图像仅捕获空白

Question

我正在尝试尝试捕获网页中特定区域的图像。下面的代码工作正常，但问题是它每次只给我一个大小为1.64kb的空白图像。

函数调用

 protected void btn_Click(object sender, EventArgs e)
        {
            System.Drawing.Image image = CaptureScreen(50, 99, 930, 450);
            image.Save(Server.MapPath("/img/screen.jpg"));
        }

功能定义

public System.Drawing.Image CaptureScreen(int sourceX, int sourceY, int destX, int destY)
        {
            Bitmap bmp = new Bitmap(930, 430);
            Graphics g = Graphics.FromImage(bmp);
            Size xx=new Size();
            xx.Width=930;
            xx.Height=450;
            g.CopyFromScreen(sourceX, sourceY, destX, destY, xx);
            return bmp;
        }