asp.net生成缩略图示例方法分享

做站的时候经常会遇到要生成缩略图的功能，因为可能不同的情况需要用来不同大小的缩略图。

本文生成的图片都为正方形，只有正方形的缩略图才是保证图片足够清晰。

当我我这里说的正方形是先按比例压缩，然后加一个固定的白底 然后居中显示。

代码：

新建outputimg.ashx

//调整图片大小
private static size newsize(int maxwidth, int maxheight, int width, int height)
        {
            double w = 0.0;
            double h = 0.0;
            double sw = convert.todouble(width);
            double sh = convert.todouble(height);
            double mw = convert.todouble(maxwidth);
            double mh = convert.todouble(maxheight);

            if (sw < mw && sh < mh)//如果maxwidth和maxheight大于源图像，则缩略图的长和高不变
            {
                w = sw;
                h = sh;
            }
            else if ((sw / sh) > (mw / mh))
            {
                w = maxwidth;
                h = (w * sh) / sw;
            }
            else
            {
                h = maxheight;
                w = (h * sw) / sh;
            }
            return new size(convert.toint32(w), convert.toint32(h));
        }

//生成缩略图
public static void sendsmallimage(string filename, string newfile, int maxheight, int maxwidth, string mode)
        {
            system.drawing.image img = system.drawing.image.fromfile(filename);//源图像的信息
            system.drawing.imaging.imageformat thisformat = img.rawformat; //源图像的格式
            size newsize = newsize(maxwidth, maxheight, img.width, img.height); //返回调整后的图像width与height
            bitmap outbmp = new bitmap(maxwidth, maxheight);
            graphics g = graphics.fromimage(outbmp);
            //设置画布的描绘质量
            g.compositingquality = compositingquality.highquality;
            g.smoothingmode = smoothingmode.highquality;
            g.interpolationmode = interpolationmode.highqualitybicubic;
            g.clear(color.white);
            g.drawimage(img, new rectangle(((maxwidth - newsize.width) / 2), ((maxheight - newsize.height) / 2), newsize.width, newsize.height), 0, 0, img.width, img.height, graphicsunit.pixel);
            g.dispose();
            //以下代码为保存图片时，设置压缩质量
            encoderparameters encoderparams = new encoderparameters();
            long[] quality = new long[1];
            quality[0] = 100;
            encoderparameter encoderparam = new encoderparameter(system.drawing.imaging.encoder.quality, quality);
            encoderparams.param[0] = encoderparam;
            //获取包含有关内置图像编码解码器的信息的imagecodecinfo对象。
            imagecodecinfo[] arrayici = imagecodecinfo.getimageencoders();
            imagecodecinfo jpegici = null;
            for (int x = 0; x < arrayici.length; x++)
            {
                if (arrayici[x].formatdescription.equals("jpeg"))
                {
                    jpegici = arrayici[x];//设置jpeg编码
                    break;
                }
            }
            if (jpegici != null)
            {
                outbmp.save(newfile, jpegici, encoderparams);
            }
            else
            {
                outbmp.save(newfile, thisformat);
            }
            img.dispose();
            outbmp.dispose();
        }

输出图片:

//输出图片
        public static void outputimg(string imgfilepath)
        {
            filestream fs = new filestream(httpcontext.current.server.mappath(imgfilepath), filemode.open, fileaccess.read);
            datetime contentmodified = system.io.file.getlastwritetime(httpcontext.current.server.mappath(imgfilepath));
            if (isclientcached(contentmodified))
            {
                httpcontext.current.response.statuscode = 304;
                httpcontext.current.response.suppresscontent = true;
            }
            else
            {
                byte[] mydata = new byte[fs.length];
                int length = convert.toint32(fs.length);
                fs.read(mydata, 0, length);
                fs.close();
                httpcontext.current.response.outputstream.write(mydata, 0, length);
                httpcontext.current.response.contenttype = "image/jpeg";
                httpcontext.current.response.end();
                httpcontext.current.response.cache.setetagfromfiledependencies();
                httpcontext.current.response.cache.setallowresponseinbrowserhistory(true);
                httpcontext.current.response.cache.setlastmodified(contentmodified);
            }
        }

//outpuimg.ashx?src=/images/weimeidesc/8af30049-797e-4eb4-8a54-cc4de47c1694.jpg!100x100.jpg
        public void processrequest(httpcontext context)
        {
            //获取图片
            string imgurl = context.request.querystring["src"];
            string truefilepath = imgurl.split('!')[0];
            //获取图片大小
            int width = convert.toint32(imgurl.split('!')[1].replace(".jpg", "").split('x')[0]);
            int height = convert.toint32(imgurl.split('!')[1].replace(".jpg", "").split('x')[1]);

            //图片已经存在直接输出
            if (file.exists(context.server.mappath("~/" + imgurl)))
            {
                outputimg("~/"+imgurl);
            }
            else
            {
                if (!string.isnullorempty(imgurl) && file.exists(context.server.mappath("~/" + truefilepath)))
                {
                    image originalimage = system.drawing.image.fromfile(context.server.mappath("~/" + truefilepath));
                    var newbitmap = new bitmap(originalimage);
                    //生成相应的小图并保存
                    sendsmallimage(context.server.mappath("~/" + truefilepath),context.server.mappath("~/" + imgurl), width, height, "meiyouyisi");
                    //输出
                    outputimg("~/" + imgurl);
                }
                else//图片如果不存在 输出默认图片
                {
                    //outputimg(imgurl);
                }
            }
        }