传统的GDI API (BitBlt)虽然可以完美的完成后台截图的任务，但是归根结底效率还是太低。直接使用DXGI方法截图只能完成前台窗口的截图，而DX HOOK的截图方法平添风险，以及很多场景不现实。本文讲介绍使用 DwmGetDxSharedSurface 函数，优雅的完成后台截图的工作。

API介绍
函数定义

BOOL WINAPI DwmGetDxSharedSurface (
    HWND hwnd,
    HANDLE* phSurface,
    LUID* pAdapterLuid,
    ULONG* pFmtWindow,
    ULONG* pPresentFlags,
    ULONGLONG* pWin32kUpdateId
)

来自于user32.dll（很离谱是吧，DwmApi不在DwmApi.dll里）。由于是ms没有公开的API，需要使用动态方法加载。

调用函数方法

//堆代码 duidaima.com
//动态载入该函数
typedef HRESULT(WINAPI* DwmGetDxSharedSurface_t)(HWND, HANDLE*, LUID*, ULONG*, ULONG*, ULONGLONG*);
DwmGetDxSharedSurface_t DwmGetDxSharedSurface = NULL;
//获取地址
HMODULE hUser32 = LoadLibraryA("user32.dll");
if (hUser32 == NULL)
{
std::cout << "LoadLibraryA failed" << std::endl;
return 0;
}
DwmGetDxSharedSurface = (DwmGetDxSharedSurface_t)GetProcAddress(hUser32, "DwmGetDxSharedSurface");
//Dwm函数 在 user32.dll 中，真是离谱
if (DwmGetDxSharedSurface == NULL)
{
std::cout << "GetProcAddress failed" << std::endl;
return 0;
}
std::cout << DwmGetDxSharedSurface << std::endl;

参数含义
hwnd:被截图窗口的句柄
phSurface: 被截图窗口的共享画面的句柄（应该是这么翻译吧）

其他，暂时还没了解。

API调用
问题
显然这个API不能一步到位获得到BMP或者其他类型的图像数据。和BitBlt一样，这个API只是拿到了对应画面的副本（？，不清楚这样描述是否准确）。参照唯一有官方信息的API得到的是DX的一个对象，那就应该从DX下手。
初始化DX
备注：这里讲个遇到的坑，DX设备的初始化不能在dllmain里进行，否则会失败。

HRESULT hr = S_OK;

hr = CreateDXGIFactory1(__uuidof(IDXGIFactory1), (void**)(&pFactory));
if (FAILED(hr))
{
throw "CreateDXGIFactory1 failed";
return 0;
}
pFactory->EnumAdapters(0, &pAdapter);

const D3D_FEATURE_LEVEL featureLevels[] = {
D3D_FEATURE_LEVEL_11_0,
D3D_FEATURE_LEVEL_10_1,
D3D_FEATURE_LEVEL_10_0,
D3D_FEATURE_LEVEL_9_3,
D3D_FEATURE_LEVEL_9_2,
D3D_FEATURE_LEVEL_9_1
};

D3D11CreateDevice(pAdapter, D3D_DRIVER_TYPE_UNKNOWN, NULL, D3D11_CREATE_DEVICE_BGRA_SUPPORT, featureLevels, 6, D3D11_SDK_VERSION, &device, NULL, NULL);

if (device == NULL)
{
throw "D3D11CreateDevice failed";
return 0;
}

获取phSurface

HANDLE phSurface = NULL;
// 使用DWM截取屏幕
DwmGetDxSharedSurface(hWnd, &phSurface, NULL, NULL, NULL, NULL);
if (phSurface == NULL)
{
throw "Get Shared Surface Failded";
return 0;
}

将数据载入

HRESULT hr = S_OK;

ID3D11Texture2D* sharedSurface = NULL;
hr = device->OpenSharedResource(phSurface, __uuidof(ID3D11Texture2D), (void**)&sharedSurface);//打开对应资源
if (FAILED(hr))
{
throw "OpenSharedResource failed";
return 0;
}

D3D11_TEXTURE2D_DESC shared_desc;
sharedSurface->GetDesc(&shared_desc);

D3D11_TEXTURE2D_DESC description;

description.ArraySize = 1;
description.BindFlags = 0;
description.CPUAccessFlags = D3D11_CPU_ACCESS_READ;
description.Format = DXGI_FORMAT_B8G8R8A8_UNORM;
description.Height = shared_desc.Height;
description.MipLevels = 1;
description.SampleDesc = { 1, 0 };
description.Usage = D3D11_USAGE_STAGING;
description.Width = shared_desc.Width;
description.MiscFlags = 0;

hr = S_OK;

ID3D11Texture2D* texture = NULL;
hr = device->CreateTexture2D(&description, NULL, &texture);
if (FAILED(hr))
{
sharedSurface->Release();
throw "CreateTexture2D failed";
return 0;
}
ID3D11DeviceContext* context = NULL;
device->GetImmediateContext(&context);
context->CopyResource(texture, sharedSurface);

D3D11_MAPPED_SUBRESOURCE mappedResource;
context->Map(texture, 0, D3D11_MAP_READ, 0, &mappedResource);

这里我们其实就已经拿到了对应的图片资源

数据转化
根据DX设备填入的D3D11_CREATE_DEVICE_BGRA_SUPPORT。可以知

typedef struct D3D11_MAPPED_SUBRESOURCE {
  void *pData;
  UINT RowPitch;
  UINT DepthPitch;
} D3D11_MAPPED_SUBRESOURCE;

其中的pData应该是一段对应像素排列位BGRA的位图。RowPitch是每行数据站的字长。为了方便我采用的是用OpenCV直接读入这段数据

cv::Mat mat(shared_desc.Height, shared_desc.Width, CV_8UC4, mappedResource.pData, mappedResource.RowPitch);
cv::imshow("mat", mat);
cv::waitKey(0);
//转BMP写出
std::vector<uchar> buffer;
cv::imencode(".bmp", mat, buffer);

当然也能用MFC

HBITMAP hbmp = CreateBitmap(shared desc.Width, shared desc.Height, 1 32, mappedResource.pData);
CImage img;
img.Attach(hbmp);
img.Save(L"233.bmp");
img.Detach();
DeleteObject(hbmp);

资源释放
最后别忘记了

context->Release();
texture->Release();
sharedSurface->Release();

device->Release();
pAdapter->Release();
pFactory->Release();

FreeLibrary(hUser32);

采用CloseHandle没法正常关掉phSurface，暂时不知道什么解决或方法，或是需不需要关掉。

库的链接
用到了DX方面的库，当然要把他们的lib给链接上，在cpp文件中添加以下代码

#pragma comment(lib, "d3d11.lib")
#pragma comment(lib, "dxgi.lib")

问题
这个API截取不到标题栏。另外也可能是本人对API和DX的理解水平还不到位D2D/D3D渲染的窗口截图是全黑的。