asp.net下如何从一张PNG图片里截取一部分图片出来声成预览图

Question

简单的示例Bitmap objBitmap1=new Bitmap(600,300,PixelFormat.Format32bppArgb);
Graphics objGraphics=Graphics.FromImage(objBitmap1);
Bitmap objBitmap=new Bitmap(Server.MapPath("test.jpg"),true);//原始图片Rectangle sr=new Rectangle(450,180,200,100);//截取的区域
Rectangle dr1=new Rectangle(0,0,600,300);//生成预览objGraphics.DrawImage(objBitmap,dr1,sr,GraphicsUnit.Pixel);objBitmap.Dispose();objBitmap1.Save(Response.OutputStream,ImageFormat.Jpeg);
objBitmap1.Dispose();
objGraphics.Dispose();