DirectX12的初始化

DirectX12的初始化主要分为以下若干步骤：

创建device和gifactory
创建与GPU通信同步相关的objects，command和fence
创建swap chain
为render target view和depth stencil view创建descriptor heap
创建back buffer和与之绑定的render target view
创建depth stencil buffer和与之绑定的depth stencil view
填充viewport和屏幕剪裁区域结构
绘制前：设置buffer，设置view，设置viewport和屏幕剪裁区域
绘制后：提交绘制，设置buffer

创建device和gifactory

主要调用如下两个接口：

HRESULT D3D12CreateDevice(

  IUnknown          *pAdapter,

  D3D_FEATURE_LEVEL MinimumFeatureLevel,

  REFIID            riid,

  void              **ppDevice

);

HRESULT CreateDXGIFactory1(

  REFIID riid,

  void   **ppFactory

);

这里，我们可以借用微软提供的ComPtr来管理要创建的相关对象：

ComPtr提供了一些常用的接口，重载了一些常用的操作符：

Get()：获取原始的指针
GetAddressOf()：获取原始指针的地址
operator&：释放原始指针指向的对象，然而再返回原始指针的地址

			ComPtr<ID3D12Device> mDevice = nullptr;

			ComPtr<IDXGISwapChain> mSwapChain = nullptr;

同时，为了方便填充这些创建接口，微软提供了一个宏定义IID_PPV_ARGS：

	ThrowIfFailed(D3D12CreateDevice(nullptr, D3D_FEATURE_LEVEL_11_0, IID_PPV_ARGS(&mDevice)));

	ThrowIfFailed(CreateDXGIFactory1(IID_PPV_ARGS(&mDxGiFactory)));

创建与GPU通信同步相关的objects，command和fence

command是用来从CPU提交相关指令给GPU执行的。这里需要三个objects：queue，allocator，list。queue是用来最终提交给CPU的数据结构，allocator负责为list提供空间管理，list提供了若干CPU提交相关指令的接口。

ThrowIfFailed(mDevice->CreateCommandQueue(&queueDesc, IID_PPV_ARGS(&mCommandQueue)));

ThrowIfFailed(mDevice->CreateCommandAllocator(D3D12_COMMAND_LIST_TYPE_DIRECT, IID_PPV_ARGS(&mCommandAlloc)));

ThrowIfFailed(mDevice->CreateCommandList(0, D3D12_COMMAND_LIST_TYPE_DIRECT, mCommandAlloc.Get(), nullptr, IID_PPV_ARGS(&mCommandList)));

// must close otherwise call reset will raise a error

mCommandList->Close();

创建完之后记得调用Close()方法，否则后面调用Reset()时会因为状态未关闭而出错。

fence是用来管理CPU和GPU同步的对象，防止对同一资源进行冲突访问，创建接口如下：

HRESULT CreateFence(

  UINT64            InitialValue,

  D3D12_FENCE_FLAGS Flags,

  REFIID            riid,

  void              **ppFence

);

创建swap chain

HRESULT CreateSwapChain(

  IUnknown             *pDevice,

  DXGI_SWAP_CHAIN_DESC *pDesc,

  IDXGISwapChain       **ppSwapChain

);

值得一提的是，DirectX12不支持用该API创建MSAA swap chain：

Direct3D 12 don't support creating MSAA swap chains--attempts to create a swap chain with SampleDesc.Count > 1 will fail. Instead, you create your own MSAA render target and explicitly resolve to the DXGI back-buffer for presentation as shown here.

为render target view和depth stencil view创建descriptor heap

heap就是用来存放管理view的结构，相关接口如下：

UINT GetDescriptorHandleIncrementSize(

  D3D12_DESCRIPTOR_HEAP_TYPE DescriptorHeapType

);

HRESULT CreateDescriptorHeap(

  const D3D12_DESCRIPTOR_HEAP_DESC *pDescriptorHeapDesc,

  REFIID                           riid,

  void                             **ppvHeap

);

通过GetDescriptorHandleIncrementSize可以知道heap上每个handle的大小，方便在取一个heap上多个view进行偏移。

创建back buffer和与之绑定的render target view

	ThrowIfFailed(mSwapChain->ResizeBuffers(mBackBufferCount, windowWidth, windowHeight, mBackBufferFormat,

		DXGI_SWAP_CHAIN_FLAG_ALLOW_MODE_SWITCH));

	CD3DX12_CPU_DESCRIPTOR_HANDLE handle = CD3DX12_CPU_DESCRIPTOR_HANDLE(

		mRtvHeap->GetCPUDescriptorHandleForHeapStart());

	for (int i = 0; i < mBackBufferCount; i++)

	{

		ThrowIfFailed(mSwapChain->GetBuffer(i, IID_PPV_ARGS(&mBackBuffer[i])));

		mDevice->CreateRenderTargetView(mBackBuffer[i].Get(), nullptr, handle);

		handle.Offset(1, mRtvHeapIncSize);

	}

CD3DX12_CPU_DESCRIPTOR_HANDLE也是微软提供的便利数据结构，包含一些简单的接口。这里用了Offset方法来取得正确的heap位置。

创建depth stencil buffer和与之绑定的depth stencil view

	ThrowIfFailed(mDevice->CreateCommittedResource(&CD3DX12_HEAP_PROPERTIES(D3D12_HEAP_TYPE_DEFAULT),

		D3D12_HEAP_FLAG_NONE, &desc, D3D12_RESOURCE_STATE_COMMON, &clearValue, IID_PPV_ARGS(&mDepthStencilBuffer)));

	handle = CD3DX12_CPU_DESCRIPTOR_HANDLE(mDsvHeap->GetCPUDescriptorHandleForHeapStart());

	mDevice->CreateDepthStencilView(mDepthStencilBuffer.Get(), &viewDesc, handle);

与back buffer的过程类似，只不过depth stencil buffer只有一个，而back buffer可能有多个。CD3DX12_HEAP_PROPERTIES同样也是便利数据结构。

注意到buffer创建完后处于D3D12_RESOURCE_STATE_COMMON状态，我们需要将其转为D3D12_RESOURCE_STATE_DEPTH_WRITE状态，使用CD3DX12_RESOURCE_BARRIER::Transition进行便捷操作：

	mCommandList->ResourceBarrier(1, &CD3DX12_RESOURCE_BARRIER::Transition(mDepthStencilBuffer.Get(),

		D3D12_RESOURCE_STATE_COMMON, D3D12_RESOURCE_STATE_DEPTH_WRITE));

	ThrowIfFailed(mCommandList->Close());

	ID3D12CommandList *cmdList[] = { mCommandList.Get() };

	mCommandQueue->ExecuteCommandLists(_countof(cmdList), cmdList);

	FlushCommandQueue();

void DirectX12::FlushCommandQueue()

{

	mCurrentFence++;

	ThrowIfFailed(mCommandQueue->Signal(mFence.Get(), mCurrentFence));

	if (mFence->GetCompletedValue() < mCurrentFence)

	{

		HANDLE handle = CreateEventEx(nullptr, false, false, EVENT_ALL_ACCESS);

		ThrowIfFailed(mFence->SetEventOnCompletion(mCurrentFence, handle));

		WaitForSingleObject(handle, INFINITE);

		CloseHandle(handle);

	}

}

资源状态转换需要向GPU提交指令，因此涉及到同步问题，即CPU需要等待GPU完成指令之后才能继续执行其他操作。这里用了FlushCommandQueue来完成同步。这个方法的实现一目了然：

首先将当前fence计数值增加，signal提交到GPU，然后一直等待，直到获取到与计数值相同的值为止。

填充viewport和屏幕剪裁区域结构

这一步骤只需填充D3D12_VIEWPORT和D3D12_RECT结构即可。

	mViewport.TopLeftX = 0;

	mViewport.TopLeftY = 0;

	mViewport.Width = windowWidth;

	mViewport.Height = windowHeight;

	mViewport.MinDepth = 0.0f;

	mViewport.MaxDepth = 1.0f;

	mScissorRect.left = 0;

	mScissorRect.right = windowWidth;

	mScissorRect.top = 0;

	mScissorRect.bottom = windowHeight;

绘制前：设置buffer，设置view，设置viewport和屏幕剪裁区域

	mCommandList->ResourceBarrier(1, &CD3DX12_RESOURCE_BARRIER::Transition(mBackBuffer[mCurBackBuffer].Get(),

		D3D12_RESOURCE_STATE_PRESENT, D3D12_RESOURCE_STATE_RENDER_TARGET));

	mCommandList->RSSetScissorRects(1, &mScissorRect);

	mCommandList->RSSetViewports(1, &mViewport);

	CD3DX12_CPU_DESCRIPTOR_HANDLE dsv = CD3DX12_CPU_DESCRIPTOR_HANDLE(

		mDsvHeap->GetCPUDescriptorHandleForHeapStart());

	mCommandList->ClearDepthStencilView(dsv, D3D12_CLEAR_FLAG_DEPTH | D3D12_CLEAR_FLAG_STENCIL, 1.0f, 0, 0,

		nullptr);

	CD3DX12_CPU_DESCRIPTOR_HANDLE rtv = CD3DX12_CPU_DESCRIPTOR_HANDLE(

		mRtvHeap->GetCPUDescriptorHandleForHeapStart(), mCurBackBuffer, mRtvHeapIncSize);

	mCommandList->ClearRenderTargetView(rtv, LightSteelBlue, 0, nullptr);

	mCommandList->OMSetRenderTargets(1, &rtv, true, &dsv);

这里将backBuffer的状态从D3D12_RESOURCE_STATE_PRESENT转移到了D3D12_RESOURCE_STATE_RENDER_TARGET，这样后面的设置和绘制工作才会有效，否则会出错。实际上，状态定义里，D3D12_RESOURCE_STATE_PRESENT和D3D12_RESOURCE_STATE_COMMON是等价的，都为0，这里只是换了一种说法。

绘制后：提交绘制，设置buffer

	mCommandList->ResourceBarrier(1, &CD3DX12_RESOURCE_BARRIER::Transition(mBackBuffer[mCurBackBuffer].Get(),

		D3D12_RESOURCE_STATE_RENDER_TARGET, D3D12_RESOURCE_STATE_PRESENT));

	ThrowIfFailed(mCommandList->Close());

	ID3D12CommandList *cmdList[] = { mCommandList.Get() };

	mCommandQueue->ExecuteCommandLists(_countof(cmdList), cmdList);

	ThrowIfFailed(mSwapChain->Present(0, 0));

	mCurBackBuffer = (mCurBackBuffer + 1) % mBackBufferCount;

在调用swapChain的Present方法之前，必须将backbuffer的状态设置为D3D12_RESOURCE_STATE_PRESENT。如果使用了多个backbuffer，别忘记切换到下一个backbuffer。

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