d3d12龍書閱讀----繪製幾何體（下）

dyccyber發表於2024-05-10
3D

d3d12龍書閱讀----繪製幾何體（下）

本節在上一節的基礎上，對整個繪製過程進行最佳化，將繪製單個幾何體的內容擴充到了多個幾何體，同時對根簽名進行了進一步地探索。

幀資源

在之前繪製每幀的結尾，我們都要使用flushingcommandqueue方法，要一直等待gpu執行完所有命令，才會繼續繪製下一幀，此時cpu處於空閒時間，同時，在繪製每一幀的初始階段，gpu要等待cpu提交命令，此時gpu處於空閒時間
解決上述問題的一種方法是：
構建以cpu每幀都要更新的資源為陣列元素的環形陣列，這些資源被稱為幀資源，一般迴圈陣列由3個幀資源元素構成
當gpu在處理上一幀的命令時，cpu可以為下一幀更新資源，並構建並提交相應的命令列表，如果環形陣列有三個元素，則令cpu比gpu提前處理兩幀，這樣可以確保gpu持續工作
幀資源定義：

針對每個物體/幾何體的常量緩衝區定義
目前儲存的是每個物體的世界矩陣 即 模型矩陣 將物體從區域性座標系轉換到世界座標系 代表著物體的位置

struct ObjectConstants
{
    DirectX::XMFLOAT4X4 World = MathHelper::Identity4x4();
};
針對每次渲染過程（rendering pass）所要用到的資料
比如 觀察矩陣 投影矩陣 時間 等等
struct PassConstants
{
    DirectX::XMFLOAT4X4 View = MathHelper::Identity4x4();
    DirectX::XMFLOAT4X4 InvView = MathHelper::Identity4x4();
    DirectX::XMFLOAT4X4 Proj = MathHelper::Identity4x4();
    DirectX::XMFLOAT4X4 InvProj = MathHelper::Identity4x4();
    DirectX::XMFLOAT4X4 ViewProj = MathHelper::Identity4x4();
    DirectX::XMFLOAT4X4 InvViewProj = MathHelper::Identity4x4();
    DirectX::XMFLOAT3 EyePosW = { 0.0f, 0.0f, 0.0f };
    float cbPerObjectPad1 = 0.0f;
    DirectX::XMFLOAT2 RenderTargetSize = { 0.0f, 0.0f };
    DirectX::XMFLOAT2 InvRenderTargetSize = { 0.0f, 0.0f };
    float NearZ = 0.0f;
    float FarZ = 0.0f;
    float TotalTime = 0.0f;
    float DeltaTime = 0.0f;
};
頂點定義
struct Vertex
{
    DirectX::XMFLOAT3 Pos;
    DirectX::XMFLOAT4 Color;
};

儲存cpu為一幀構建命令列表所需資源
struct FrameResource
{
public:
    
    FrameResource(ID3D12Device* device, UINT passCount, UINT objectCount);
    FrameResource(const FrameResource& rhs) = delete;
    FrameResource& operator=(const FrameResource& rhs) = delete;
    ~FrameResource();

    每一幀都要有自己的命令分配器
    因為當上一幀的gpu還在處理命令時 我們不能重置命令分配器
    Microsoft::WRL::ComPtr<ID3D12CommandAllocator> CmdListAlloc;

    同理 每個幀資源也要有自己的常量緩衝區
    std::unique_ptr<UploadBuffer<PassConstants>> PassCB = nullptr;
    std::unique_ptr<UploadBuffer<ObjectConstants>> ObjectCB = nullptr;

    圍欄點可以幫助檢測 gpu是否仍然使用著幀資源
    UINT64 Fence = 0;
};

可以看到我們在幀資源中將常量緩衝區分為pass 與 object， 這是基於資源的更新頻率對常量資源進行分組，每次渲染過程我們都要更新pass緩衝區，而對於object來說，只有當發生變化的時候才需要更新，具體程式碼我們待會再看。

回到cpu與gpu的同步上來，首先建立初始化幀資源陣列：

void ShapesApp::BuildFrameResources()
{
    for(int i = 0; i < gNumFrameResources; ++i)
    {
        mFrameResources.push_back(std::make_unique<FrameResource>(md3dDevice.Get(),
            1, (UINT)mAllRitems.size()));
            其中1代表著一個幀資源1個pass緩衝區 第二個是所有渲染物體的數目
    }
}

cpu端更新第n幀：

void ShapesApp::Update(const GameTimer& gt)
{
    OnKeyboardInput(gt);
	UpdateCamera(gt);

    迴圈幀資源陣列
    mCurrFrameResourceIndex = (mCurrFrameResourceIndex + 1) % gNumFrameResources;
    mCurrFrameResource = mFrameResources[mCurrFrameResourceIndex].get();

    等待gpu完成圍欄點之前的所有命令
    if(mCurrFrameResource->Fence != 0 && mFence->GetCompletedValue() < mCurrFrameResource->Fence)
    {
        HANDLE eventHandle = CreateEventEx(nullptr, false, false, EVENT_ALL_ACCESS);
        ThrowIfFailed(mFence->SetEventOnCompletion(mCurrFrameResource->Fence, eventHandle));
        WaitForSingleObject(eventHandle, INFINITE);
        CloseHandle(eventHandle);
    }
    更新常量緩衝區
	UpdateObjectCBs(gt);
	UpdateMainPassCB(gt);
}

繪製第n幀：

void ShapesApp::draw(const GameTimer& gt){
新增圍欄值 將命令標記到此圍欄點
mCurrFrameResource->Fence = ++mCurrentFence;

向命令佇列中新增一條設定新圍欄點的命令
由於這條命令要交給gpu處理，所以gpu處理完signal之前的所有命令之前，它不會設定新的圍欄點
mCommandQueue->Signal(mFence.Get(), mCurrentFence);
}

其實這種方法也有著缺陷，如果gpu處理命令的速度大於cpu提交命令列表的速度，則還是要等待cpu，理想的情況是cpu處理幀的速度大於gpu，這樣cpu可以有空閒時間來處理遊戲邏輯的其它部分，此方法的最大好處是cpu可以持續向gpu提供資料

渲染項

渲染項是一個輕量型結構 用於儲存繪製物體所需要資料：

struct RenderItem
{
	RenderItem() = default;

    世界矩陣
    XMFLOAT4X4 World = MathHelper::Identity4x4();

	// 一個髒標記用於記錄是否需要更新物體緩衝區 因為每個幀資源都有各自獨立的物體緩衝區 所以髒標記的數目要設定和幀資源數目一致
	int NumFramesDirty = gNumFrameResources;

	// 當前渲染項對應object緩衝區索引
	UINT ObjCBIndex = -1;

    該渲染項參與繪製的幾何體
	MeshGeometry* Geo = nullptr;

    //圖元拓撲型別
    D3D12_PRIMITIVE_TOPOLOGY PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;

    // DrawIndexedInstanced 方法的引數
    UINT IndexCount = 0;
    UINT StartIndexLocation = 0;
    int BaseVertexLocation = 0;
};

渲染項的具體使用之後介紹

渲染過程中用到的常量資料

我們需要更新hlsl中用到的cbuffer：

cbuffer cbPerObject : register(b0)
{
	float4x4 gWorld; 
};

cbuffer cbPass : register(b1)
{
    float4x4 gView;
    float4x4 gInvView;
    float4x4 gProj;
    float4x4 gInvProj;
    float4x4 gViewProj;
    float4x4 gInvViewProj;
    float3 gEyePosW;
    float cbPerObjectPad1;
    float2 gRenderTargetSize;
    float2 gInvRenderTargetSize;
    float gNearZ;
    float gFarZ;
    float gTotalTime;
    float gDeltaTime;
};

更新object緩衝區 與 pass緩衝區 這裡利用了前一節介紹的uploadbuffer的方法 從cpu端更新資料：

void ShapesApp::UpdateObjectCBs(const GameTimer& gt)
{
	auto currObjectCB = mCurrFrameResource->ObjectCB.get();
	for(auto& e : mAllRitems)
	{
		每個幀資源都需要更新物體緩衝區
		if(e->NumFramesDirty > 0)
		{
			XMMATRIX world = XMLoadFloat4x4(&e->World);

			ObjectConstants objConstants;
			XMStoreFloat4x4(&objConstants.World, XMMatrixTranspose(world));

			currObjectCB->CopyData(e->ObjCBIndex, objConstants);

			// Next FrameResource need to be updated too.
			e->NumFramesDirty--;
		}
	}
}

void ShapesApp::UpdateMainPassCB(const GameTimer& gt)
{
	XMMATRIX view = XMLoadFloat4x4(&mView);
	XMMATRIX proj = XMLoadFloat4x4(&mProj);

	XMMATRIX viewProj = XMMatrixMultiply(view, proj);
	XMMATRIX invView = XMMatrixInverse(&XMMatrixDeterminant(view), view);
	XMMATRIX invProj = XMMatrixInverse(&XMMatrixDeterminant(proj), proj);
	XMMATRIX invViewProj = XMMatrixInverse(&XMMatrixDeterminant(viewProj), viewProj);

	XMStoreFloat4x4(&mMainPassCB.View, XMMatrixTranspose(view));
	XMStoreFloat4x4(&mMainPassCB.InvView, XMMatrixTranspose(invView));
	XMStoreFloat4x4(&mMainPassCB.Proj, XMMatrixTranspose(proj));
	XMStoreFloat4x4(&mMainPassCB.InvProj, XMMatrixTranspose(invProj));
	XMStoreFloat4x4(&mMainPassCB.ViewProj, XMMatrixTranspose(viewProj));
	XMStoreFloat4x4(&mMainPassCB.InvViewProj, XMMatrixTranspose(invViewProj));
	mMainPassCB.EyePosW = mEyePos;
	mMainPassCB.RenderTargetSize = XMFLOAT2((float)mClientWidth, (float)mClientHeight);
	mMainPassCB.InvRenderTargetSize = XMFLOAT2(1.0f / mClientWidth, 1.0f / mClientHeight);
	mMainPassCB.NearZ = 1.0f;
	mMainPassCB.FarZ = 1000.0f;
	mMainPassCB.TotalTime = gt.TotalTime();
	mMainPassCB.DeltaTime = gt.DeltaTime();

	auto currPassCB = mCurrFrameResource->PassCB.get();
	currPassCB->CopyData(0, mMainPassCB);
}

繪製多種幾何體

在這裡就不再介紹柱體 球體 正方體的過程 設計到一些幾何知識
直接進入幾何體的繪製階段

建立頂點與索引緩衝區

將所有幾何體的頂點緩衝區 與 索引緩衝區，合成一個大的頂點緩衝區與 索引緩衝區，之後使用drawindexinstanced方法繪製 需要記錄每個幾何體起始索引 索引數 以及起始頂點

void ShapesApp::BuildShapeGeometry()
{
    GeometryGenerator geoGen;
	GeometryGenerator::MeshData box = geoGen.CreateBox(1.5f, 0.5f, 1.5f, 3);
	GeometryGenerator::MeshData grid = geoGen.CreateGrid(20.0f, 30.0f, 60, 40);
	GeometryGenerator::MeshData sphere = geoGen.CreateSphere(0.5f, 20, 20);
	GeometryGenerator::MeshData cylinder = geoGen.CreateCylinder(0.5f, 0.3f, 3.0f, 20, 20);

	// 計算各幾何體的起始頂點
	UINT boxVertexOffset = 0;
	UINT gridVertexOffset = (UINT)box.Vertices.size();
	UINT sphereVertexOffset = gridVertexOffset + (UINT)grid.Vertices.size();
	UINT cylinderVertexOffset = sphereVertexOffset + (UINT)sphere.Vertices.size();

	// 儲存起始索引
	UINT boxIndexOffset = 0;
	UINT gridIndexOffset = (UINT)box.Indices32.size();
	UINT sphereIndexOffset = gridIndexOffset + (UINT)grid.Indices32.size();
	UINT cylinderIndexOffset = sphereIndexOffset + (UINT)sphere.Indices32.size();

    定義各子網格結構體
	SubmeshGeometry boxSubmesh;
	boxSubmesh.IndexCount = (UINT)box.Indices32.size();
	boxSubmesh.StartIndexLocation = boxIndexOffset;
	boxSubmesh.BaseVertexLocation = boxVertexOffset;

	SubmeshGeometry gridSubmesh;
	gridSubmesh.IndexCount = (UINT)grid.Indices32.size();
	gridSubmesh.StartIndexLocation = gridIndexOffset;
	gridSubmesh.BaseVertexLocation = gridVertexOffset;

	SubmeshGeometry sphereSubmesh;
	sphereSubmesh.IndexCount = (UINT)sphere.Indices32.size();
	sphereSubmesh.StartIndexLocation = sphereIndexOffset;
	sphereSubmesh.BaseVertexLocation = sphereVertexOffset;

	SubmeshGeometry cylinderSubmesh;
	cylinderSubmesh.IndexCount = (UINT)cylinder.Indices32.size();
	cylinderSubmesh.StartIndexLocation = cylinderIndexOffset;
	cylinderSubmesh.BaseVertexLocation = cylinderVertexOffset;

    將各頂點 各索引合併
    子網格合併為一個大的meshgeometry
	auto totalVertexCount =
		box.Vertices.size() +
		grid.Vertices.size() +
		sphere.Vertices.size() +
		cylinder.Vertices.size();

	std::vector<Vertex> vertices(totalVertexCount);

	UINT k = 0;
	for(size_t i = 0; i < box.Vertices.size(); ++i, ++k)
	{
		vertices[k].Pos = box.Vertices[i].Position;
        vertices[k].Color = XMFLOAT4(DirectX::Colors::DarkGreen);
	}

	for(size_t i = 0; i < grid.Vertices.size(); ++i, ++k)
	{
		vertices[k].Pos = grid.Vertices[i].Position;
        vertices[k].Color = XMFLOAT4(DirectX::Colors::ForestGreen);
	}

	for(size_t i = 0; i < sphere.Vertices.size(); ++i, ++k)
	{
		vertices[k].Pos = sphere.Vertices[i].Position;
        vertices[k].Color = XMFLOAT4(DirectX::Colors::Crimson);
	}

	for(size_t i = 0; i < cylinder.Vertices.size(); ++i, ++k)
	{
		vertices[k].Pos = cylinder.Vertices[i].Position;
		vertices[k].Color = XMFLOAT4(DirectX::Colors::SteelBlue);
	}

	std::vector<std::uint16_t> indices;
	indices.insert(indices.end(), std::begin(box.GetIndices16()), std::end(box.GetIndices16()));
	indices.insert(indices.end(), std::begin(grid.GetIndices16()), std::end(grid.GetIndices16()));
	indices.insert(indices.end(), std::begin(sphere.GetIndices16()), std::end(sphere.GetIndices16()));
	indices.insert(indices.end(), std::begin(cylinder.GetIndices16()), std::end(cylinder.GetIndices16()));

    const UINT vbByteSize = (UINT)vertices.size() * sizeof(Vertex);
    const UINT ibByteSize = (UINT)indices.size()  * sizeof(std::uint16_t);

	auto geo = std::make_unique<MeshGeometry>();
	geo->Name = "shapeGeo";

	ThrowIfFailed(D3DCreateBlob(vbByteSize, &geo->VertexBufferCPU));
	CopyMemory(geo->VertexBufferCPU->GetBufferPointer(), vertices.data(), vbByteSize);

	ThrowIfFailed(D3DCreateBlob(ibByteSize, &geo->IndexBufferCPU));
	CopyMemory(geo->IndexBufferCPU->GetBufferPointer(), indices.data(), ibByteSize);

	geo->VertexBufferGPU = d3dUtil::CreateDefaultBuffer(md3dDevice.Get(),
		mCommandList.Get(), vertices.data(), vbByteSize, geo->VertexBufferUploader);

	geo->IndexBufferGPU = d3dUtil::CreateDefaultBuffer(md3dDevice.Get(),
		mCommandList.Get(), indices.data(), ibByteSize, geo->IndexBufferUploader);

	geo->VertexByteStride = sizeof(Vertex);
	geo->VertexBufferByteSize = vbByteSize;
	geo->IndexFormat = DXGI_FORMAT_R16_UINT;
	geo->IndexBufferByteSize = ibByteSize;

	geo->DrawArgs["box"] = boxSubmesh;
	geo->DrawArgs["grid"] = gridSubmesh;
	geo->DrawArgs["sphere"] = sphereSubmesh;
	geo->DrawArgs["cylinder"] = cylinderSubmesh;

	mGeometries[geo->Name] = std::move(geo);
}

定義具體渲染項

在完成構建幾何體之後 我們根據上一步建立的meshgeometry 來提取submeshgeometry 然後 裡面的資訊 根據需要建立相應的渲染項 並填寫相應的內容

void ShapesApp::BuildRenderItems()
{
	auto boxRitem = std::make_unique<RenderItem>();
	XMStoreFloat4x4(&boxRitem->World, XMMatrixScaling(2.0f, 2.0f, 2.0f)*XMMatrixTranslation(0.0f, 0.5f, 0.0f));
	boxRitem->ObjCBIndex = 0;
	boxRitem->Geo = mGeometries["shapeGeo"].get();
	boxRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
	boxRitem->IndexCount = boxRitem->Geo->DrawArgs["box"].IndexCount;
	boxRitem->StartIndexLocation = boxRitem->Geo->DrawArgs["box"].StartIndexLocation;
	boxRitem->BaseVertexLocation = boxRitem->Geo->DrawArgs["box"].BaseVertexLocation;
	mAllRitems.push_back(std::move(boxRitem));

    auto gridRitem = std::make_unique<RenderItem>();
    gridRitem->World = MathHelper::Identity4x4();
	gridRitem->ObjCBIndex = 1;
	gridRitem->Geo = mGeometries["shapeGeo"].get();
	gridRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
    gridRitem->IndexCount = gridRitem->Geo->DrawArgs["grid"].IndexCount;
    gridRitem->StartIndexLocation = gridRitem->Geo->DrawArgs["grid"].StartIndexLocation;
    gridRitem->BaseVertexLocation = gridRitem->Geo->DrawArgs["grid"].BaseVertexLocation;
	mAllRitems.push_back(std::move(gridRitem));

	UINT objCBIndex = 2;
	for(int i = 0; i < 5; ++i)
	{
		auto leftCylRitem = std::make_unique<RenderItem>();
		auto rightCylRitem = std::make_unique<RenderItem>();
		auto leftSphereRitem = std::make_unique<RenderItem>();
		auto rightSphereRitem = std::make_unique<RenderItem>();

		XMMATRIX leftCylWorld = XMMatrixTranslation(-5.0f, 1.5f, -10.0f + i*5.0f);
		XMMATRIX rightCylWorld = XMMatrixTranslation(+5.0f, 1.5f, -10.0f + i*5.0f);

		XMMATRIX leftSphereWorld = XMMatrixTranslation(-5.0f, 3.5f, -10.0f + i*5.0f);
		XMMATRIX rightSphereWorld = XMMatrixTranslation(+5.0f, 3.5f, -10.0f + i*5.0f);

		XMStoreFloat4x4(&leftCylRitem->World, rightCylWorld);
		leftCylRitem->ObjCBIndex = objCBIndex++;
		leftCylRitem->Geo = mGeometries["shapeGeo"].get();
		leftCylRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
		leftCylRitem->IndexCount = leftCylRitem->Geo->DrawArgs["cylinder"].IndexCount;
		leftCylRitem->StartIndexLocation = leftCylRitem->Geo->DrawArgs["cylinder"].StartIndexLocation;
		leftCylRitem->BaseVertexLocation = leftCylRitem->Geo->DrawArgs["cylinder"].BaseVertexLocation;
        此處省略
		
	}

	for(auto& e : mAllRitems)
		mOpaqueRitems.push_back(e.get());
}

定義常量緩衝區檢視

之後由於我們現在有3個pass常量緩衝區 3n個object常量緩衝區 總共3n+3個常量緩衝區 所以就需要 3n+3個cbv 同時也要擴充描述符堆的大小：

void ShapesApp::BuildDescriptorHeaps()
{
    UINT objCount = (UINT)mOpaqueRitems.size();

    UINT numDescriptors = (objCount+1) * gNumFrameResources;
    mPassCbvOffset = objCount * gNumFrameResources;

    D3D12_DESCRIPTOR_HEAP_DESC cbvHeapDesc;
    cbvHeapDesc.NumDescriptors = numDescriptors;
    cbvHeapDesc.Type = D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV;
    cbvHeapDesc.Flags = D3D12_DESCRIPTOR_HEAP_FLAG_SHADER_VISIBLE;
    cbvHeapDesc.NodeMask = 0;
    ThrowIfFailed(md3dDevice->CreateDescriptorHeap(&cbvHeapDesc,
        IID_PPV_ARGS(&mCbvHeap)));
}

void ShapesApp::BuildConstantBufferViews()
{
    UINT objCBByteSize = d3dUtil::CalcConstantBufferByteSize(sizeof(ObjectConstants));

    UINT objCount = (UINT)mOpaqueRitems.size();

    每個幀資源中的每個object都需要一個cbv
    for(int frameIndex = 0; frameIndex < gNumFrameResources; ++frameIndex)
    {
        auto objectCB = mFrameResources[frameIndex]->ObjectCB->Resource();
        for(UINT i = 0; i < objCount; ++i)
        {
            D3D12_GPU_VIRTUAL_ADDRESS cbAddress = objectCB->GetGPUVirtualAddress();

            // 每個物體的偏移
            cbAddress += i*objCBByteSize;

            // 計算在描述符堆中的偏移
            int heapIndex = frameIndex*objCount + i;
            auto handle = CD3DX12_CPU_DESCRIPTOR_HANDLE(mCbvHeap->GetCPUDescriptorHandleForHeapStart());
            handle.Offset(heapIndex, mCbvSrvUavDescriptorSize);

            D3D12_CONSTANT_BUFFER_VIEW_DESC cbvDesc;
            cbvDesc.BufferLocation = cbAddress;
            cbvDesc.SizeInBytes = objCBByteSize;

            md3dDevice->CreateConstantBufferView(&cbvDesc, handle);
        }
    }

    UINT passCBByteSize = d3dUtil::CalcConstantBufferByteSize(sizeof(PassConstants));
    每個幀資源都要一個pass 描述符
    for(int frameIndex = 0; frameIndex < gNumFrameResources; ++frameIndex)
    {
        auto passCB = mFrameResources[frameIndex]->PassCB->Resource();
        D3D12_GPU_VIRTUAL_ADDRESS cbAddress = passCB->GetGPUVirtualAddress();

        計算偏移
        int heapIndex = mPassCbvOffset + frameIndex;
        auto handle = CD3DX12_CPU_DESCRIPTOR_HANDLE(mCbvHeap->GetCPUDescriptorHandleForHeapStart());
        handle.Offset(heapIndex, mCbvSrvUavDescriptorSize);

        D3D12_CONSTANT_BUFFER_VIEW_DESC cbvDesc;
        cbvDesc.BufferLocation = cbAddress;
        cbvDesc.SizeInBytes = passCBByteSize;
        
        md3dDevice->CreateConstantBufferView(&cbvDesc, handle);
    }
}

繪製

最後一步是繪製每個渲染項 ：

void ShapesApp::DrawRenderItems(ID3D12GraphicsCommandList* cmdList, const std::vector<RenderItem*>& ritems)
{
    UINT objCBByteSize = d3dUtil::CalcConstantBufferByteSize(sizeof(ObjectConstants));
 
	auto objectCB = mCurrFrameResource->ObjectCB->Resource();

    for(size_t i = 0; i < ritems.size(); ++i)
    {
        auto ri = ritems[i];

        cmdList->IASetVertexBuffers(0, 1, &ri->Geo->VertexBufferView());
        cmdList->IASetIndexBuffer(&ri->Geo->IndexBufferView());
        cmdList->IASetPrimitiveTopology(ri->PrimitiveType);

        
        UINT cbvIndex = mCurrFrameResourceIndex*(UINT)mOpaqueRitems.size() + ri->ObjCBIndex;
        auto cbvHandle = CD3DX12_GPU_DESCRIPTOR_HANDLE(mCbvHeap->GetGPUDescriptorHandleForHeapStart());
        cbvHandle.Offset(cbvIndex, mCbvSrvUavDescriptorSize);

        cmdList->SetGraphicsRootDescriptorTable(0, cbvHandle);

        cmdList->DrawIndexedInstanced(ri->IndexCount, 1, ri->StartIndexLocation, ri->BaseVertexLocation, 0);
    }
}

細探根簽名

根簽名由一系列根引數構成 根引數主要有以下三種型別
img

我們可以建立出任意組合的根簽名 只要不超過64 DWORD大小 根常量使用方便 無需使用相應的常量緩衝區 與 cbv堆，但是假如我們使用根常量儲存mvp矩陣，16個float元素需要16個DWORD 即需要16個根常量 大幅消耗了根簽名的空間 所以在使用時我們要靈活組合

根簽名結構體定義：

typedef struct D3D12_ROOT_PARAMETER
    {
    D3D12_ROOT_PARAMETER_TYPE ParameterType;
    union 
        {
        D3D12_ROOT_DESCRIPTOR_TABLE DescriptorTable;
        D3D12_ROOT_CONSTANTS Constants;
        D3D12_ROOT_DESCRIPTOR Descriptor;
        } 	;
    D3D12_SHADER_VISIBILITY ShaderVisibility;
    } 	D3D12_ROOT_PARAMETER;

其中ParameterType的定義是根引數的型別，包括描述符表，根常量，cbv根描述符，srv根描述符，uav根描述符：
img
ShaderVisibility代表著著色器可見性：
img

建立 DescriptorTable Constants Descriptor

DescriptorTable :
描述符表的定義可以藉助CD3DX12_DESCRIPTOR_RANGE的init方法

struct CD3DX12_DESCRIPTOR_RANGE : public D3D12_DESCRIPTOR_RANGE
{
    CD3DX12_DESCRIPTOR_RANGE() { }
    explicit CD3DX12_DESCRIPTOR_RANGE(const D3D12_DESCRIPTOR_RANGE &o) :
        D3D12_DESCRIPTOR_RANGE(o)
    {}
    CD3DX12_DESCRIPTOR_RANGE(
        D3D12_DESCRIPTOR_RANGE_TYPE rangeType,
        UINT numDescriptors,
        UINT baseShaderRegister,
        UINT registerSpace = 0,
        UINT offsetInDescriptorsFromTableStart =
        D3D12_DESCRIPTOR_RANGE_OFFSET_APPEND)
    {
        Init(rangeType, numDescriptors, baseShaderRegister, registerSpace, offsetInDescriptorsFromTableStart);
    }
    
    inline void Init(
        D3D12_DESCRIPTOR_RANGE_TYPE rangeType,
        UINT numDescriptors,
        UINT baseShaderRegister,
        UINT registerSpace = 0,
        UINT offsetInDescriptorsFromTableStart =
        D3D12_DESCRIPTOR_RANGE_OFFSET_APPEND)
    {
        Init(*this, rangeType, numDescriptors, baseShaderRegister, registerSpace, offsetInDescriptorsFromTableStart);
    }
}

其中D3D12_DESCRIPTOR_RANGE_TYPE rangeType定義為：
img
numDescriptors代表著範圍內描述符的數量
baseShaderRegister：
img

img
然後使用InitAsDescriptorTable建立 ：

 CD3DX12_DESCRIPTOR_RANGE cbvTable0;
 cbvTable0.Init(D3D12_DESCRIPTOR_RANGE_TYPE_CBV, 1, 0);

 CD3DX12_DESCRIPTOR_RANGE cbvTable1;
 cbvTable1.Init(D3D12_DESCRIPTOR_RANGE_TYPE_CBV, 1, 1);

CD3DX12_ROOT_PARAMETER slotRootParameter[2];
 slotRootParameter[0].InitAsDescriptorTable(1, &cbvTable0);
 slotRootParameter[1].InitAsDescriptorTable(1, &cbvTable1);

根描述符與根常量的定義可以直接使用如下方法建立：

static inline void InitAsConstants(
    _Out_ D3D12_ROOT_PARAMETER &rootParam,
    UINT num32BitValues,
    UINT shaderRegister,
    UINT registerSpace = 0,
    D3D12_SHADER_VISIBILITY visibility = D3D12_SHADER_VISIBILITY_ALL)
{
    rootParam.ParameterType = D3D12_ROOT_PARAMETER_TYPE_32BIT_CONSTANTS;
    rootParam.ShaderVisibility = visibility;
    CD3DX12_ROOT_CONSTANTS::Init(rootParam.Constants, num32BitValues, shaderRegister, registerSpace);
}

static inline void InitAsConstantBufferView(
    _Out_ D3D12_ROOT_PARAMETER &rootParam,
    UINT shaderRegister,
    UINT registerSpace = 0,
    D3D12_SHADER_VISIBILITY visibility = D3D12_SHADER_VISIBILITY_ALL)
{
    rootParam.ParameterType = D3D12_ROOT_PARAMETER_TYPE_CBV;
    rootParam.ShaderVisibility = visibility;
    CD3DX12_ROOT_DESCRIPTOR::Init(rootParam.Descriptor, shaderRegister, registerSpace);
}

例子：
img
img

不同型別的根簽名繫結著色器暫存器

將不同型別的根簽名繫結著色器暫存器需要使用不同的命令：

根常量：ID3D12GraphicsCommandList：：SetComputeRoot32BitConstants
https://learn.microsoft.com/zh-cn/windows/win32/api/d3d12/nf-d3d12-id3d12graphicscommandlist-setcomputeroot32bitconstants
根描述符：ID3D12GraphicsCommandList：：SetComputeRootConstantBufferView
https://learn.microsoft.com/zh-cn/windows/win32/api/d3d12/nf-d3d12-id3d12graphicscommandlist-setcomputerootconstantbufferview
描述符表：ID3D12GraphicsCommandList：：SetComputeRootDescriptorTable
https://learn.microsoft.com/zh-cn/windows/win32/api/d3d12/nf-d3d12-id3d12graphicscommandlist-setcomputerootdescriptortable
其中根常量與根描述符都不需要涉及描述符堆

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