WinAPI BitBlt freezes for minutes

Question

I have a little modified QScreen::grabWindow function. And on some computers, unfortunately I didn't find relationship between them, BitBlt freezes for even minutes! Why this can happen and what can I do?

QPixmap DetectionFlow::grabScreen(HWND h)
{
    RECT rect;
    GetClientRect(h, (LPRECT)&rect);

    // get the height and width of the screen
    int height = rect.bottom - rect.top;
    int width = rect.right - rect.left;

    // Create and setup bitmap
    HDC display_dc = GetDC(0);

    HDC bitmap_dc = CreateCompatibleDC(display_dc);

    HBITMAP bitmap = CreateCompatibleBitmap(display_dc, width, height);

    HGDIOBJ null_bitmap = SelectObject(bitmap_dc, bitmap);

    HDC window_dc = GetDC(h);
    BitBlt(bitmap_dc, 0, 0, width, height, window_dc, 0, 0, SRCCOPY);

    // clean up all but bitmap
    ReleaseDC(h, window_dc);

    SelectObject(bitmap_dc, null_bitmap);

    DeleteDC(bitmap_dc);


    const QPixmap pixmap = qt_pixmapFromWinHBITMAP(bitmap);

    DeleteObject(bitmap);

    ReleaseDC(0, display_dc);

    return pixmap;
}

PS. What is interesting, on computers, where freezes, it freezes randomly. So usually there it works fast (a couple of ms) and then freeze.

Answer 1

Problem was in Aero. Computers that freezes with BitBlt has Win7 and Aero. Without Aero all ok.