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On this page you can learn how to do a screen dump.
The program will dump the complete screen to a bmp-file.
First we must get a Device Context for the entire screen, this is done by getting a DC with HWND set to 0:
   HDC dc = GetDC(0);
If you want to dump a specific window to the file, you simply use the window's HWND in the call to GetDC.
Then we must get the size of the screen, here GetDeviceCaps comes handy:
   int Width = GetDeviceCaps(dc, HORZRES);
   int Height = GetDeviceCaps(dc, VERTRES);
Next we create a memory DC:
   HDC MemDc = CreateCompatibleDC(dc);
Then we create a bitmap, comatible with, and the size off, the screen DC, and tells windows to use this bitmap with the memory DC:
   HBITMAP BitMap = CreateCompatibleBitmap(dc, Width, Height);
   SelectObject(MemDc, BitMap);
Then we copy the image from the screen DC to the memory DC, and thus to our bitmap,
   BitBlt(MemDc, 0, 0, Width, Height, dc, 0, 0, SRCCOPY);
Then we are ready to start writing:
   std::ofstream File("screen.bmp", std::ios::binary);
Then we must know how many bits is used for each pixel on the screen:
   GetObject(BitMap, sizeof(Bm), &Bm);
A bmp files contains tree parts, two headers and the data. The two headers can be written with WriteHeader which I created for the tutorial. To write the header is simply:
      WriteHeader(File, Width, Height, Bm.bmBitsPixel);
Then we get the data from the bitmap and write them to the file:
   char *Buffer = new char [Width*Height*Bm.bmBitsPixel/8];
             (BITMAPINFO *)&Header,
   File.write(Buffer, Height*Width*Bm.bmBitsPixel/8);
And with a bit cleanup we are done:
   ReleaseDC(0, dc);
   ReleaseDC(0, MemDc);
   delete [] Buffer;