How do I detect whether a graphic will be dithered?

Question

Our application draws drop shadows beneath thumbnails. It does this by creating a bitmap, getting a Graphics object using Graphics.FromImage , and then overlaying images using Graphics.DrawImage . I'm using it today for the first time over remote desktop, and it looks awful , because the shadows are being dithered. I don't know whether this is happening during the overlaying, or in the RDP client. Is there a way for me to determine whether the final image will be dithered, either by looking at the image, the graphics object, or the screen settings, so I can omit the shadow?

Answer 1

you can use the System.Windows.Forms.SystemInformation.TerminalServerSession variable to detect if you are in RDP mode and degrade accordingly.

I do not know a way to detect whether the RDP client does dithering or whether the deskto's colour depth is shifted to match it but you can detect the latter via the GetDeviceCaps function:

using System.Runtime.InteropServices;

public class DeviceCaps
{
    private const int PLANES = 14;
    private const int BITSPIXEL = 12;
    [DllImport("gdi32", CharSet = CharSet.Ansi, 
        SetLastError = true, ExactSpelling = true)]
    private static extern int GetDeviceCaps(int hdc, int nIndex);
    [DllImport("user32", CharSet = CharSet.Ansi, 
        SetLastError = true, ExactSpelling = true)]
    private static extern int GetDC(int hWnd);
    [DllImport("user32", CharSet = CharSet.Ansi, 
        SetLastError = true, ExactSpelling = true)]
    private static extern int ReleaseDC(int hWnd, int hdc);
    
    public short ColorDepth()
    {
        int dc = 0;
        try 
        {
            dc = GetDC(0);
            var nPlanes = GetDeviceCaps(dc, PLANES);
            var bitsPerPixel = GetDeviceCaps(dc, BITSPIXEL);
            return nPlanes * bitsPerPixel;                 
        }
        finally
        {
            if (dc != 0)
                ReleaseDC(0, dc);
        }
    }
}

Rendering based on the colour depth is preferable to speculatively degrading by assuming the user is doing it because of RDP but may be sufficient for you.