More bugfixes for volume texture sampling

com.unity.render-pipelines.core/CoreRP/ShaderLibrary/Common.hlsl

     return ComputeTextureLOD(uv);
 }
 
+// LOD clamp is optional and happens outside the function.
-    return max(0.0, 0.5 * log2(d * (scale * scale)));
+    return 0.5 * log2(d * (scale * scale));
 }
 
 

com.unity.render-pipelines.high-definition/HDRP/Lighting/Volumetrics/VolumeVoxelization.compute

     float4   _VBufferSampleOffset;     // Not used by this shader
     float    _CornetteShanksConstant;  // Not used by this shader
     uint     _NumVisibleDensityVolumes;
-    float3   _VolumeMaskDimensions;    //x = 1/totalTextures , y = 1/textureSize, z = textureSize
+    float4   _VolumeMaskDimensions;    //x = 1/numTextures , y = width, z = depth = width * numTextures, w = maxLod
 CBUFFER_END
 
 //--------------------------------------------------------------------------------------------------
 {
-    float offset = volumeData.textureIndex * _VolumeMaskDimensions.x;
+    // Scale and bias the UVWs and then take fractional part, will be in [0,1] range.
+    voxelCenterUVW = frac(voxelCenterUVW * volumeData.textureTiling + volumeData.textureScroll);
-    //scale and bias the UVWs and then take fractional part, will be in [0,1] range
-    voxelCenterUVW = frac(voxelCenterUVW * volumeData.textureTiling + volumeData.textureScroll);
+    float rcpNumTextures = _VolumeMaskDimensions.x;
+    float textureWidth   = _VolumeMaskDimensions.y;
+    float textureDepth   = _VolumeMaskDimensions.z;
+    float maxLod         = _VolumeMaskDimensions.w;
-    voxelCenterUVW.z = voxelCenterUVW.z * _VolumeMaskDimensions.x;
-    voxelCenterUVW.z += offset;
+    float offset = volumeData.textureIndex * rcpNumTextures;
+    voxelCenterUVW.z = voxelCenterUVW.z * rcpNumTextures + offset;
-    float lod = ComputeTextureLOD(duvw_dx, duvw_dy, duvw_dz, _VolumeMaskDimensions.z);
+    float lod = ComputeTextureLOD(duvw_dx, duvw_dy, duvw_dz, textureWidth);
+    lod = clamp(lod, 0, maxLod);
-    // Secondly, for trilinear filering, which of the two LoDs should you choose to compute the distance to the edge?
-    // If you use floor(lod), the lower LoD may cause the leak across the edge from the neighbour texture.
-    // If you use ceil(lod), the upper LoD effectively loses a texel at the border, which may break tilable textures.
+    // Secondly, for trilinear filtering, which of the two LoDs should you choose to compute the distance to the edge?
+    // If you use floor(lod), the lower LoD may cause a leak across the edge from the neighbor texture.
+    // If you use ceil(lod), the upper LoD effectively loses a texel at the border, which may break tileable textures.
-    int   textureSize = (int)_VolumeMaskDimensions.z;
-    int   mipSize     = textureSize >> (int)ceil(lod);
-    float clampBorder = 0.5f * rcp(mipSize);
-    voxelCenterUVW.z  = clamp(voxelCenterUVW.z, offset + clampBorder, offset + _VolumeMaskDimensions.x - clampBorder);
+    // We support texture filtering across the wrap in Z in neither case.
+    int   textureSize   = (int)textureDepth;
+    int   mipSize       = textureSize >> (int)ceil(lod);
+    float halfTexelSize = 0.5f * rcp(mipSize);
+    voxelCenterUVW.z    = clamp(voxelCenterUVW.z, offset + halfTexelSize, offset + rcpNumTextures - halfTexelSize);
     
     return SAMPLE_TEXTURE3D_LOD(_VolumeMaskAtlas, s_trilinear_clamp_sampler, voxelCenterUVW, lod).a;
 }

com.unity.render-pipelines.high-definition/HDRP/Lighting/Volumetrics/VolumetricLighting.cs

                 Matrix4x4 transform   = HDUtils.ComputePixelCoordToWorldSpaceViewDirectionMatrix(vFoV, resolution, hdCamera.viewMatrix, false);
 
                 Texture3D volumeAtlas = DensityVolumeManager.manager.volumeAtlas.volumeAtlas;
-                Vector3 volumeAtlasDimensions = new Vector3(0.0f, 0.0f, 0.0f);
+                Vector4 volumeAtlasDimensions = new Vector4(0.0f, 0.0f, 0.0f, 0.0f);
-                    volumeAtlasDimensions.y = 1.0f / volumeAtlas.width;
-                    volumeAtlasDimensions.z = volumeAtlas.width;
+                    volumeAtlasDimensions.y = volumeAtlas.width;
+                    volumeAtlasDimensions.z = volumeAtlas.depth;
+                    volumeAtlasDimensions.w = Mathf.Log(volumeAtlas.width, 2);              // Max LoD
                 }
                 else
                 {