Question

I am just starting out with win32 GDI programming and finding good references hard to come by. I have a simple application that captures the screen by doing the following:

UINT32 x,y;
x = GetSystemMetrics(SM_CXSCREEN);
y = GetSystemMetrics(SM_CYSCREEN);

HDC hdc = GetDC(NULL);
HDC hdcScreen = CreateCompatibleDC(hdc);

HBITMAP hbmp = CreateCompatibleBitmap(hdc, x, y);

SelectObject(hdcScreen, hbmp);

BitBlt(hdcScreen, 0, 0, x, y, hdc, 0, 0, SRCCOPY)

ReleaseDC(NULL, hdc);

I am capturing a compatible bitmap which on my machine is 32-bit. Using same/similar calls how would I capture the screen in 8-bit? What about as 16-bit?

Answer 1

Use CreateDIBSection to create a 8bpp bitmap, and BitBlt onto that.

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Filling in the BITMAPINFO structure is going to be the interesting. You can’t use a normal BITMAPINFO struct as it only allocates space for a single palette entry and, with a 8bpp image – you will need the full 256 entries.

If you want to cheat a bit, one can use a anonymous union to declare a BITMAPINFO with enough space for its palette.

union
{
  BITMAPINFO bmi;
  struct {
    BITMAPINFOHEADER bmih;
    RGBQUAD extra[256];
  } dummy;
};

bmi.bmiHeader.biSize = sizeof (bmi.bmiHeader);
bmi.biBitCount = 8;
// and so on.

As to the values to initialize in the color table… I can’t think of an easy way to get a default 8bpp palette from GDI when its not in 8bpp mode. I have a suspicion that CreateHalftonePalette isn’t going to do anything on a non palette device.