生成高清晰缩略图

private static Bitmap CreateThumbnail(Bitmap source, int thumbWi, int thumbHi, bool maintainAspect)
    {
        // return the source image if it's smaller than the designated thumbnail
        if (source.Width < thumbWi && source.Height < thumbHi) return source;

        System.Drawing.Bitmap ret = null;
        try
        {
            int wi, hi;

            wi = thumbWi;
            hi = thumbHi;

            if (maintainAspect)
            {
                // maintain the aspect ratio despite the thumbnail size parameters
                if (source.Width > source.Height)
                {
                    wi = thumbWi;
                    hi = (int)(source.Height * ((decimal)thumbWi / source.Width));
                }
                else
                {
                    hi = thumbHi;
                    wi = (int)(source.Width * ((decimal)thumbHi / source.Height));
                }
            }

            // original code that creates lousy thumbnails
            // System.Drawing.Image ret = source.GetThumbnailImage(wi,hi,null,IntPtr.Zero);
            ret = new Bitmap(wi, hi);
            using (Graphics g = Graphics.FromImage(ret))
            {
                g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                g.FillRectangle(Brushes.White, 0, 0, wi, hi);
                g.DrawImage(source, 0, 0, wi, hi);
                g.Dispose();
            }
        }
        catch
        {
            ret = null;
        }
        return ret;
    }
    public static void CreateCompressThumbnail(Bitmap source, int thumbWi, int thumbHi, bool maintainAspect, string SavePathThumb, string fileName)
    {
        using (System.Drawing.Image myThumbnail = CreateThumbnail(source, thumbWi, thumbHi, maintainAspect))
        {
            System.Drawing.Imaging.EncoderParameters encoderParams = new System.Drawing.Imaging.EncoderParameters();
            long[] quality = new long[1];
            quality[0] = 75;
            System.Drawing.Imaging.EncoderParameter encoderParam = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
            encoderParams.Param[0] = encoderParam;

            System.Drawing.Imaging.ImageCodecInfo[] arrayICI = System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders();
            System.Drawing.Imaging.ImageCodecInfo jpegICI = null;
            for (int x = 0; x < arrayICI.Length; x++)
            {
                if (arrayICI[x].FormatDescription.Equals("JPEG"))
                {
                    jpegICI = arrayICI[x];
                    break;
                }
            }
            Bitmap bmp = new Bitmap(myThumbnail);
            bmp.Save(SavePathThumb+fileName, System.Drawing.Imaging.ImageFormat.Jpeg);
            //myThumbnail.Save(Path.Combine(SavePathThumb, fileName + ".jpg"), jpegICI, encoderParams);
            bmp.Dispose();
            myThumbnail.Dispose();

        }

调用方法：

 Bitmap bitmap = new Bitmap("f://1.jpg");

 CreateCompressThumbnail(bitmap, 600, 600, true, "f://", "2.jpg");