如何在win32中捕获透明或半透明窗口

Question

我希望有一个透明窗口，它只有边框而不是捕获它（并查看它背后的所有窗口 - 因为它是透明的）到位图。 我设法创建透明窗口，但是当我尝试捕获它时，它不再是透明的，我得到它的原始背景。

这是我到目前为止所拥有的，

1. 创建窗口：

   //registering the window class with white backgournd
   WNDCLASSEX wcex;
   wcex.cbSize = sizeof(WNDCLASSEX);
   wcex.style          = CS_HREDRAW | CS_VREDRAW;
   wcex.lpfnWndProc    = ScreenCaptureROIWinProc;
   wcex.cbClsExtra     = 0;
   wcex.cbWndExtra     = 0;
   wcex.hInstance      = _WPModule.GetHInstance();
   wcex.hIcon          = NULL;
   wcex.hCursor        = NULL;
   wcex.hbrBackground  = CreateSolidBrush(0x00FFFFFF);//white brush;
   wcex.lpszMenuName   = NULL;
   wcex.lpszClassName  = g_lpszClassName;
   wcex.hIconSm        = NULL;
   
   RegisterClassEx(&wcex);
   
   //creating a layered window
   m_hWnd = CreateWindowEx(WS_EX_CLIENTEDGE | WS_EX_LAYERED,g_lpszClassName, NULL,
                           WS_THICKFRAME, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL,
                           NULL, hInstance(), NULL);
   
   // Set white brushed sections of the window to be transparent
   SetLayeredWindowAttributes(m_hWnd, 0x00FFFFFF, 0, LWA_COLORKEY);
   

2. 捕捉窗口：

   RECT ROIRect;
   GetWindowRect(m_hWnd,ROIRect);
   HDC hdcScreen = GetDC(m_hWnd);          
   HDC hdcMem = CreateCompatibleDC(hdcScreen);
   
   BitBlt(
   hdcMem,
   0,
   0,
   ROIRect.right - ROIRect.left, //width of window
   ROIRect.bottom - ROIRect.top, //height of window
   hdcScreen,
   0,
   0,
   SRCCOPY);