Recently I am using an Win API called PrintWindow, which can screenshot a specific window. In some windows that enabled hardware acceleration, the image will be blank. However, if I changed the third parameter from 0 to 3, it can work again.

I also posted a demo code written in C#:

        [DllImport("user32.dll")]
        public static extern bool GetWindowRect(IntPtr hWnd, out RECT lpRect);
        [DllImport("user32.dll")]
        public static extern bool PrintWindow(IntPtr hWnd, IntPtr hdcBlt, int nFlags);
        public struct RECT
        {
            public int Left;
            public int Top;
            public int Right;
            public int Bottom;
        }
        public static Bitmap GetScreenshot_PrintWindow(IntPtr hWnd, Rectangle screenBounds)
        {
            RECT windowRect;
            if (!GetWindowRect(hWnd, out windowRect))
                return null;
            Rectangle windowBounds = new Rectangle(windowRect.Left, windowRect.Top, windowRect.Right - windowRect.Left, windowRect.Bottom - windowRect.Top);
            Rectangle intersection = Rectangle.Intersect(screenBounds, windowBounds);
            if (intersection.IsEmpty)
                return null;
            Bitmap screenshot = new Bitmap(intersection.Width, intersection.Height);
            using (Graphics g = Graphics.FromImage(screenshot))
            {
                IntPtr hdc = g.GetHdc();
                PrintWindow(hWnd, hdc, 3);
                g.ReleaseHdc(hdc);
            }

            return screenshot;
        }

In the official document, Microsoft only listed PW_CLIENTONLY, and there is no much information about this parameter, so I am wondering what it is and what values can I use (and their effect).