d3d12龙书阅读----绘制几何体（下）

d3d12龙书阅读----绘制几何体（下）

本节在上一节的基础上，对整个绘制过程进行优化，将绘制单个几何体的内容拓展到了多个几何体，同时对根签名进行了进一步地探索。

帧资源

在之前绘制每帧的结尾，我们都要使用flushingcommandqueue方法，要一直等待gpu执行完所有命令，才会继续绘制下一帧，此时cpu处于空闲时间，同时，在绘制每一帧的初始阶段，gpu要等待cpu提交命令，此时gpu处于空闲时间

解决上述问题的一种方法是：

构建以cpu每帧都要更新的资源为数组元素的环形数组，这些资源被称为帧资源，一般循环数组由3个帧资源元素构成

当gpu在处理上一帧的命令时，cpu可以为下一帧更新资源，并构建并提交相应的命令列表，如果环形数组有三个元素，则令cpu比gpu提前处理两帧，这样可以确保gpu持续工作

帧资源定义：

针对每个物体/几何体的常量缓冲区定义

目前存储的是每个物体的世界矩阵 即 模型矩阵 将物体从局部坐标系转换到世界坐标系 代表着物体的位置

struct ObjectConstants
{
    DirectX::XMFLOAT4X4 World = MathHelper::Identity4x4();
};
针对每次渲染过程（rendering pass）所要用到的数据
比如 观察矩阵 投影矩阵 时间 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;
};
顶点定义
struct Vertex
{
    DirectX::XMFLOAT3 Pos;
    DirectX::XMFLOAT4 Color;
};

存储cpu为一帧构建命令列表所需资源
struct FrameResource
{
public:
    
    FrameResource(ID3D12Device* device, UINT passCount, UINT objectCount);
    FrameResource(const FrameResource& rhs) = delete;
    FrameResource& operator=(const FrameResource& rhs) = delete;
    ~FrameResource();

    每一帧都要有自己的命令分配器
    因为当上一帧的gpu还在处理命令时 我们不能重置命令分配器
    Microsoft::WRL::ComPtr<ID3D12CommandAllocator> CmdListAlloc;

    同理 每个帧资源也要有自己的常量缓冲区
    std::unique_ptr<UploadBuffer<PassConstants>> PassCB = nullptr;
    std::unique_ptr<UploadBuffer<ObjectConstants>> ObjectCB = nullptr;

    围栏点可以帮助检测 gpu是否仍然使用着帧资源
    UINT64 Fence = 0;
};

可以看到我们在帧资源中将常量缓冲区分为pass 与 object， 这是基于资源的更新频率对常量资源进行分组，每次渲染过程我们都要更新pass缓冲区，而对于object来说，只有当发生变化的时候才需要更新，具体代码我们待会再看。

回到cpu与gpu的同步上来，首先创建初始化帧资源数组：

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代表着一个帧资源1个pass缓冲区 第二个是所有渲染物体的数目
    }
}

cpu端更新第n帧：

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

    循环帧资源数组
    mCurrFrameResourceIndex = (mCurrFrameResourceIndex + 1) % gNumFrameResources;
    mCurrFrameResource = mFrameResources[mCurrFrameResourceIndex].get();

    等待gpu完成围栏点之前的所有命令
    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);
    }
    更新常量缓冲区
	UpdateObjectCBs(gt);
	UpdateMainPassCB(gt);
}

绘制第n帧：

void ShapesApp::draw(const GameTimer& gt){
添加围栏值 将命令标记到此围栏点
mCurrFrameResource->Fence = ++mCurrentFence;

向命令队列中添加一条设置新围栏点的命令
由于这条命令要交给gpu处理，所以gpu处理完signal之前的所有命令之前，它不会设置新的围栏点
mCommandQueue->Signal(mFence.Get(), mCurrentFence);
}

其实这种方法也有着缺陷，如果gpu处理命令的速度大于cpu提交命令列表的速度，则还是要等待cpu，理想的情况是cpu处理帧的速度大于gpu，这样cpu可以有空闲时间来处理游戏逻辑的其它部分，此方法的最大好处是cpu可以持续向gpu提供数据

渲染项

渲染项是一个轻量型结构 用于存储绘制物体所需要数据：

struct RenderItem
{
	RenderItem() = default;

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

	// 一个脏标记用于记录是否需要更新物体缓冲区 因为每个帧资源都有各自独立的物体缓冲区 所以脏标记的数目要设置和帧资源数目一致
	int NumFramesDirty = gNumFrameResources;

	// 当前渲染项对应object缓冲区索引
	UINT ObjCBIndex = -1;

    该渲染项参与绘制的几何体
	MeshGeometry* Geo = nullptr;

    //图元拓扑类型
    D3D12_PRIMITIVE_TOPOLOGY PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;

    // DrawIndexedInstanced 方法的参数
    UINT IndexCount = 0;
    UINT StartIndexLocation = 0;
    int BaseVertexLocation = 0;
};

渲染项的具体使用之后介绍

渲染过程中用到的常量数据

我们需要更新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缓冲区 与 pass缓冲区 这里利用了前一节介绍的uploadbuffer的方法 从cpu端更新数据：

void ShapesApp::UpdateObjectCBs(const GameTimer& gt)
{
	auto currObjectCB = mCurrFrameResource->ObjectCB.get();
	for(auto& e : mAllRitems)
	{
		每个帧资源都需要更新物体缓冲区
		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);
}

绘制多种几何体

在这里就不再介绍柱体 球体 正方体的过程 设计到一些几何知识

直接进入几何体的绘制阶段

创建顶点与索引缓冲区

将所有几何体的顶点缓冲区 与 索引缓冲区，合成一个大的顶点缓冲区与 索引缓冲区，之后使用drawindexinstanced方法绘制 需要记录每个几何体起始索引 索引数 以及起始顶点

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);

	// 计算各几何体的起始顶点
	UINT boxVertexOffset = 0;
	UINT gridVertexOffset = (UINT)box.Vertices.size();
	UINT sphereVertexOffset = gridVertexOffset + (UINT)grid.Vertices.size();
	UINT cylinderVertexOffset = sphereVertexOffset + (UINT)sphere.Vertices.size();

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

    定义各子网格结构体
	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;

    将各顶点 各索引合并
    子网格合并为一个大的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);
}

定义具体渲染项

在完成构建几何体之后 我们根据上一步创建的meshgeometry 来提取submeshgeometry 然后 里面的信息 根据需要创建相应的渲染项 并填写相应的内容

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;
        此处省略
		
	}

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

定义常量缓冲区视图

之后由于我们现在有3个pass常量缓冲区 3n个object常量缓冲区 总共3n+3个常量缓冲区 所以就需要 3n+3个cbv 同时也要拓展描述符堆的大小：

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();

    每个帧资源中的每个object都需要一个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();

            // 每个物体的偏移
            cbAddress += i*objCBByteSize;

            // 计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));
    每个帧资源都要一个pass 描述符
    for(int frameIndex = 0; frameIndex < gNumFrameResources; ++frameIndex)
    {
        auto passCB = mFrameResources[frameIndex]->PassCB->Resource();
        D3D12_GPU_VIRTUAL_ADDRESS cbAddress = passCB->GetGPUVirtualAddress();

        计算偏移
        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);
    }
}

绘制

最后一步是绘制每个渲染项 ：

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);
    }
}

细探根签名

根签名由一系列根参数构成 根参数主要有以下三种类型

我们可以创建出任意组合的根签名 只要不超过64 DWORD大小 根常量使用方便 无需使用相应的常量缓冲区 与 cbv堆，但是假如我们使用根常量存储mvp矩阵，16个float元素需要16个DWORD 即需要16个根常量 大幅消耗了根签名的空间 所以在使用时我们要灵活组合

根签名结构体定义：

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的定义是根参数的类型，包括描述符表，根常量，cbv根描述符，srv根描述符，uav根描述符：

ShaderVisibility代表着着色器可见性：

创建 DescriptorTable Constants Descriptor

DescriptorTable :

描述符表的定义可以借助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定义为：

numDescriptors代表着范围内描述符的数量

baseShaderRegister：

然后使用InitAsDescriptorTable创建 ：

 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);

根描述符与根常量的定义可以直接使用如下方法创建：

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：

不同类型的根签名绑定着色器寄存器

将不同类型的根签名绑定着色器寄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

其中根常量与根描述符都不需要涉及描述符堆