怎样用 c# GDI 截取一张图片的一部分，然后再替换另一张图片的一部分

主要利用://在指定位置画图
            g.DrawImage(image, new System.Drawing.Rectangle(0, 0, bitmap.Width, bitmap.Height),
                new System.Drawing.Rectangle(0, 0, image.Width, image.Height),
                System.Drawing.GraphicsUnit.Pixel);
image:你的原图,
System.Drawing.Rectangle:位置与长宽,根据自己需要设置!
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实例如下:         


   public void MakeSmallImg(string filePath, string saveImg)
        {
            //从文件取得图片对象
            System.Drawing.Image image = System.Drawing.Image.FromFile(filePath, true);

           
            //取得图片大小
            System.Drawing.Size size = new System.Drawing.Size((int)image.Width, (int)image.Height);
            //新建一个bmp图片
            System.Drawing.Image bitmap = new System.Drawing.Bitmap(size.Width, size.Height);
            //新建一个画板
            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
            //设置高质量插值法
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.Default;
            //设置高质量,低速度呈现平滑程度
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.Default;
            //清空一下画布
            g.Clear(System.Drawing.Color.White);
            //在指定位置画图
            g.DrawImage(image, new System.Drawing.Rectangle(0, 0, bitmap.Width, bitmap.Height),
                new System.Drawing.Rectangle(0, 0, image.Width, image.Height),
                System.Drawing.GraphicsUnit.Pixel);


            ///文字水印
            //System.Drawing.Graphics G=System.Drawing.Graphics.FromImage(bitmap);
            //System.Drawing.Font f=new Font("宋体",10);
            //System.Drawing.Brush b=new SolidBrush(Color.Black);
            //G.DrawString("myohmine",f,b,10,10);
            //G.Dispose();

 
            ///图片水印
            //System.Drawing.Image copyImage = System.Drawing.Image.FromFile(System.Web.HttpContext.Current.Server.MapPath("pic/1.gif"));
            //Graphics a = Graphics.FromImage(bitmap);
            //a.DrawImage(copyImage, new Rectangle(bitmap.Width-copyImage.Width,bitmap.Height-copyImage.Height,copyImage.Width, copyImage.Height),0,0, copyImage.Width, copyImage.Height, GraphicsUnit.Pixel);

            //copyImage.Dispose();
            //a.Dispose();
            //copyImage.Dispose();


            //保存高清晰度的缩略图
            //   bitmap.Save(strGoodFile, System.Drawing.Imaging.ImageFormat.Jpeg);
            //   加个a表示是缩略图
            bitmap.Save(saveImg, System.Drawing.Imaging.ImageFormat.Jpeg);
            g.Dispose();
            image.Dispose();
            bitmap.Dispose();
        }

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c#缩小图片后不清晰，要怎么缩小才能跟原来一样清晰。我用：System.Drawing.Bitmap缩小的。

正好之前有写过这个，不过是用的.net自带的api写的，如果想要弄的很好的话非常难，有兴趣可以看一下插值法，有若干种办法不一一列举。
        /// <summary>
        /// 获取缩小后的图片
        /// </summary>
        /// <param name="bm">要缩小的图片</param>
        /// <param name="times">要缩小的倍数</param>
        /// <returns></returns>
        private Bitmap GetSmall(Bitmap bm, double times)
        {
            int nowWidth = (int)(bm.Width / times);
            int nowHeight = (int)(bm.Height / times);
            Bitmap newbm = new Bitmap(nowWidth, nowHeight);//新建一个放大后大小的图片

            if (times >= 1 && times <= 1.1)
            {
                newbm = bm;
            }
            else
            {
                Graphics g = Graphics.FromImage(newbm);
                g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                g.SmoothingMode = SmoothingMode.HighQuality;
                g.CompositingQuality = CompositingQuality.HighQuality;
                g.DrawImage(bm, new Rectangle(0, 0, nowWidth, nowHeight), new Rectangle(0, 0, bm.Width, bm.Height), GraphicsUnit.Pixel);
                g.Dispose();
            }
            return newbm;
        }