C#更改图的PixelFormat

Bitmap clone = new Bitmap(bitmap.Width, bitmap.Height, System.Drawing.Imaging.PixelFormat.Format32bppRgb);
using (Graphics gr = Graphics.FromImage(clone))
{
    gr.DrawImage(bitmap, new Rectangle(0, 0, clone.Width, clone.Height));
    clone.SetResolution(xDpi, yDpi);
 }