Directx10和Directx11很多resource类都有个Map()函数，个人感觉它是一个蛮重要的CPU和GPU沟通桥梁。下面以ID3D11DeviceContext::Map()为例讲下。
ID3D11DeviceContext::Map官方解释是：ID3D11DeviceContext::Map Method
Get a pointer to the data contained in a subresource, and deny the GPU access to that subresource.
1 HRESULT Map(
2 ID3D11Resource *pResource,
3 UINT Subresource,
4 D3D11_MAP MapType,
5 UINT MapFlags,
6 D3D11_MAPPED_SUBRESOURCE *pMappedResource
7);
个人理解总结是当需要用CPU读写（GPU正在读写的）subresouce（最常用如buffer）时，就用Map()得到该subresource的pointer（D3D11_MAPPED_SUBRESOURCE*），然后将D3D11_MAPPED_SUBRESOURCE::pData强制转换成CPU理解的类型（struct，class），CPU就可以对得到的struct*或class*读写了。
1// 附录D3D11_MAPPED_SUBRESOURCE结构体
2typedef struct D3D11_MAPPED_SUBRESOURCE
3   {
4 void *pData;
5 UINT RowPitch;
6 UINT DepthPitch;
7 }D3D11_MAPPED_SUBRESOURCE;
Map用法总结1：
debug时用CPU读写GPU之前读写的buffer（比如GPU计算输出的结果），然后再通过CPU输出来检验对错。
1//将GPU读或读写的buffer *pGBuffer拷贝到可给CPU读的buffer并返回，以便调试
2
3ID3D11Buffer* GPUBaseApp::CreateAndCopyToDebugBuf(ID3D11Buffer* pGBuffer)
4{
5 D3D11_BUFFER_DESC bufferDesc;
6 ZeroMemory(&bufferDesc,sizeof(bufferDesc));
7 pGBuffer->GetDesc(&bufferDesc);
8 bufferDesc.CPUAccessFlags=D3D11_CPU_ACCESS_READ;
9 bufferDesc.Usage=D3D11_USAGE_STAGING;
10 bufferDesc.BindFlags=0;
11 bufferDesc.MiscFlags=0;
12
13 ID3D11Buffer* pDebugBuffer=NULL;
14
15 if( SUCCEEDED(m_pDevice->CreateBuffer(&bufferDesc,NULL,&pDebugBuffer)) )
16  {
17 m_pDeviceContext->CopyResource(pDebugBuffer,pGBuffer);
18 }
19 return pDebugBuffer;
20}
21
22HRESULT Function()
23{
24 //看Direct Compute的计算结果
25 ID3D11Buffer* debugOutBuffer=NULL;
26 ID3D11Buffer* debugInBuffer=NULL;
27
28 debugOutBuffer=CreateAndCopyToDebugBuf(m_pOutBuffer);
29 debugInBuffer=CreateAndCopyToDebugBuf(m_pInputBuffer);
30
31 VoronoiOutBufType* pOut=NULL;
32 VoronoiInBufType* pIn=NULL;
33
34 D3D11_MAPPED_SUBRESOURCE mappedOutResource;
35 D3D11_MAPPED_SUBRESOURCE mappedInResource;
36
37 m_pDeviceContext->Map(debugOutBuffer,0,D3D11_MAP_READ,0,&mappedOutResource);
38 pOut=(VoronoiOutBufType*)mappedOutResource.pData;
39
40 m_pDeviceContext->Map(debugInBuffer,0,D3D11_MAP_READ,0,&mappedInResource);
41 pIn=(VoronoiInBufType*)mappedInResource.pData;
42
43 //printf("OutBuffer的结果如下:\n");
44 //for(int i=0;i<30;i++)
45 // printf("%d %f\n",pOut[i].index,pOut[i].dist);
46
47 m_pDeviceContext->Unmap(debugOutBuffer,0);
48 SAFE_RELEASE(debugOutBuffer);
49
50 m_pDeviceContext->Unmap(debugInBuffer,0);
51 SAFE_RELEASE(debugInBuffer);
52}
53
Map用法2：
更新Constant buffer，把CPU中新得到的CB_MouseInfo类型数据mouseInfo写入GPU中Constant Buffer。
1struct CB_MouseInfo
2{
3 UINT x;
4 UINT y;
5 UINT btDown;
6};
7
8CB_MouseInfo mouseInfo；
9
10HRESULT GPUBaseApp::UpDateMouseInfoCB()
11{
12 HRESULT hr=S_OK;
14 D3D11_MAPPED_SUBRESOURCE subData;
16ZeroMemory(&subData,sizeof(subData));
17m_pDeviceContext->Map(m_pConstantBuffer,0,D3D11_MAP_WRITE_DISCARD,0,&subData);
19
20 memcpy(subData.pData,&mouseInfo,sizeof(CB_MouseInfo));
21
22 m_pDeviceContext->Unmap(&subData,0);
23
24 return hr;
25}
注意点，当调用了Map()，最后记得调用ID3D11DeviceContext::Unmap Method使得GPU重新获取对该resouce的读写权。
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