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

//--------------------------------------------------------------------------------------------------
// Definitions
//--------------------------------------------------------------------------------------------------

#pragma kernel VolumeVoxelizationBruteforceMQ VolumeVoxelization=VolumeVoxelizationBruteforceMQ VL_PRESET_MQ LIGHTLOOP_SINGLE_PASS
#pragma kernel VolumeVoxelizationClusteredMQ  VolumeVoxelization=VolumeVoxelizationClusteredMQ  VL_PRESET_MQ LIGHTLOOP_TILE_PASS   USE_CLUSTERED_LIGHTLIST
#pragma kernel VolumeVoxelizationBruteforceHQ VolumeVoxelization=VolumeVoxelizationBruteforceHQ VL_PRESET_HQ LIGHTLOOP_SINGLE_PASS
#pragma kernel VolumeVoxelizationClusteredHQ  VolumeVoxelization=VolumeVoxelizationClusteredHQ  VL_PRESET_HQ LIGHTLOOP_TILE_PASS   USE_CLUSTERED_LIGHTLIST

// #pragma enable_d3d11_debug_symbols

#ifdef VL_PRESET_MQ
    // E.g. for 1080p: (1920/8)x(1080/8)x(64)  =  2,073,600 voxels
    #define VBUFFER_TILE_SIZE 8
#endif
#ifdef VL_PRESET_HQ
    // E.g. for 1080p: (1920/4)x(1080/4)x(128) = 16,588,800 voxels
    #define VBUFFER_TILE_SIZE 4
#endif

#define GROUP_SIZE_1D     8
#define SOFT_VOXELIZATION 1 // Hack which attempts to determine the partial coverage of the voxel

#include "../../ShaderPass/ShaderPass.cs.hlsl"
#define SHADERPASS SHADERPASS_VOLUME_VOXELIZATION

//--------------------------------------------------------------------------------------------------
// Included headers
//--------------------------------------------------------------------------------------------------

#include "CoreRP/ShaderLibrary/Common.hlsl"
#include "CoreRP/Utilities/GeometryUtils.cs.hlsl"

#include "../../ShaderVariables.hlsl"
#include "VolumetricLighting.cs.hlsl"

#define UNITY_MATERIAL_VOLUMETRIC // Define before including Lighting.hlsl and Material.hlsl
#include "../Lighting.hlsl"       // Includes Material.hlsl

#pragma only_renderers d3d11 ps4 xboxone vulkan metal

//--------------------------------------------------------------------------------------------------
// Inputs & outputs
//--------------------------------------------------------------------------------------------------

StructuredBuffer<OrientedBBox>            _VolumeBounds;
StructuredBuffer<DensityVolumeData>       _VolumeData;

TEXTURE3D(_VolumeMaskAtlas);

RW_TEXTURE3D(float4, _VBufferDensity); // RGB = sqrt(scattering), A = sqrt(extinction)

// TODO: avoid creating another Constant Buffer...
CBUFFER_START(UnityVolumetricLighting)
    float4x4 _VBufferCoordToViewDirWS; // Actually just 3x3, but Unity can only set 4x4
    float4   _VBufferSampleOffset;     // Not used by this shader
    float    _CornetteShanksConstant;  // Not used by this shader
    uint     _NumVisibleDensityVolumes;
    float4   _VolumeMaskDimensions;    //x = 1/numTextures , y = width, z = depth = width * numTextures, w = maxLod
CBUFFER_END

//--------------------------------------------------------------------------------------------------
// Implementation
//--------------------------------------------------------------------------------------------------
float SampleVolumeMask(DensityVolumeData volumeData, float3 voxelCenterUVW, float3 duvw_dx, float3 duvw_dy, float3 duvw_dz)
{
    // 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;

    float offset = volumeData.textureIndex * rcpNumTextures;
    voxelCenterUVW.z = voxelCenterUVW.z * rcpNumTextures + offset;

    // TODO: expose the LoD bias parameter.
    float lod = ComputeTextureLOD(duvw_dx, duvw_dy, duvw_dz, textureWidth);
    lod = clamp(lod, 0, maxLod);

    // TODO: bugfix.
    // Note that this clamping to edge doesn't quite work.
    // First of all, the distance to the edge should depend on the LoD.
    // 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.
    // For now, we choose the second option.
    // 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);
    
    // Reminder: still no filtering across the the wrap in Z.
    return SAMPLE_TEXTURE3D_LOD(_VolumeMaskAtlas, s_trilinear_repeat_sampler, voxelCenterUVW, lod).a;
}


void FillVolumetricDensityBuffer(PositionInputs posInput, float3 rayOriginWS, float3 rayUnDirWS,
                                 float3 voxelAxisRight, float3 voxelAxisUp, float3 voxelAxisForward)
{
    float n  = _VBufferDepthDecodingParams.x + _VBufferDepthDecodingParams.z;
    float z0 = n;                    // Start the computation from the near plane
    float de = _VBufferSliceCount.y; // Log-encoded distance between slices

#ifdef USE_CLUSTERED_LIGHTLIST
    // The voxel can overlap up to 2 light clusters along Z, so we have to iterate over both.
    // TODO: implement Z-binning which makes Z-range queries easy.
    uint volumeStarts[2], volumeCounts[2];

    GetCountAndStartCluster(posInput.tileCoord, GetLightClusterIndex(posInput.tileCoord, z0),
                            LIGHTCATEGORY_DENSITY_VOLUME, volumeStarts[0], volumeCounts[0]);
#endif // USE_CLUSTERED_LIGHTLIST

    for (uint slice = 0; slice < (uint)_VBufferSliceCount.x; slice++)
    {
        uint3 voxelCoord = uint3(posInput.positionSS, slice);

        float e1 = slice * de + de; // (slice + 1) / sliceCount
    #if defined(SHADER_API_METAL)
        // Warning: this compiles, but it's nonsense. Use DecodeLogarithmicDepthGeneralized().
        float z1 = DecodeLogarithmicDepth(e1, _VBufferDepthDecodingParams);
    #else
        float z1 = DecodeLogarithmicDepthGeneralized(e1, _VBufferDepthDecodingParams);
    #endif

        float  halfDZ        = 0.5 * (z1 - z0);
        float  z             = z0 + halfDZ;
        float3 voxelCenterWS = rayOriginWS + z * rayUnDirWS; // Works due to the length of of the dir

        // TODO: define a function ComputeGlobalFogCoefficients(float3 voxelCenterWS),
        // which allows procedural definition of extinction and scattering.
        float3 voxelScattering = _GlobalScattering;
        float  voxelExtinction = _GlobalExtinction;

    #ifdef USE_CLUSTERED_LIGHTLIST

        GetCountAndStartCluster(posInput.tileCoord, GetLightClusterIndex(posInput.tileCoord, z1),
                                LIGHTCATEGORY_DENSITY_VOLUME, volumeStarts[1], volumeCounts[1]);

        // Iterate over all volumes within 2 (not necessarily unique) clusters overlapping the voxel along Z.
        // We need to skip duplicates, but it's not too difficult since volumes are sorted by index.
        uint i = 0, j = 0;

        if (i < volumeCounts[0] || j < volumeCounts[1])
        {
            // At least one of the clusters is non-empty.
            uint volumeIndices[2];

            // Fetch two initial indices from both clusters.
            volumeIndices[0] = FetchIndexWithBoundsCheck(volumeStarts[0], volumeCounts[0], i);
            volumeIndices[1] = FetchIndexWithBoundsCheck(volumeStarts[1], volumeCounts[1], j);

            do
            {
                // Process volumes in order.
                uint volumeIndex = min(volumeIndices[0], volumeIndices[1]);

    #else  // USE_CLUSTERED_LIGHTLIST
        {
            for (uint volumeIndex = 0; volumeIndex < _NumVisibleDensityVolumes; volumeIndex++)
            {
    #endif // USE_CLUSTERED_LIGHTLIST

                const OrientedBBox obb = _VolumeBounds[volumeIndex];

                const float3x3 obbFrame   = float3x3(obb.right, obb.up, cross(obb.up, obb.right));
                const float3   obbExtents = float3(obb.extentX, obb.extentY, obb.extentZ);

                // Express the voxel center in the local coordinate system of the box.
                const float3 voxelCenterBS = mul(voxelCenterWS - obb.center, transpose(obbFrame));
                const float3 voxelCenterCS = (voxelCenterBS / obbExtents);

                const float3 voxelAxisRightBS   = mul(voxelAxisRight,   transpose(obbFrame));
                const float3 voxelAxisUpBS      = mul(voxelAxisUp,      transpose(obbFrame));
                const float3 voxelAxisForwardBS = mul(voxelAxisForward, transpose(obbFrame));

            #if SOFT_VOXELIZATION
                // We need to determine which is the face closest to 'voxelCenterBS'.
                float minFaceDist = abs(obbExtents.x - abs(voxelCenterBS.x));

                // TODO: use v_cubeid_f32.
                uint axisIndex; float faceDist;

                faceDist    = abs(obbExtents.y - abs(voxelCenterBS.y));
                axisIndex   = (faceDist < minFaceDist) ? 1 : 0;
                minFaceDist = min(faceDist, minFaceDist);

                faceDist    = abs(obbExtents.z - abs(voxelCenterBS.z));
                axisIndex   = (faceDist < minFaceDist) ? 2 : axisIndex;

                float3 N = float3(axisIndex == 0 ? 1 : 0, axisIndex == 1 ? 1 : 0, axisIndex == 2 ? 1 : 0);

                // We have determined the normal of the closest face.
                // We now have to construct the diagonal of the voxel with the longest extent along this normal.
                float3 minDiagPointBS, maxDiagPointBS;

                // Start at the center of the voxel.
                minDiagPointBS = maxDiagPointBS = voxelCenterBS;

                bool  normalFwd  = dot(voxelAxisForwardBS, N) >= 0;
                float mulForward = normalFwd ? halfDZ : -halfDZ;
                float mulMin     = normalFwd ? z0 : z1;
                float mulMax     = normalFwd ? z1 : z0;

                minDiagPointBS -= mulForward * voxelAxisForwardBS;
                maxDiagPointBS += mulForward * voxelAxisForwardBS;

                float mulUp = dot(voxelAxisUpBS, N) >= 0 ? 1 : -1;

                minDiagPointBS -= (mulMin * mulUp) * voxelAxisUpBS;
                maxDiagPointBS += (mulMax * mulUp) * voxelAxisUpBS;

                float mulRight = dot(voxelAxisRightBS, N) >= 0 ? 1 : -1;

                minDiagPointBS -= (mulMin * mulRight) * voxelAxisRightBS;
                maxDiagPointBS += (mulMax * mulRight) * voxelAxisRightBS;

                // We want to determine the fractional overlap of the diagonal and the box.
                float3 diagOriginBS = minDiagPointBS;
                float3 diagUnDirBS  = maxDiagPointBS - minDiagPointBS;

                float tEntr, tExit;

                IntersectRayAABB(diagOriginBS, diagUnDirBS,
                                 -obbExtents, obbExtents,
                                 0, 1,
                                 tEntr, tExit);

                float overlapFraction = tExit - tEntr;

            #else  // SOFT_VOXELIZATION

                bool overlap = Max3(abs(voxelCenterCS.x), abs(voxelCenterCS.y), abs(voxelCenterCS.z)) <= 1;

                float overlapFraction = overlap ? 1 : 0;

            #endif // SOFT_VOXELIZATION

                if (overlapFraction > 0)
                {
                    float densityMask = 1.0f;
                    //Sample the volumeMask
                    if (_VolumeData[volumeIndex].textureIndex != -1)
                    {
                        // We divide extents (half-sizes) by extents here, obtaining full-sized gradients.
                        float3 voxelGradRightUVW   = z      * voxelAxisRightBS   / obbExtents;
                        float3 voxelGradUpUVW      = z      * voxelAxisUpBS      / obbExtents;
                        float3 voxelGradForwardUVW = halfDZ * voxelAxisForwardBS / obbExtents;
                        float3 voxelCenterUVW      = voxelCenterCS * 0.5 + 0.5;

                        densityMask = SampleVolumeMask(_VolumeData[volumeIndex], voxelCenterUVW, voxelGradRightUVW, voxelGradUpUVW, voxelGradForwardUVW);
                    }

                    // There is an overlap. Sample the 3D texture, or load the constant value.
                    voxelScattering += overlapFraction * _VolumeData[volumeIndex].scattering * densityMask;
                    voxelExtinction += overlapFraction * _VolumeData[volumeIndex].extinction * densityMask;
                }

    #ifndef USE_CLUSTERED_LIGHTLIST
            }
        }
    #else // USE_CLUSTERED_LIGHTLIST

                // Advance to the next volume in one (or both at the same time) clusters.
                if (volumeIndex == volumeIndices[0])
                {
                    i++;
                    volumeIndices[0] = FetchIndexWithBoundsCheck(volumeStarts[0], volumeCounts[0], i);
                }

                if (volumeIndex == volumeIndices[1])
                {
                    j++;
                    volumeIndices[1] = FetchIndexWithBoundsCheck(volumeStarts[1], volumeCounts[1], j);
                }
            } while (i < volumeCounts[0] || j < volumeCounts[1]);
        }

        // We don't need to carry over the cluster index, only the start and the count.
        volumeStarts[0] = volumeStarts[1];
        volumeCounts[0] = volumeCounts[1];

    #endif // USE_CLUSTERED_LIGHTLIST

        _VBufferDensity[voxelCoord] = float4(voxelScattering, voxelExtinction);

        z0 = z1;
    }
}

[numthreads(GROUP_SIZE_1D, GROUP_SIZE_1D, 1)]
void VolumeVoxelization(uint2 groupId       : SV_GroupID,
                        uint2 groupThreadId : SV_GroupThreadID)
{
    // Perform compile-time checks.
    if (!IsPower2(VBUFFER_TILE_SIZE) || !IsPower2(TILE_SIZE_CLUSTERED)) return;

    uint2 groupCoord  = groupThreadId;
    uint2 groupOffset = groupId * GROUP_SIZE_1D;
    uint2 voxelCoord  = groupOffset + groupCoord;
    uint2 tileCoord   = voxelCoord * VBUFFER_TILE_SIZE / TILE_SIZE_CLUSTERED;

    uint voxelsPerClusterTile = Sq((uint)(TILE_SIZE_CLUSTERED / VBUFFER_TILE_SIZE));

    if (voxelsPerClusterTile >= 64)
    {
        // TODO: this is a compile-time test, make sure the compiler actually scalarizes.
        tileCoord = groupOffset * VBUFFER_TILE_SIZE / TILE_SIZE_CLUSTERED;
    }

    UNITY_BRANCH
    if (voxelCoord.x >= (uint)_VBufferResolution.x ||
        voxelCoord.y >= (uint)_VBufferResolution.y)
    {
        return;
    }

    // Reminder: our voxel is a skewed pyramid frustum with square front and back faces.

    // Compute 3x orthogonal directions.
    float2 centerCoord = voxelCoord + float2( 0.5,  0.5);
    float2 leftCoord   = voxelCoord + float2(-0.5,  0.5);
    float2 upCoord     = voxelCoord + float2( 0.5, -0.5);

    // TODO: avoid 2x matrix multiplications by precomputing the world-space offset on the vs_Z=1 plane.
    // Compute 3x ray directions s.t. its ViewSpace(rayDirWS).z = 1.
    float3 centerDirWS = mul(-float3(centerCoord, 1), (float3x3)_VBufferCoordToViewDirWS);
    float3 leftDirWS   = mul(-float3(leftCoord,   1), (float3x3)_VBufferCoordToViewDirWS);
    float3 upDirWS     = mul(-float3(upCoord,     1), (float3x3)_VBufferCoordToViewDirWS);

    // Compute the axes of the voxel. These are not normalized, but rather computed to scale with Z.
    // This coordinate system is generally not orthogonal.
    float3 voxelAxisForward = centerDirWS;
    float3 voxelAxisUp      = 0.5 * (upDirWS - centerDirWS);
    float3 voxelAxisRight   = 0.5 * (centerDirWS - leftDirWS);

    PositionInputs posInput = GetPositionInput(voxelCoord, _VBufferResolution.zw, tileCoord);

    FillVolumetricDensityBuffer(posInput, GetCurrentViewPosition(), centerDirWS,
                                voxelAxisRight, voxelAxisUp, voxelAxisForward);
}