在網站開發時,生成縮略圖是一個非常常見和實用的功能.以前在asp里只能借助com組件實現,現在在.net里可以利用框架的強大的類庫輕松實現.下面帖出完整的代碼(帶詳細注釋),參考了網上的一些文章及.net sdk相關內容.qqroom網絡家園的圖片上傳用到了所有的4種生成方式.
        /// <summary>
        /// 生成縮略圖
        /// </summary>
        /// <param name="originalimagepath">源圖路徑（物理路徑）</param>
        /// <param name="thumbnailpath">縮略圖路徑（物理路徑）</param>
        /// <param name="width">縮略圖寬度</param>
        /// <param name="height">縮略圖高度</param>
        /// <param name="mode">生成縮略圖的方式</param>   
        public static void makethumbnail(string originalimagepath, string thumbnailpath, int width, int height, string mode)
        {
            image originalimage = image.fromfile(originalimagepath);
           
            int towidth = width;
            int toheight = height;
       
            int x = 0;
            int y = 0;
            int ow = originalimage.width;
            int oh = originalimage.height;       

            switch (mode)
            {       
                case "hw"://指定高寬縮放（可能變形）               
                    break;
                case "w"://指定寬，高按比例                   
                    toheight = originalimage.height * width/originalimage.width;
                    break;
                case "h"://指定高，寬按比例
                    towidth = originalimage.width * height/originalimage.height;                   
                    break;       
                case "cut"://指定高寬裁減（不變形）               
                    if((double)originalimage.width/(double)originalimage.height > (double)towidth/(double)toheight)
                    {
                        oh = originalimage.height;
                        ow = originalimage.height*towidth/toheight;
                        y = 0;
                        x = (originalimage.width - ow)/2;
                    }
                    else
                    {
                        ow = originalimage.width;
                        oh = originalimage.width*height/towidth;
                        x = 0;
                        y = (originalimage.height - oh)/2;
                    }
                    break;                   
                default :
                    break;
            }   
           
            //新建一個bmp圖片
            image bitmap = new system.drawing.bitmap(towidth,toheight);

            //新建一個畫板
            graphics g = system.drawing.graphics.fromimage(bitmap);

            //設置高質量插值法
            g.interpolationmode = system.drawing.drawing2d.interpolationmode.high;

            //設置高質量,低速度呈現平滑程度
            g.smoothingmode = system.drawing.drawing2d.smoothingmode.highquality;

            //清空畫布并以透明背景色填充
            g.clear(color.transparent);       

            //在指定位置并且按指定大小繪制原圖片的指定部分
            g.drawimage(originalimage, new rectangle(0, 0, towidth, toheight),
                new rectangle(x, y, ow,oh),
                graphicsunit.pixel);

            try
            {           
                //以jpg格式保存縮略圖
                bitmap.save(thumbnailpath, system.drawing.imaging.imageformat.jpeg);
            }
            catch(system.exception e)
            {
                throw e;
            }
            finally
            {
                originalimage.dispose();
                bitmap.dispose();                       
                g.dispose();
            }
        }