BVH 建置器

D3D12_RAYTRACING_INSTANCE_DESC desc[3] = {};
// This example enables TAPU4 on every instance, this probably will not help performance, and will have a memory cost.
// You'll want a more targeted solution, such as marking up problematic instances manually by name,
// or enabling based on world-space-size or volume.
for (unsigned i=0; i < _countof(desc); ++i)
{
    desc[i].AccelerationStructure = blas->GetGPUVirtualAddress();
    // Use TAPU16 to break up the object by a further level.
    desc[i].Flags = D3D12XBOX_RAYTRACING_INSTANCE_FLAG_TAPU4;
    desc[i].InstanceMask = 0xff;
    desc[i].InstanceContributionToHitGroupIndex = 0;
    desc[i].Transform[0][0] = desc[i].Transform[1][1] = desc[i].Transform[2][2] = 1.0f;
}

ComPtr<ID3D12Resource> pInstanceDescs;
UploadBuffer(desc, sizeof(desc), &pInstanceDescs);

// Set special options
struct
{
D3D12XBOX_BUILD_RAYTRACING_ACCELERATION_STRUCTURE_PARAMS1 params1;
D3D12XBOX_BUILD_RAYTRACING_ACCELERATION_STRUCTURE_PARAMS2 params2;
} options;
D3D12XboxMakeDefault_D3D12XBOX_BUILD_RAYTRACING_ACCELERATION_STRUCTURE_PARAMS1(&options.params1);
D3D12XboxMakeDefault_D3D12XBOX_BUILD_RAYTRACING_ACCELERATION_STRUCTURE_PARAMS2(&options.params2);

// Access via a batched build
D3D12XBOX_BUILD_RAYTRACING_ACCELERATION_STRUCTURE_BATCHED_DESC2 tlasBatchedBuild {};
tlasBatchedBuild.NumPostbuildInfoDescs = 0;
tlasBatchedBuild.pPostbuildInfoDescs = nullptr;
tlasBatchedBuild.ExtraOptions.SizeInBytes = sizeof(options);
tlasBatchedBuild.ExtraOptions.pXdxrStreamDesc = &options;

// Tell the builder that this is how many outputs slots it can rely on!
// This should be the total number of instance nodes in your build.
// E.G. instances without a TAPU flag take up 1 element, instances with TAPU4 take up 4 elements, and instances with TAPU16 take up 16 elements.
options.params2.NumberOfElementsAllocated = _countof(desc) * 4;

D3D12_BUILD_RAYTRACING_ACCELERATION_STRUCTURE_DESC buildDesc {};
D3D12_BUILD_RAYTRACING_ACCELERATION_STRUCTURE_INPUTS &topLevelInputs = buildDesc.Inputs;
topLevelInputs.DescsLayout = D3D12_ELEMENTS_LAYOUT_ARRAY;
topLevelInputs.Flags = D3D12_RAYTRACING_ACCELERATION_STRUCTURE_BUILD_FLAG_PREFER_FAST_TRACE;
// This is the number of instance descs, NOT multiplied by the TAPU factor.
topLevelInputs.NumDescs = _countof(desc); 
topLevelInputs.Type = D3D12_RAYTRACING_ACCELERATION_STRUCTURE_TYPE_TOP_LEVEL;
topLevelInputs.InstanceDescs = pInstanceDescs->GetGPUVirtualAddress();

// Get total size required including additional nodes for TAPU.
D3D12_RAYTRACING_ACCELERATION_STRUCTURE_PREBUILD_INFO topLevelPrebuildInfo;
m_pDxrDevice->GetRaytracingAccelerationStructurePrebuildInfoX(&topLevelInputs,
&tlasBatchedBuild.ExtraOptions,
&topLevelPrebuildInfo);

ComPtr<ID3D12Resource> pScratchResource;
ComPtr<ID3D12Resource> tlas;

AllocateRawUAVBuffer(topLevelPrebuildInfo.ScratchDataSizeInBytes, &pScratchResource);
auto tlasPointer = AllocateAccelerationStructureBuffer(topLevelPrebuildInfo.ResultDataMaxSizeInBytes, &tlas);

buildDesc.DestAccelerationStructureData = tlasPointer;
buildDesc.ScratchAccelerationStructureData = pScratchResource->GetGPUVirtualAddress();

// Make sure the number of incoming instances is clearly communicated!
tlasBatchedBuild.Desc = buildDesc;

// Build acceleration structure.
((ID3D12GraphicsCommandList6*)m_CommandList.Get())->BuildRaytracingAccelerationStructureBatched2X(1, &tlasBatchedBuild);