绘制多种几何体演示程序（第七章内容）
7.5 绘制多种几何体演示程序
在本小节中，我们将示范生成球体和柱体的代码。在探究该示例的过程中，我们可以学习到：
- 如何在场景中利用创建多个世界变换矩阵绘制对个物体并对其进行定位
- 如何将场景中所有几何体都放置在一个大的顶点缓冲区和一个索引缓冲区中
7.5.1 顶点缓冲区和索引缓冲区
在本节的演示程序中，我们要绘制长方体、栅格、柱体以及球体，在示例中我们需要绘制多个球体和柱体，但实际上我们只需要对一组球体和柱体进行复制即可，其间只需要运用不同的世界矩阵。所以说，这是一个实例化的过程，实例化技术可以大大减少资源的占用
通过将不同物体的顶点和索引合并起来，我们就可以把所有的几何体网格的顶点和索引数据都装进一个顶点缓冲区和一个索引缓冲区中，这意味着，在绘制一个物体时，我们只需要绘制该物体在顶点缓冲区和索引缓冲区的数据子集即可，为了调用ID3D12GraphicsCommandList::DrawIndexedInstanced方法，我们需要掌握三个数据（如果忘了可以回顾6.3节内容）：
- 物体在索引缓冲区中的起始索引位置
- 待绘制物体的索引数量
- 基准顶点地址
下面代码将展示创建几何体缓冲区，缓存绘制调用所需参数以及绘制物体的具体过程：
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);
//
// 将所有的几何数据都合并到一个大的顶点/索引缓冲区中
// 以此来定义每个子网格数据在缓冲区中所占的范围
//
// 对合并缓冲区中每个物体的顶点偏移量进行缓存
UINT boxVertexOffset = 0;
UINT gridVertexOffset = (UINT)box.Vertices.size();
UINT sphereVertexOffset = gridVertexOffset + (UINT)grid.Vertices.size();
UINT cylinderVertexOffset = sphereVertexOffset + (UINT)sphere.Vertices.size();
// 对合并缓冲区中每个物体的起始索引进行缓存
UINT boxIndexOffset = 0;
UINT gridIndexOffset = (UINT)box.Indices32.size();
UINT sphereIndexOffset = gridIndexOffset + (UINT)grid.Indices32.size();
UINT cylinderIndexOffset = sphereIndexOffset + (UINT)sphere.Indices32.size();
// 定义的多个SubmeshGeometry结构体中包换了顶点/索引缓冲区内不同几何体的子网格数据
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;
//
// 提取出所有的顶点元素，再将所有网格的顶点装进一个顶点缓冲区中
//
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);
}
上述方法中的最后一行所使用的变量mGeometries被定义为：
std::unordered_map<std::string,std::unique_ptr<MeshGeometry>> mGeoMetries;
这是我们在后面的示例代码中会经常使用到的一种模式，因为为每个几何体，PSO，纹理和着色器等创建新的变量名是一件很烦的事情，所以我们会使用无序映射表，并根据名称在常数时间内寻找和引用所需的对象。以下是一些相关的示例：
std::unordered_map<std::string, std::unique_ptr<MeshGeometry>> mGeometries;
std::unordered_map<std::string, ComPtr<ID3DBlob>> mShaders;
std::unordered_map<std::string, ComPtr<ID3D12PipelineState>> mPSOs;
7.5.2 渲染项
现在我们来定义场景中的渲染项，通过观察可知，所有的渲染项都使用同一个MeshGeometry，所以我们可以使用DrawArgs来获取DrawIndexedInstanced方法的参数，并以此来绘制顶点/索引缓冲区的子区域（即单个几何体）：
// 所有的渲染项
std::vector<std::unique_ptr<RenderItem>> mAllRitems;
// 根据不同的PSO来划分渲染项
std::vector<RenderItem*> mOpaqueRitems;
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;
XMStoreFloat4x4(&rightCylRitem->World, leftCylWorld);
rightCylRitem->ObjCBIndex = objCBIndex++;
rightCylRitem->Geo = mGeometries["shapeGeo"].get();
rightCylRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
rightCylRitem->IndexCount = rightCylRitem->Geo->DrawArgs["cylinder"].IndexCount;
rightCylRitem->StartIndexLocation = rightCylRitem->Geo->DrawArgs["cylinder"].StartIndexLocation;
rightCylRitem->BaseVertexLocation = rightCylRitem->Geo->DrawArgs["cylinder"].BaseVertexLocation;
XMStoreFloat4x4(&leftSphereRitem->World, leftSphereWorld);
leftSphereRitem->ObjCBIndex = objCBIndex++;
leftSphereRitem->Geo = mGeometries["shapeGeo"].get();
leftSphereRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
leftSphereRitem->IndexCount = leftSphereRitem->Geo->DrawArgs["sphere"].IndexCount;
leftSphereRitem->StartIndexLocation = leftSphereRitem->Geo->DrawArgs["sphere"].StartIndexLocation;
leftSphereRitem->BaseVertexLocation = leftSphereRitem->Geo->DrawArgs["sphere"].BaseVertexLocation;
XMStoreFloat4x4(&rightSphereRitem->World, rightSphereWorld);
rightSphereRitem->ObjCBIndex = objCBIndex++;
rightSphereRitem->Geo = mGeometries["shapeGeo"].get();
rightSphereRitem->PrimitiveType = D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST;
rightSphereRitem->IndexCount = rightSphereRitem->Geo->DrawArgs["sphere"].IndexCount;
rightSphereRitem->StartIndexLocation = rightSphereRitem->Geo->DrawArgs["sphere"].StartIndexLocation;
rightSphereRitem->BaseVertexLocation = rightSphereRitem->Geo->DrawArgs["sphere"].BaseVertexLocation;
mAllRitems.push_back(std::move(leftCylRitem));
mAllRitems.push_back(std::move(rightCylRitem));
mAllRitems.push_back(std::move(leftSphereRitem));
mAllRitems.push_back(std::move(rightSphereRitem));
}
// All the render items are opaque.
for (auto& e : mAllRitems)
mOpaqueRitems.push_back(e.get());
}
7.5.3 帧资源和常量缓冲区视图
回顾前文可知，我们已经创建一个由FrameResource类型元素所构成的向量，每个FrameReousrce中都有上传缓冲区，用于为场景中每一个渲染项存储渲染过程常量和物体常量数据。
如果有3个帧资源和n个渲染项，那么我们需要创建3n个物体常量缓冲区（object constant buffer）以及3个渲染过程常量缓冲区（pass 从constant buffer），因此我们需要创建3(n+1)个常量缓冲区视图。所以我们要修改CBV堆以容纳额外的描述符：
void ShapesApp::BuildDescriptorHeaps()
{
UINT objCount = (UINT)mOpaqueRitems.size();
// 我们需要为每个帧资源中的每一个物体都创建一个CBV描述符
// 为了容纳每个帧资源中的渲染过程CBV + 1
UINT numDescriptors = (objCount + 1) * gNumFrameResources;
// 保存渲染过程CBV的起始偏移量，在示例程序中，渲染过程CBV排在最后三个
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)));
}
现在我们可以使用下列代码来填充CBV堆了，其中0-n-1描述符为第0个帧资源的物体CBV……3n、3n + 1、3n + 2分别为帧资源的渲染过程CBV：
void ShapesApp::BuildConstantBufferViews()
{
UINT objCBByteSize = d3dUtil::CalcConstantBufferByteSize(sizeof(ObjectConstants));
UINT objCount = (UINT)mOpaqueRitems.size();
// 每一个帧资源中的每一个物体都需要一个对应的CBV描述符
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();
// 偏移到第i个物体的常量缓冲区
cbAddress += i * objCBByteSize;
// 偏移到该物体在描述符堆中的CBV
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));
// 最后三个是帧资源的渲染过程CBV
for (int frameIndex = 0; frameIndex < gNumFrameResources; ++frameIndex)
{
auto passCB = mFrameResources[frameIndex]->PassCB->Resource();
D3D12_GPU_VIRTUAL_ADDRESS cbAddress = passCB->GetGPUVirtualAddress();
// 偏移到描述符堆中对应的渲染过程CBV
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);
}
}
在前文中，通过调用ID3D12DescriptorHeap::GetCPUDescriptorHandleForHeapStart方法可以获得堆中的第一个描述符的句柄，但是现在堆中存储的不是一个描述符了，如果想要访问到堆中的其他描述符，必须进行偏移，为此，我们需要了解堆内相邻描述符的增量。现在让我们重温一下D3DApp类中缓存的下列信息：
//渲染目标视图的大小
mRtvDescriptorSize = md3dDevice->GetDescriptorHandleIncrementSize(D3D12_DESCRIPTOR_HEAP_TYPE_RTV);
//深度模板视图的大小
mDsvDescriptorSize = md3dDevice->GetDescriptorHandleIncrementSize(D3D12_DESCRIPTOR_HEAP_TYPE_DSV);
//常量缓冲区/着色器资源/无序访问视图的大小
mCbvSrvUavDescriptorSize = md3dDevice->GetDescriptorHandleIncrementSize(D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV);
只要知道了相邻描述符堆的增量，我们便可以使用CD3DX12_CPU_DESCRIPTOR_HANDLE：：Offset方法来偏移到第n个描述符的句柄处了：
CD3DX12_CPU_DESCRIPTOR_HANDLE handle = CD3DX12_CPU_DESCRIPTOR_HANDLE(mCbvHeap->GetCPUDescriptorHandleForHeapStart());
//第一种方法
handle.Offset(n*mCbvSrvUavDescriptorSize);
//第二种方法
handle.Offset(n, mCbvSrvUavDescriptorSize);
7.5.4 绘制场景
最后一步便是绘制场景，在该步骤中，最复杂的一个部分便是为了绘制物体而根据偏移量找到它在描述符堆中对应的CBV了：
void ShapesApp::DrawRenderItems(ID3D12GraphicsCommandList* cmdList, const std::vector<RenderItem*>& ritems)
{
UINT objCBByteSize = d3dUtil::CalcConstantBufferByteSize(sizeof(ObjectConstants));
auto objectCB = mCurrFrameResource->ObjectCB->Resource();
// 对于每一个渲染项来说...
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);
// 为了绘制当前的帧资源和当前的物体，偏移到描述符堆中对应的CBV处
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);
}
}
在执行主要绘制函数Draw的过程中调用DrawRenderItems方法：
void ShapesApp::Draw(const GameTimer& gt)
{
auto cmdListAlloc = mCurrFrameResource->CmdListAlloc;
// 复用与记录命令有关的内存
// 在GPU执行完与该内存相关联的命令列表时，才可以对此命令分配器进行重置
ThrowIfFailed(cmdListAlloc->Reset());
if (mIsWireframe)
{
ThrowIfFailed(mCommandList->Reset(cmdListAlloc.Get(), mPSOs["opaque_wireframe"].Get()));
}
else
{
ThrowIfFailed(mCommandList->Reset(cmdListAlloc.Get(), mPSOs["opaque"].Get()));
}
// 重置视口和裁剪矩形
mCommandList->RSSetViewports(1, &mScreenViewport);
mCommandList->RSSetScissorRects(1, &mScissorRect);
// 根据资源的用途进行资源转换
mCommandList->ResourceBarrier(1, &CD3DX12_RESOURCE_BARRIER::Transition(CurrentBackBuffer(),
D3D12_RESOURCE_STATE_PRESENT, D3D12_RESOURCE_STATE_RENDER_TARGET));
// 清除后台和深度模板缓冲区
mCommandList->ClearRenderTargetView(CurrentBackBufferView(), Colors::LightSteelBlue, 0, nullptr);
mCommandList->ClearDepthStencilView(DepthStencilView(), D3D12_CLEAR_FLAG_DEPTH | D3D12_CLEAR_FLAG_STENCIL, 1.0f, 0, 0, nullptr);
// 指定要渲染的缓冲区
mCommandList->OMSetRenderTargets(1, &CurrentBackBufferView(), true, &DepthStencilView());
ID3D12DescriptorHeap* descriptorHeaps[] = { mCbvHeap.Get() };
mCommandList->SetDescriptorHeaps(_countof(descriptorHeaps), descriptorHeaps);
mCommandList->SetGraphicsRootSignature(mRootSignature.Get());
int passCbvIndex = mPassCbvOffset + mCurrFrameResourceIndex;
auto passCbvHandle = CD3DX12_GPU_DESCRIPTOR_HANDLE(mCbvHeap->GetGPUDescriptorHandleForHeapStart());
passCbvHandle.Offset(passCbvIndex, mCbvSrvUavDescriptorSize);
mCommandList->SetGraphicsRootDescriptorTable(1, passCbvHandle);
DrawRenderItems(mCommandList.Get(), mOpaqueRitems);
// 将资源转换为呈现状态
mCommandList->ResourceBarrier(1, &CD3DX12_RESOURCE_BARRIER::Transition(CurrentBackBuffer(),
D3D12_RESOURCE_STATE_RENDER_TARGET, D3D12_RESOURCE_STATE_PRESENT));
// 结束命令的记录
ThrowIfFailed(mCommandList->Close());
// 将命令列表加入到命令队列中
ID3D12CommandList* cmdsLists[] = { mCommandList.Get() };
mCommandQueue->ExecuteCommandLists(_countof(cmdsLists), cmdsLists);
// 交换前后台缓冲区
ThrowIfFailed(mSwapChain->Present(0, 0));
mCurrBackBuffer = (mCurrBackBuffer + 1) % SwapChainBufferCount;
// 增加围栏值，将之前的命令标记到该围栏点上
mCurrFrameResource->Fence = ++mCurrentFence;
//设置新的围栏点（我们将命令提交到GPU后，GPU并不会马上执行
//而是在执行之前的命令，所以我们需要设置新的围栏点，让GPU
//区分新提交的命令和之前的命令）
mCommandQueue->Signal(mFence.Get(), mCurrentFence);
}
7.5.5 演示效果
由于完整的源码过于庞大，这里就不展示所有的源代码了。如果想要完整的源码，可以在GitHub上搜索，这里就不给链接了！
