//制作縮略圖（壓縮圖），可接收兩種參數(shù)，物理路徑文件 originalImagePath 或 Byte[]型數(shù)據(jù) bvalue，兩者同時(shí)傳遞以物理路徑文件優(yōu)先。
//方法返回壓縮后的 Byte[]數(shù)組數(shù)據(jù)，如果同時(shí)指定輸出文件路徑thumbnailPath，則同時(shí)生成這個(gè)文件。
//指定輸出縮略圖的寬width和高h(yuǎn)eight，如果為0，則默認(rèn)為原圖的寬或高
//mode為壓縮方法："HW":指定高寬縮放（可能變形）,"W":指定寬，高按比例 ,"H":指定高，寬按比例 , "Cut":指定高寬裁減（不變形）
 
public  Byte[] MakeThumbnail(string originalImagePath, Byte[] bvalue, string thumbnailPath, int width=0, int height=0, string mode="Cut", System.Drawing.Drawing2D.InterpolationMode interpolationMode= System.Drawing.Drawing2D.InterpolationMode.High)
{
                System.Drawing.Image originalImage;
                if (originalImagePath != "")
                {
                    originalImage = System.Drawing.Image.FromFile(originalImagePath);
                }
                else
                {
                    originalImage = System.Drawing.Image.FromStream(new System.IO.MemoryStream(bvalue));
                }
 
                int towidth = width;
                int toheight = height;
 
                int x = 0;
                int y = 0;
                int ow = originalImage.Width;
                int oh = originalImage.Height;
 
                if (towidth == 0)
                {
                    towidth = ow;
                }
                if (toheight == 0)
                {
                    toheight = oh;
                }
 
                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 * towidth / originalImage.Width;
                        break;
                    case "H"://指定高，寬按比例 
                        towidth = originalImage.Width * toheight / 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 * toheight / towidth;
                            x = 0;
                            y = (originalImage.Height - oh) / 2;
                        }
                        break;
                    default:
                        break;
                }
                //新建一個(gè)bmp圖片 
                System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
 
                //新建一個(gè)畫(huà)板 
                System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
 
                //設(shè)置高質(zhì)量插值法 
                g.InterpolationMode = interpolationMode ;
 
                //設(shè)置高質(zhì)量,低速度呈現(xiàn)平滑程度 
                g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
 
                //清空畫(huà)布并以透明背景色填充 
                g.Clear(System.Drawing.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 System.Drawing.Rectangle(0, 0, towidth, toheight),
                    new System.Drawing.Rectangle(x, y, ow, oh),
                    System.Drawing.GraphicsUnit.Pixel);
 
 
                try
                {
                    //以jpg格式保存縮略圖 
                    System.IO.MemoryStream mstream = new System.IO.MemoryStream();
                    System.Drawing.Imaging.ImageFormat format = originalImage.RawFormat;
                    System.Drawing.Imaging.ImageFormat toFormat = System.Drawing.Imaging.ImageFormat.Jpeg;
                    if (format.Equals(System.Drawing.Imaging.ImageFormat.Jpeg))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Jpeg;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Png))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Png;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Bmp))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Bmp;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Gif))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Gif;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Icon))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Icon;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Emf))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Emf;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Exif))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Exif;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Tiff))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Tiff;
                    }
                    else if (format.Equals(System.Drawing.Imaging.ImageFormat.Wmf))
                    {
                        toFormat = System.Drawing.Imaging.ImageFormat.Wmf;
                    }
 
                    bitmap.Save(mstream, toFormat);
                    
                    byte[] byData = new Byte[mstream.Length];
                    mstream.Position = 0;
                    mstream.Read(byData, 0, byData.Length);
                    mstream.Close();
                    if (thumbnailPath != "")
                    {
                        bitmap.Save(thumbnailPath, toFormat);
                    }
                    return byData;
                }
                catch (System.Exception e)
                {
                    throw e;
                }
                finally
                {
                    originalImage.Dispose();
                    bitmap.Dispose();
                    g.Dispose();
                }
}

string upfilename = Request.PhysicalApplicationPath + "\\upload.jpg"; //上傳的圖片路徑
string mtfilename = Request.PhysicalApplicationPath + "\\mt.jpg";  //縮略圖的圖片路徑
if (System.IO.File.Exists(upfilename))
{
    FileInfo fileInfo = new FileInfo(upfilename);
    float _fsize = fileInfo.Length / (1024*1024);
    if (_fsize >= 2)
    {
        MakeThumbnail(upfilename, null, mtfilename);
    }
    else
    {
        mtfilename = upfilename;
    }
    Response.Write("Result Filename is :"+mtfilename);
}