.NET生成缩略图（转载）

    /// <summary>
    /// 生成缩略图
    /// </summary>
    /// <param name="originalImagePath"></param>
    /// <param name="thumbnailPath"></param>
    /// <param name="width"></param>
    /// <param name="height"></param>
    public void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height)
    {
        //获取原始图片  
        System.Drawing.Image originalImage = System.Drawing.Image.FromFile(originalImagePath);
        //缩略图画布宽高  
        int towidth = width;
        int toheight = height;
        //原始图片写入画布坐标和宽高(用来设置裁减溢出部分)  
        int x = 0;
        int y = 0;
        int ow = originalImage.Width;
        int oh = originalImage.Height;
        //原始图片画布,设置写入缩略图画布坐标和宽高(用来原始图片整体宽高缩放)  
        int bg_x = 0;
        int bg_y = 0;
        int bg_w = towidth;
        int bg_h = toheight;
        //倍数变量  
        double multiple = 0;
        //获取宽长的或是高长与缩略图的倍数  
        if (originalImage.Width >= originalImage.Height)
            multiple = (double)originalImage.Width / (double)width;
        else
            multiple = (double)originalImage.Height / (double)height;
        //上传的图片的宽和高小等于缩略图  
        if (ow <= width && oh <= height)
        {
            //缩略图按原始宽高  
            bg_w = originalImage.Width;
            bg_h = originalImage.Height;
            //空白部分用背景色填充  
            bg_x = Convert.ToInt32(((double)towidth - (double)ow) / 2);
            bg_y = Convert.ToInt32(((double)toheight - (double)oh) / 2);
        }
        //上传的图片的宽和高大于缩略图  
        else
        {
            //宽高按比例缩放  
            bg_w = Convert.ToInt32((double)originalImage.Width / multiple);
            bg_h = Convert.ToInt32((double)originalImage.Height / multiple);
            //空白部分用背景色填充  
            bg_y = Convert.ToInt32(((double)height - (double)bg_h) / 2);
            bg_x = Convert.ToInt32(((double)width - (double)bg_w) / 2);
        }
        //新建一个bmp图片,并设置缩略图大小.  
        System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
        //新建一个画板  
        System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
        //设置高质量插值法  
        g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBilinear;
        //设置高质量,低速度呈现平滑程度  
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
        //清空画布并设置背景色  
        g.Clear(System.Drawing.ColorTranslator.FromHtml("#F2F2F2"));
        //在指定位置并且按指定大小绘制原图片的指定部分  
        //第一个System.Drawing.Rectangle是原图片的画布坐标和宽高,第二个是原图片写在画布上的坐标和宽高,最后一个参数是指定数值单位为像素  
        g.DrawImage(originalImage, new System.Drawing.Rectangle(bg_x, bg_y, bg_w, bg_h), new System.Drawing.Rectangle(x, y, ow, oh), System.Drawing.GraphicsUnit.Pixel);
        try
        {
            //获取图片类型  
            string fileExtension = System.IO.Path.GetExtension(originalImagePath).ToLower();
            //按原图片类型保存缩略图片,不按原格式图片会出现模糊,锯齿等问题.  
            switch (fileExtension)
            {
                case ".gif": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Gif); break;
                case ".jpg": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg); break;
                case ".bmp": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Bmp); break;
                case ".png": bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Png); break;
            }
        }
        catch (System.Exception e)
        {
            throw e;
        }
        finally
        {
            originalImage.Dispose();
            bitmap.Dispose();
            g.Dispose();
        }
    }