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();
            }
        }

關鍵方法Graphics.DrawImage見ms-help://MS.NETFrameworkSDKv1.1.CHS/cpref/html/frlrfsystemdrawinggraphicsclassdrawimagetopic11.htm