using System;
using UnityEngine.Rendering;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace UnityEngine.Experimental.Rendering.HDPipeline
{
    [GenerateHLSL]
    public struct DensityVolumeData
    {
        public Vector3 scattering; // [0, 1], prefer sRGB
        public float   extinction;// [0, 1], prefer sRGB
        public Vector3 textureTiling;
        public int     textureIndex;//
        public Vector3 textureScroll;

        public static DensityVolumeData GetNeutralValues()
        {
            DensityVolumeData data;

            data.scattering = Vector3.zero;
            data.extinction = 0;
            data.textureIndex = -1;

            data.textureTiling = Vector3.one;
            data.textureScroll = Vector3.zero;

            return data;
        }
    } // struct VolumeProperties

    public class VolumeRenderingUtils
    {
        public static float MeanFreePathFromExtinction(float extinction)
        {
            return 1.0f / extinction;
        }

        public static float ExtinctionFromMeanFreePath(float meanFreePath)
        {
            return 1.0f / meanFreePath;
        }

        public static Vector3 AbsorptionFromExtinctionAndScattering(float extinction, Vector3 scattering)
        {
            return new Vector3(extinction, extinction, extinction) - scattering;
        }

        public static Vector3 ScatteringFromExtinctionAndAlbedo(float extinction, Vector3 albedo)
        {
            return extinction * albedo;
        }

        public static Vector3 AlbedoFromMeanFreePathAndScattering(float meanFreePath, Vector3 scattering)
        {
            return meanFreePath * scattering;
        }
    }

    public struct DensityVolumeList
    {
        public List<OrientedBBox>      bounds;
        public List<DensityVolumeData> density;
    }

    public class VolumetricLightingSystem
    {
        public enum VolumetricLightingPreset
        {
            Off,
            Normal,
            Ultra,
            Count
        } // enum VolumetricLightingPreset

        public struct VBufferParameters
        {
            public Vector4 resolution;
            public Vector2 sliceCount;
            public Vector4 uvScaleAndLimit; // Necessary to work with sub-allocation (resource aliasing) in the RTHandle system
            public Vector4 depthEncodingParams;
            public Vector4 depthDecodingParams;

            public VBufferParameters(Vector3Int viewportResolution, Vector3Int bufferResolution, Vector2 depthRange, float depthDistributionUniformity)
            {
                int w = viewportResolution.x;
                int h = viewportResolution.y;
                int d = viewportResolution.z;

                // The depth is fixed for now.
                Vector2 uvScale = new Vector2((float)w / (float)bufferResolution.x,
                        (float)h / (float)bufferResolution.y);

                // vp_scale = vp_dim / tex_dim.
                // clamp to (vp_dim - 0.5) / tex_dim = vp_scale - 0.5 * (1 / tex_dim) =
                // vp_scale - 0.5 * (vp_scale / vp_dim) = vp_scale * (1 - 0.5 / vp_dim).
                Vector2 uvLimit = new Vector2((w - 0.5f) / (float)bufferResolution.x,
                        (h - 0.5f) / (float)bufferResolution.y);

                resolution          = new Vector4(w, h, 1.0f / w, 1.0f / h);
                sliceCount          = new Vector2(d, 1.0f / d);
                uvScaleAndLimit     = new Vector4(uvScale.x, uvScale.y, uvLimit.x, uvLimit.y);

                float n = depthRange.x;
                float f = depthRange.y;
                float c = 2 - 2 * depthDistributionUniformity; // remap [0, 1] -> [2, 0]

                depthEncodingParams = ComputeLogarithmicDepthEncodingParams(n, f, c);
                depthDecodingParams = ComputeLogarithmicDepthDecodingParams(n, f, c);
            }
        } // struct Parameters

        public VolumetricLightingPreset preset { get { return (VolumetricLightingPreset)Math.Min(ShaderConfig.s_VolumetricLightingPreset, (int)VolumetricLightingPreset.Count); } }

        static ComputeShader    m_VolumeVoxelizationCS      = null;
        static ComputeShader    m_VolumetricLightingCS      = null;

        List<OrientedBBox>      m_VisibleVolumeBounds       = null;
        List<DensityVolumeData> m_VisibleVolumeData         = null;
        public const int        k_MaxVisibleVolumeCount     = 512;

        // Static keyword is required here else we get a "DestroyBuffer can only be called from the main thread"
        static ComputeBuffer    s_VisibleVolumeBoundsBuffer = null;
        static ComputeBuffer    s_VisibleVolumeDataBuffer   = null;

        // These two buffers do not depend on the frameID and are therefore shared by all views.
        RTHandleSystem.RTHandle m_DensityBufferHandle;
        RTHandleSystem.RTHandle m_LightingBufferHandle;

        // Do we support volumetric or not?
        bool m_supportVolumetric = false;

        public void Build(HDRenderPipelineAsset asset)
        {
            m_supportVolumetric = asset.renderPipelineSettings.supportVolumetric;

            if (!m_supportVolumetric)
                return;

            m_VolumeVoxelizationCS = asset.renderPipelineResources.volumeVoxelizationCS;
            m_VolumetricLightingCS = asset.renderPipelineResources.volumetricLightingCS;

            CreateBuffers();
        }

        // RTHandleSystem API expects a function which computes the resolution. We define it here.
        Vector2Int ComputeVBufferResolutionXY(Vector2Int screenSize)
        {
            Vector3Int resolution = ComputeVBufferResolution(preset, screenSize.x, screenSize.y);

            return new Vector2Int(resolution.x, resolution.y);
        }

        // RTHandleSystem API expects a function which computes the resolution. We define it here.
        Vector2Int ComputeHistoryVBufferResolutionXY(Vector2Int screenSize)
        {
            Vector2Int resolution = ComputeVBufferResolutionXY(screenSize);

            // Since the buffers owned by the VolumetricLightingSystem may have different lifetimes compared
            // to those owned by the HDCamera, we need to make sure that the buffer resolution is the same
            // (in order to share the UV scale and the UV limit).
            if (m_LightingBufferHandle != null)
            {
                resolution.x = Math.Max(resolution.x, m_LightingBufferHandle.rt.width);
                resolution.y = Math.Max(resolution.y, m_LightingBufferHandle.rt.height);
            }

            return resolution;
        }

        // BufferedRTHandleSystem API expects an allocator function. We define it here.
        RTHandleSystem.RTHandle HistoryBufferAllocatorFunction(string viewName, int frameIndex, RTHandleSystem rtHandleSystem)
        {
            frameIndex &= 1;

            int d = ComputeVBufferSliceCount(preset);

            return rtHandleSystem.Alloc(scaleFunc:         ComputeHistoryVBufferResolutionXY,
                slices:            d,
                dimension:         TextureDimension.Tex3D,
                colorFormat:       RenderTextureFormat.ARGBHalf,
                sRGB:              false,
                enableRandomWrite: true,
                enableMSAA:        false,
                /* useDynamicScale: true, // <- TODO */
                name: string.Format("{0}_VBufferHistory{1}", viewName, frameIndex)
                );
        }

        void CreateBuffers()
        {
            Debug.Assert(m_VolumetricLightingCS != null);

            m_VisibleVolumeBounds       = new List<OrientedBBox>();
            m_VisibleVolumeData         = new List<DensityVolumeData>();
            s_VisibleVolumeBoundsBuffer = new ComputeBuffer(k_MaxVisibleVolumeCount, Marshal.SizeOf(typeof(OrientedBBox)));
            s_VisibleVolumeDataBuffer   = new ComputeBuffer(k_MaxVisibleVolumeCount, Marshal.SizeOf(typeof(DensityVolumeData)));

            int d = ComputeVBufferSliceCount(preset);

            m_DensityBufferHandle = RTHandles.Alloc(scaleFunc:         ComputeVBufferResolutionXY,
                    slices:            d,
                    dimension:         TextureDimension.Tex3D,
                    colorFormat:       RenderTextureFormat.ARGBHalf,
                    sRGB:              false,
                    enableRandomWrite: true,
                    enableMSAA:        false,
                    /* useDynamicScale: true, // <- TODO */
                    name:              "VBufferDensity");

            m_LightingBufferHandle = RTHandles.Alloc(scaleFunc:         ComputeVBufferResolutionXY,
                    slices:            d,
                    dimension:         TextureDimension.Tex3D,
                    colorFormat:       RenderTextureFormat.ARGBHalf,
                    sRGB:              false,
                    enableRandomWrite: true,
                    enableMSAA:        false,
                    /* useDynamicScale: true, // <- TODO */
                    name:              "VBufferIntegral");
        }

        // For the initial allocation, no suballocation happens (the texture is full size).
        VBufferParameters ComputeVBufferParameters(HDCamera hdCamera, bool isInitialAllocation)
        {
            Vector3Int viewportResolution = ComputeVBufferResolution(preset, hdCamera.camera.pixelWidth, hdCamera.camera.pixelHeight);
            Vector3Int bufferResolution; // Could be higher due to sub-allocation (resource aliasing) in the RTHandle system

            if (isInitialAllocation)
            {
                bufferResolution = viewportResolution;
            }
            else
            {
                // All V-Buffers of the current frame should have the same size (you have to double-buffer history, of course).
                bufferResolution = new Vector3Int(m_LightingBufferHandle.rt.width, m_LightingBufferHandle.rt.height, m_LightingBufferHandle.rt.volumeDepth);
            }

            var controller = VolumeManager.instance.stack.GetComponent<VolumetricLightingController>();

            // We must not allow the V-Buffer to extend outside of the camera's frustum.
            float n = hdCamera.camera.nearClipPlane;
            float f = hdCamera.camera.farClipPlane;

            Vector2 vBufferDepthRange = controller.depthRange.value;
            vBufferDepthRange.y = Mathf.Clamp(vBufferDepthRange.y, n, f);               // far
            vBufferDepthRange.x = Mathf.Clamp(vBufferDepthRange.x, n, vBufferDepthRange.y); // near
            float vBufferDepthDistributionUniformity = controller.depthDistributionUniformity.value;

            return new VBufferParameters(viewportResolution, bufferResolution, vBufferDepthRange, vBufferDepthDistributionUniformity);
        }

        public void InitializePerCameraData(HDCamera hdCamera)
        {
            // Note: Here we can't test framesettings as they are not initialize yet
            // TODO: Here we allocate history even for camera that may not use volumetric
            if (!m_supportVolumetric)
                return;

            // Start with the same parameters for both frames. Then update them one by one every frame.
            var parameters          = ComputeVBufferParameters(hdCamera, true);
            hdCamera.vBufferParams    = new VBufferParameters[2];
            hdCamera.vBufferParams[0] = parameters;
            hdCamera.vBufferParams[1] = parameters;

            if (hdCamera.camera.cameraType == CameraType.Game ||
                hdCamera.camera.cameraType == CameraType.SceneView)
            {
                // We don't need reprojection for other view types, such as reflection and preview.
                hdCamera.AllocHistoryFrameRT((int)HDCameraFrameHistoryType.VolumetricLighting, HistoryBufferAllocatorFunction);
            }
        }

        // This function relies on being called once per camera per frame.
        // The results are undefined otherwise.
        public void UpdatePerCameraData(HDCamera hdCamera)
        {
            if (!hdCamera.frameSettings.enableVolumetric)
                return;

            var parameters = ComputeVBufferParameters(hdCamera, false);

            // Double-buffer. I assume the cost of copying is negligible (don't want to use the frame index).
            hdCamera.vBufferParams[1] = hdCamera.vBufferParams[0];
            hdCamera.vBufferParams[0] = parameters;

            // Note: resizing of history buffer is automatic (handled by the BufferedRTHandleSystem).
        }

        void DestroyBuffers()
        {
            if (m_DensityBufferHandle != null)
                RTHandles.Release(m_DensityBufferHandle);
            if (m_LightingBufferHandle != null)
                RTHandles.Release(m_LightingBufferHandle);

            CoreUtils.SafeRelease(s_VisibleVolumeBoundsBuffer);
            CoreUtils.SafeRelease(s_VisibleVolumeDataBuffer);

            m_VisibleVolumeBounds = null;
            m_VisibleVolumeData   = null;
        }

        public void Cleanup()
        {
            // Note: No need to test for support volumetric here, we do saferelease and null assignation
            DestroyBuffers();

            m_VolumeVoxelizationCS = null;
            m_VolumetricLightingCS = null;
        }

        static int ComputeVBufferTileSize(VolumetricLightingPreset preset)
        {
            switch (preset)
            {
                case VolumetricLightingPreset.Normal:
                    return 8;
                case VolumetricLightingPreset.Ultra:
                    return 4;
                case VolumetricLightingPreset.Off:
                    return 0;
                default:
                    Debug.Assert(false, "Encountered an unexpected VolumetricLightingPreset.");
                    return 0;
            }
        }

        static int ComputeVBufferSliceCount(VolumetricLightingPreset preset)
        {
            switch (preset)
            {
                case VolumetricLightingPreset.Normal:
                    return 64;
                case VolumetricLightingPreset.Ultra:
                    return 128;
                case VolumetricLightingPreset.Off:
                    return 0;
                default:
                    Debug.Assert(false, "Encountered an unexpected VolumetricLightingPreset.");
                    return 0;
            }
        }

        static Vector3Int ComputeVBufferResolution(VolumetricLightingPreset preset,
            int screenWidth, int screenHeight)
        {
            int t = ComputeVBufferTileSize(preset);

            // ceil(ScreenSize / TileSize).
            int w = (screenWidth  + (t - 1)) / t;
            int h = (screenHeight + (t - 1)) / t;
            int d = ComputeVBufferSliceCount(preset);

            return new Vector3Int(w, h, d);
        }

        // See EncodeLogarithmicDepthGeneralized().
        static Vector4 ComputeLogarithmicDepthEncodingParams(float nearPlane, float farPlane, float c)
        {
            Vector4 depthParams = new Vector4();

            float n = nearPlane;
            float f = farPlane;

            c = Mathf.Max(c, 0.001f); // Avoid NaNs

            depthParams.y = 1.0f / Mathf.Log(c * (f - n) + 1, 2);
            depthParams.x = Mathf.Log(c, 2) * depthParams.y;
            depthParams.z = n - 1.0f / c; // Same
            depthParams.w = 0.0f;

            return depthParams;
        }

        // See DecodeLogarithmicDepthGeneralized().
        static Vector4 ComputeLogarithmicDepthDecodingParams(float nearPlane, float farPlane, float c)
        {
            Vector4 depthParams = new Vector4();

            float n = nearPlane;
            float f = farPlane;

            c = Mathf.Max(c, 0.001f); // Avoid NaNs

            depthParams.x = 1.0f / c;
            depthParams.y = Mathf.Log(c * (f - n) + 1, 2);
            depthParams.z = n - 1.0f / c; // Same
            depthParams.w = 0.0f;

            return depthParams;
        }

        void SetPreconvolvedAmbientLightProbe(CommandBuffer cmd, float anisotropy)
        {
            SphericalHarmonicsL2 probeSH = SphericalHarmonicMath.UndoCosineRescaling(RenderSettings.ambientProbe);
            ZonalHarmonicsL2     phaseZH = ZonalHarmonicsL2.GetCornetteShanksPhaseFunction(anisotropy);
            SphericalHarmonicsL2 finalSH = SphericalHarmonicMath.PremultiplyCoefficients(SphericalHarmonicMath.Convolve(probeSH, phaseZH));

            cmd.SetGlobalVectorArray(HDShaderIDs._AmbientProbeCoeffs, SphericalHarmonicMath.PackCoefficients(finalSH));
        }

        float CornetteShanksPhasePartConstant(float anisotropy)
        {
            float g = anisotropy;

            return (1.0f / (4.0f * Mathf.PI)) * 1.5f * (1.0f - g * g) / (2.0f + g * g);
        }

        public void PushGlobalParams(HDCamera hdCamera, CommandBuffer cmd, uint frameIndex)
        {
            if (!hdCamera.frameSettings.enableVolumetric)
                return;

            var visualEnvironment = VolumeManager.instance.stack.GetComponent<VisualEnvironment>();

            // VisualEnvironment sets global fog parameters: _GlobalAnisotropy, _GlobalScattering, _GlobalExtinction.

            if (visualEnvironment.fogType != FogType.Volumetric)
            {
                // Set the neutral black texture.
                cmd.SetGlobalTexture(HDShaderIDs._VBufferLighting, CoreUtils.blackVolumeTexture);
                return;
            }

            // Get the interpolated anisotropy value.
            var fog = VolumeManager.instance.stack.GetComponent<VolumetricFog>();

            SetPreconvolvedAmbientLightProbe(cmd, fog.anisotropy);

            var currFrameParams = hdCamera.vBufferParams[0];
            var prevFrameParams = hdCamera.vBufferParams[1];

            cmd.SetGlobalVector(HDShaderIDs._VBufferResolution,              currFrameParams.resolution);
            cmd.SetGlobalVector(HDShaderIDs._VBufferSliceCount,              currFrameParams.sliceCount);
            cmd.SetGlobalVector(HDShaderIDs._VBufferUvScaleAndLimit,         currFrameParams.uvScaleAndLimit);
            cmd.SetGlobalVector(HDShaderIDs._VBufferDepthEncodingParams,     currFrameParams.depthEncodingParams);
            cmd.SetGlobalVector(HDShaderIDs._VBufferDepthDecodingParams,     currFrameParams.depthDecodingParams);

            cmd.SetGlobalVector(HDShaderIDs._VBufferPrevResolution,          prevFrameParams.resolution);
            cmd.SetGlobalVector(HDShaderIDs._VBufferPrevSliceCount,          prevFrameParams.sliceCount);
            cmd.SetGlobalVector(HDShaderIDs._VBufferPrevUvScaleAndLimit,     prevFrameParams.uvScaleAndLimit);
            cmd.SetGlobalVector(HDShaderIDs._VBufferPrevDepthEncodingParams, prevFrameParams.depthEncodingParams);
            cmd.SetGlobalVector(HDShaderIDs._VBufferPrevDepthDecodingParams, prevFrameParams.depthDecodingParams);

            cmd.SetGlobalTexture(HDShaderIDs._VBufferLighting,                m_LightingBufferHandle);
        }

        public DensityVolumeList PrepareVisibleDensityVolumeList(HDCamera hdCamera, CommandBuffer cmd, float time)
        {
            DensityVolumeList densityVolumes = new DensityVolumeList();

            if (!hdCamera.frameSettings.enableVolumetric)
                return densityVolumes;

            var visualEnvironment = VolumeManager.instance.stack.GetComponent<VisualEnvironment>();
            if (visualEnvironment.fogType != FogType.Volumetric)
                return densityVolumes;

            using (new ProfilingSample(cmd, "Prepare Visible Density Volume List"))
            {
                Vector3 camPosition = hdCamera.camera.transform.position;
                Vector3 camOffset   = Vector3.zero;// World-origin-relative

                if (ShaderConfig.s_CameraRelativeRendering != 0)
                {
                    camOffset = camPosition; // Camera-relative
                }

                m_VisibleVolumeBounds.Clear();
                m_VisibleVolumeData.Clear();

                // Collect all visible finite volume data, and upload it to the GPU.
                DensityVolume[] volumes = DensityVolumeManager.manager.PrepareDensityVolumeData(cmd, hdCamera.camera, time);

                for (int i = 0; i < Math.Min(volumes.Length, k_MaxVisibleVolumeCount); i++)
                {
                    DensityVolume volume = volumes[i];

                    // TODO: cache these?
                    var obb = OrientedBBox.Create(volume.transform);

                    // Handle camera-relative rendering.
                    obb.center -= camOffset;

                    // Frustum cull on the CPU for now. TODO: do it on the GPU.
                    // TODO: account for custom near and far planes of the V-Buffer's frustum.
                    // It's typically much shorter (along the Z axis) than the camera's frustum.
                    if (GeometryUtils.Overlap(obb, hdCamera.frustum, 6, 8))
                    {
                        // TODO: cache these?
                        var data = volume.parameters.GetData();

                        m_VisibleVolumeBounds.Add(obb);
                        m_VisibleVolumeData.Add(data);
                    }
                }

                s_VisibleVolumeBoundsBuffer.SetData(m_VisibleVolumeBounds);
                s_VisibleVolumeDataBuffer.SetData(m_VisibleVolumeData);

                // Fill the struct with pointers in order to share the data with the light loop.
                densityVolumes.bounds  = m_VisibleVolumeBounds;
                densityVolumes.density = m_VisibleVolumeData;

                return densityVolumes;
            }
        }

        public void VolumeVoxelizationPass(HDCamera hdCamera, CommandBuffer cmd, uint frameIndex, DensityVolumeList densityVolumes)
        {
            if (!hdCamera.frameSettings.enableVolumetric)
                return;

            var visualEnvironment = VolumeManager.instance.stack.GetComponent<VisualEnvironment>();
            if (visualEnvironment.fogType != FogType.Volumetric)
                return;

            using (new ProfilingSample(cmd, "Volume Voxelization"))
            {
                int numVisibleVolumes = m_VisibleVolumeBounds.Count;

                if (numVisibleVolumes == 0)
                {
                    // Clear the render target instead of running the shader.
                    // Note: the clear must take the global fog into account!
                    // CoreUtils.SetRenderTarget(cmd, vBuffer.GetDensityBuffer(), ClearFlag.Color, CoreUtils.clearColorAllBlack);
                    // return;

                    // Clearing 3D textures does not seem to work!
                    // Use the workaround by running the full shader with 0 density
                }

                bool enableClustered = hdCamera.frameSettings.lightLoopSettings.enableTileAndCluster;

                int kernel = m_VolumeVoxelizationCS.FindKernel(enableClustered ? "VolumeVoxelizationClustered"
                        : "VolumeVoxelizationBruteforce");

                var     frameParams = hdCamera.vBufferParams[0];
                Vector4 resolution  = frameParams.resolution;
                float   vFoV        = hdCamera.camera.fieldOfView * Mathf.Deg2Rad;

                // Compose the matrix which allows us to compute the world space view direction.
                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);

                if (volumeAtlas != null)
                {
                    volumeAtlasDimensions.x = volumeAtlas.width / volumeAtlas.depth; // 1 / number of textures
                    volumeAtlasDimensions.y = 1.0f / volumeAtlas.width;
                    volumeAtlasDimensions.z = volumeAtlas.width;
                }

                cmd.SetComputeTextureParam(m_VolumeVoxelizationCS, kernel, HDShaderIDs._VBufferDensity, m_DensityBufferHandle);
                cmd.SetComputeBufferParam(m_VolumeVoxelizationCS, kernel, HDShaderIDs._VolumeBounds,   s_VisibleVolumeBoundsBuffer);
                cmd.SetComputeBufferParam(m_VolumeVoxelizationCS, kernel, HDShaderIDs._VolumeData,     s_VisibleVolumeDataBuffer);
                cmd.SetComputeTextureParam(m_VolumeVoxelizationCS, kernel, HDShaderIDs._VolumeMaskAtlas, volumeAtlas);

                // TODO: set the constant buffer data only once.
                cmd.SetComputeMatrixParam(m_VolumeVoxelizationCS, HDShaderIDs._VBufferCoordToViewDirWS,  transform);
                cmd.SetComputeIntParam(m_VolumeVoxelizationCS, HDShaderIDs._NumVisibleDensityVolumes, numVisibleVolumes);
                cmd.SetComputeVectorParam(m_VolumeVoxelizationCS, HDShaderIDs._VolumeMaskDimensions, volumeAtlasDimensions);

                int w = (int)resolution.x;
                int h = (int)resolution.y;

                // The shader defines GROUP_SIZE_1D = 8.
                cmd.DispatchCompute(m_VolumeVoxelizationCS, kernel, (w + 7) / 8, (h + 7) / 8, 1);
            }
        }

        // Ref: https://en.wikipedia.org/wiki/Close-packing_of_equal_spheres
        // The returned {x, y} coordinates (and all spheres) are all within the (-0.5, 0.5)^2 range.
        // The pattern has been rotated by 15 degrees to maximize the resolution along X and Y:
        // https://www.desmos.com/calculator/kcpfvltz7c
        static Vector2[] GetHexagonalClosePackedSpheres7()
        {
            Vector2[] coords = new Vector2[7];

            float r = 0.17054068870105443882f;
            float d = 2 * r;
            float s = r * Mathf.Sqrt(3);

            // Try to keep the weighted average as close to the center (0.5) as possible.
            //  (7)(5)    ( )( )    ( )( )    ( )( )    ( )( )    ( )(o)    ( )(x)    (o)(x)    (x)(x)
            // (2)(1)(3) ( )(o)( ) (o)(x)( ) (x)(x)(o) (x)(x)(x) (x)(x)(x) (x)(x)(x) (x)(x)(x) (x)(x)(x)
            //  (4)(6)    ( )( )    ( )( )    ( )( )    (o)( )    (x)( )    (x)(o)    (x)(x)    (x)(x)
            coords[0] = new Vector2(0,  0);
            coords[1] = new Vector2(-d,  0);
            coords[2] = new Vector2(d,  0);
            coords[3] = new Vector2(-r, -s);
            coords[4] = new Vector2(r,  s);
            coords[5] = new Vector2(r, -s);
            coords[6] = new Vector2(-r,  s);

            // Rotate the sampling pattern by 15 degrees.
            const float cos15 = 0.96592582628906828675f;
            const float sin15 = 0.25881904510252076235f;

            for (int i = 0; i < 7; i++)
            {
                Vector2 coord = coords[i];

                coords[i].x = coord.x * cos15 - coord.y * sin15;
                coords[i].y = coord.x * sin15 + coord.y * cos15;
            }

            return coords;
        }

        public void VolumetricLightingPass(HDCamera hdCamera, CommandBuffer cmd, uint frameIndex)
        {
            if (!hdCamera.frameSettings.enableVolumetric)
                return;

            var visualEnvironment = VolumeManager.instance.stack.GetComponent<VisualEnvironment>();
            if (visualEnvironment.fogType != FogType.Volumetric)
                return;

            using (new ProfilingSample(cmd, "Volumetric Lighting"))
            {
                // Only available in the Play Mode because all the frame counters in the Edit Mode are broken.
                bool enableClustered    = hdCamera.frameSettings.lightLoopSettings.enableTileAndCluster;
                bool enableReprojection = Application.isPlaying && hdCamera.camera.cameraType == CameraType.Game;

                int kernel;

                if (enableReprojection)
                {
                    kernel = m_VolumetricLightingCS.FindKernel(enableClustered ? "VolumetricLightingClusteredReproj"
                            : "VolumetricLightingBruteforceReproj");
                }
                else
                {
                    kernel = m_VolumetricLightingCS.FindKernel(enableClustered ? "VolumetricLightingClustered"
                            : "VolumetricLightingBruteforce");
                }

                var       frameParams = hdCamera.vBufferParams[0];
                Vector4   resolution  = frameParams.resolution;
                float     vFoV        = hdCamera.camera.fieldOfView * Mathf.Deg2Rad;
                // Compose the matrix which allows us to compute the world space view direction.
                Matrix4x4 transform   = HDUtils.ComputePixelCoordToWorldSpaceViewDirectionMatrix(vFoV, resolution, hdCamera.viewMatrix, false);

                Vector2[] xySeq = GetHexagonalClosePackedSpheres7();

                // This is a sequence of 7 equidistant numbers from 1/14 to 13/14.
                // Each of them is the centroid of the interval of length 2/14.
                // They've been rearranged in a sequence of pairs {small, large}, s.t. (small + large) = 1.
                // That way, the running average position is close to 0.5.
                // | 6 | 2 | 4 | 1 | 5 | 3 | 7 |
                // |   |   |   | o |   |   |   |
                // |   | o |   | x |   |   |   |
                // |   | x |   | x |   | o |   |
                // |   | x | o | x |   | x |   |
                // |   | x | x | x | o | x |   |
                // | o | x | x | x | x | x |   |
                // | x | x | x | x | x | x | o |
                // | x | x | x | x | x | x | x |
                float[] zSeq = {7.0f / 14.0f, 3.0f / 14.0f, 11.0f / 14.0f, 5.0f / 14.0f, 9.0f / 14.0f, 1.0f / 14.0f, 13.0f / 14.0f};

                int sampleIndex = (int)frameIndex % 7;

                // TODO: should we somehow reorder offsets in Z based on the offset in XY? S.t. the samples more evenly cover the domain.
                // Currently, we assume that they are completely uncorrelated, but maybe we should correlate them somehow.
                Vector4 offset = new Vector4(xySeq[sampleIndex].x, xySeq[sampleIndex].y, zSeq[sampleIndex], frameIndex);

                // Get the interpolated anisotropy value.
                var fog = VolumeManager.instance.stack.GetComponent<VolumetricFog>();

                // TODO: set 'm_VolumetricLightingPreset'.
                // TODO: set the constant buffer data only once.
                cmd.SetComputeMatrixParam(m_VolumetricLightingCS,         HDShaderIDs._VBufferCoordToViewDirWS, transform);
                cmd.SetComputeVectorParam(m_VolumetricLightingCS,         HDShaderIDs._VBufferSampleOffset,     offset);
                cmd.SetComputeFloatParam(m_VolumetricLightingCS,         HDShaderIDs._CornetteShanksConstant,  CornetteShanksPhasePartConstant(fog.anisotropy));
                cmd.SetComputeTextureParam(m_VolumetricLightingCS, kernel, HDShaderIDs._VBufferDensity,          m_DensityBufferHandle);// Read
                cmd.SetComputeTextureParam(m_VolumetricLightingCS, kernel, HDShaderIDs._VBufferLightingIntegral, m_LightingBufferHandle); // Write
                if (enableReprojection)
                {
                    cmd.SetComputeTextureParam(m_VolumetricLightingCS, kernel, HDShaderIDs._VBufferLightingHistory,  hdCamera.GetPreviousFrameRT((int)HDCameraFrameHistoryType.VolumetricLighting));// Read
                    cmd.SetComputeTextureParam(m_VolumetricLightingCS, kernel, HDShaderIDs._VBufferLightingFeedback, hdCamera.GetCurrentFrameRT((int)HDCameraFrameHistoryType.VolumetricLighting)); // Write
                }

                int w = (int)resolution.x;
                int h = (int)resolution.y;

                // The shader defines GROUP_SIZE_1D = 8.
                cmd.DispatchCompute(m_VolumetricLightingCS, kernel, (w + 7) / 8, (h + 7) / 8, 1);
            }
        }
    } // class VolumetricLightingModule
} // namespace UnityEngine.Experimental.Rendering.HDPipeline