DirectX屏幕截图：为什么GetBackBuffer总是给我黑色图像？

Question

My problem is about capturing the full screen image using DirectX ,no matter what applications are running. I tried to capture the screen by using the GetBackBuffer function (because I've been warned that GetFrontBufferData is very slow ,as slow as GDI)

However, when I tried it, GetFrontBufferData works fine for capturing the screen, but GetBackBuffer always gives me a black image .

Some people told me that GetBackBuffer only gets what you draw on the screen. If you draw nothing ,you get nothing. If you want to capture the screen image by using GetBackBuffer function ,that is impossible. Is that right ?

My attempted code is:

LPDIRECT3D9 g_pD3D = NULL;
D3DDISPLAYMODE ddm;
D3DPRESENT_PARAMETERS d3dpp;
IDirect3DDevice9 * g_pd3dDevice;
IDirect3DSurface9 * pSurface;
ZeroMemory(&d3dpp, sizeof(D3DPRESENT_PARAMETERS));
ZeroMemory(&d3dpp, sizeof(d3dpp));
g_pD3D = Direct3DCreate9(D3D_SDK_VERSION);
g_pD3D->GetAdapterDisplayMode(D3DADAPTER_DEFAULT, &ddm);
d3dpp.Windowed = TRUE;
d3dpp.Flags = D3DPRESENTFLAG_LOCKABLE_BACKBUFFER;
d3dpp.BackBufferFormat = ddm.Format;
d3dpp.BackBufferHeight = ddm.Height;
d3dpp.BackBufferWidth = ddm.Width;
d3dpp.MultiSampleType = D3DMULTISAMPLE_NONE;
d3dpp.SwapEffect = D3DSWAPEFFECT_DISCARD;
d3dpp.hDeviceWindow = GetDesktopWindow();
d3dpp.PresentationInterval = D3DPRESENT_INTERVAL_DEFAULT;
d3dpp.FullScreen_RefreshRateInHz = D3DPRESENT_RATE_DEFAULT;
HRESULT hr = g_pD3D->CreateDevice(D3DADAPTER_DEFAULT, D3DDEVTYPE_HAL, GetDesktopWindow(), D3DCREATE_SOFTWARE_VERTEXPROCESSING, &d3dpp, &g_pd3dDevice);
hr = g_pd3dDevice->CreateOffscreenPlainSurface(ddm.Width, ddm.Height, D3DFMT_A8R8G8B8, D3DPOOL_SCRATCH, &pSurface, NULL);
//hr = g_pd3dDevice->GetBackBuffer(0, 0, D3DBACKBUFFER_TYPE_MONO, &pSurface);//Getbackbuffer gives me a black image
hr = g_pd3dDevice->GetFrontBufferData(0, pSurface);//getfrontbufferdata works well
hr = D3DXSaveSurfaceToFile("d:/1.png", D3DXIFF_PNG, pSurface, NULL, NULL);
pSurface->UnlockRect();
pSurface->Release();
g_pd3dDevice->Release();
g_pD3D->Release();

Answer 1

The people that told you that GetBackbuffer only gets what you've drawn are telling you the truth. Each application that uses Direct3D has its own back buffers that contain only what that application draws into them. Your application has its own back buffer that contains only what your application draws into it.