c#利用Grahics进行图片裁剪

最开始用了

/// <summary>
        /// 裁剪图片
        /// </summary>
        /// <param name="imagepath"/>
        /// <param name="savepath">"c:\images\"</param>
        private list<string> defclipimage(string imagepath, string savepath)
        {

            var fileinfo = new fileinfo(imagepath);
            if (!fileinfo.exists)
                throw new exception("图片" + imagepath + "不存在！");
            var savepathlist = new list<string>();
            var spath = savepath + fileinfo.name.replace(fileinfo.extension, string.empty);
            try
            {
                var bitmap = new bitmap(imagepath);
                var format = bitmap.pixelformat;
                bitmap clonebitmap = bitmap.clone(_clonerect1, format);
                var temppath = spath + "_1.jpg";
                clonebitmap.save(temppath);
                savepathlist.add(temppath);
                clonebitmap.dispose();
                clonebitmap = bitmap.clone(_clonerect2, format);
                temppath = spath + "_2.jpg";
                clonebitmap.save(temppath);
                savepathlist.add(temppath);
                clonebitmap.dispose();
                clonebitmap = bitmap.clone(_clonerect3, format);
                temppath = spath + "_3.jpg";
                clonebitmap.save(temppath);
                savepathlist.add(temppath);
                clonebitmap.dispose();
                clonebitmap = bitmap.clone(_clonerect4, format);
                temppath = spath + "_4.jpg";
                clonebitmap.save(temppath);
                savepathlist.add(temppath);
                clonebitmap.dispose();
                bitmap.dispose();
                return savepathlist;

            }
            catch
            {
                throw new exception("图片" + imagepath + "处理失败！");

            }

        }

但是速度太慢。

后来发现用grahics 会快很多

   private void test()
        {

            bitmap bitmap = new bitmap(application.startuppath + @"\image\1.jpg");
            var bt = new bitmap(7500, 3750);

            var grahics = graphics.fromimage(bt);
            grahics.drawimage(bitmap, _clonerect1, _clonerect1,graphicsunit.pixel);
            bt.save(application.startuppath + "1.jpg");

            grahics.drawimage(bitmap, _clonerect1, _clonerect2, graphicsunit.pixel);
            bt.save(application.startuppath + "2.jpg");

            grahics.drawimage(bitmap, _clonerect1, _clonerect3, graphicsunit.pixel);
            bt.save(application.startuppath + "3.jpg");

            grahics.drawimage(bitmap, _clonerect1, _clonerect4, graphicsunit.pixel);
            bt.save(application.startuppath + "4.jpg");

            grahics.dispose();
            bt.dispose();
        }