }
 
     // Ref: https://en.wikipedia.org/wiki/Close-packing_of_equal_spheres

ScriptableRenderPipeline/HDRenderPipeline/HDRP/RenderPipelineResources/RenderPipelineResources.cs

         public ComputeShader buildMaterialFlagsShader;
         public ComputeShader deferredComputeShader;
         public ComputeShader deferredDirectionalShadowComputeShader;
+        public ComputeShader volumeVoxelizationCS;
         public ComputeShader volumetricLightingCS;
 
         public ComputeShader subsurfaceScatteringCS; // Disney SSS

ScriptableRenderPipeline/HDRenderPipeline/HDRP/ShaderPass/ShaderPass.cs

         LightTransport,
         Shadows,
         SubsurfaceScattering,
+        VolumeVoxelization,
         VolumetricLighting,
         DBuffer
     }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/ShaderPass/ShaderPass.cs.hlsl

 #define SHADERPASS_LIGHT_TRANSPORT (7)
 #define SHADERPASS_SHADOWS (8)
 #define SHADERPASS_SUBSURFACE_SCATTERING (9)
-#define SHADERPASS_VOLUMETRIC_LIGHTING (10)
-#define SHADERPASS_DBUFFER (11)
+#define SHADERPASS_VOLUME_VOXELIZATION (10)
+#define SHADERPASS_VOLUMETRIC_LIGHTING (11)
+#define SHADERPASS_DBUFFER (12)
 
 
 #endif

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/Resources/VolumeVoxelization.compute

+//--------------------------------------------------------------------------------------------------
+// Definitions
+//--------------------------------------------------------------------------------------------------
+
+#pragma kernel VolumeVoxelization
+
+// #pragma enable_d3d11_debug_symbols
+
+#include "../../../ShaderPass/ShaderPass.cs.hlsl"
+#define SHADERPASS SHADERPASS_VOLUME_VOXELIZATION
+
+#include "../../../ShaderConfig.cs.hlsl"
+#if (SHADEROPTIONS_VOLUMETRIC_LIGHTING_PRESET == 1)
+    // E.g. for 1080p: (1920/8)x(1080/8)x(128) =  4,147,200 voxels
+    #define VBUFFER_TILE_SIZE   8
+    #define VBUFFER_SLICE_COUNT 64
+#else
+    // E.g. for 1080p: (1920/4)x(1080/4)x(256) = 33,177,600 voxels
+    #define VBUFFER_TILE_SIZE   4
+    #define VBUFFER_SLICE_COUNT 128
+#endif

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/Resources/VolumeVoxelization.compute.meta

+fileFormatVersion: 2
+guid: aab1ae3ccb26b3e448072bc56d22162d
+ComputeShaderImporter:
+  externalObjects: {}
+  currentAPIMask: 4
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: