C#图片压缩的实现方法

一般在web应用中，对客户端提交上来的图片肯定需要进行压缩的。尤其是比较大的图片，如果不经过压缩会导致页面变的很大，打开速度比较慢，当然了如果是需要高质量的图片也得需要生产缩略图。

下面贴出我自己琢磨的图片压缩算法，首先这个是未经优化的简单实现：

public static system.drawing.image getimagethumb(system.drawing.image sourceimg, int width, int height)
        {
            system.drawing.image targetimg = new system.drawing.bitmap(width, height);
            using (system.drawing.graphics g = system.drawing.graphics.fromimage(targetimg))
            {
                g.interpolationmode = system.drawing.drawing2d.interpolationmode.high;
                g.smoothingmode = system.drawing.drawing2d.smoothingmode.highquality;
                g.interpolationmode = system.drawing.drawing2d.interpolationmode.highqualitybicubic;
                g.compositingquality = system.drawing.drawing2d.compositingquality.highquality;
                g.pixeloffsetmode = system.drawing.drawing2d.pixeloffsetmode.highquality;
                g.drawimage(sourceimg, new system.drawing.rectangle(0, 0, width, height), new system.drawing.rectangle(0, 0, sourceimg.width, sourceimg.height), system.drawing.graphicsunit.pixel);
                g.dispose();
            }
            return targetimg;
        }

这个方法比较简单，用到的是高质量压缩。经过这个方法压缩后，200k的图片只能压缩到160k左右。经过改写代码实现了如下的方法：

public bitmap getimagethumb(bitmap mg, size newsize)
        {
            double ratio = 0d;
            double mythumbwidth = 0d;
            double mythumbheight = 0d;
            int x = 0;
            int y = 0;

            bitmap bp;

            if ((mg.width / convert.todouble(newsize.width)) > (mg.height /
            convert.todouble(newsize.height)))
                ratio = convert.todouble(mg.width) / convert.todouble(newsize.width);
            else
                ratio = convert.todouble(mg.height) / convert.todouble(newsize.height);
            mythumbheight = math.ceiling(mg.height / ratio);
            mythumbwidth = math.ceiling(mg.width / ratio);

            size thumbsize = new size((int)newsize.width, (int)newsize.height);
            bp = new bitmap(newsize.width, newsize.height);
            x = (newsize.width - thumbsize.width) / 2;
            y = (newsize.height - thumbsize.height);
            system.drawing.graphics g = graphics.fromimage(bp);
            g.smoothingmode = smoothingmode.highquality;
            g.interpolationmode = interpolationmode.highqualitybicubic;
            g.pixeloffsetmode = pixeloffsetmode.highquality;
            rectangle rect = new rectangle(x, y, thumbsize.width, thumbsize.height);
            g.drawimage(mg, rect, 0, 0, mg.width, mg.height, graphicsunit.pixel);

            return bp;
        }

这样实现的压缩使压缩率大幅度上升。其实代码并没有变多少，最主要的是在保存的时候要是用jpg格式，如果不指定格式，默认使用的是png格式。

下面这个是园友写的根据设置图片质量数值来压缩图片的方法：

public static bool getpicthumbnail(string sfile, string outpath, int flag)
        {
            system.drawing.image isource = system.drawing.image.fromfile(sfile);
            imageformat tformat = isource.rawformat;

            //以下代码为保存图片时，设置压缩质量 
            encoderparameters ep = new encoderparameters();
            long[] qy = new long[1];
            qy[0] = flag;//设置压缩的比例1-100 
            encoderparameter eparam = new encoderparameter(system.drawing.imaging.encoder.quality, qy);
            ep.param[0] = eparam;
            try
            {
                imagecodecinfo[] arrayici = imagecodecinfo.getimageencoders();
                imagecodecinfo jpegiciinfo = null;
                for (int x = 0; x < arrayici.length; x++)
                {
                    if (arrayici[x].formatdescription.equals("jpeg"))
                    {
                        jpegiciinfo = arrayici[x];
                        break;
                    }
                }
                if (jpegiciinfo != null)
                {
                    isource.save(outpath, jpegiciinfo, ep);//dfile是压缩后的新路径 
                }
                else
                {
                    isource.save(outpath, tformat);
                }
                return true;
            }
            catch
            {
                return false;
            }
            finally
            {
                isource.dispose();
                isource.dispose();
            }
        }

转载来源：http://www.cnblogs.com/lifeil/archive/2013/02/25/2931683.html