图片缩放

/// <summary>
        /// 图片转换
        /// </summary>
        /// <param name="image"></param>
        /// <param name="directoryPath">目录路径</param>
        /// <param name="imageenum"></param>
        /// <param name="qualityNum">图片质量</param>
        public Image ImageTransformation(Image image, string directoryPath, ImageFormatEnum imageenum,long qualityNum=100)
        {
            int oldWidth = image.Width;
            int oldHeight = image.Height;

            int newWidth = 0;
            int newHeight = 0;

            if (oldWidth > oldHeight && oldWidth > (int)imageenum)
            {
                newWidth = (int)imageenum;
                newHeight = Convert.ToInt32(oldHeight * newWidth / oldWidth);
            }
            else if (oldHeight > oldWidth && oldHeight > (int)imageenum)
            {
                newHeight = (int)imageenum;
                newWidth = Convert.ToInt32(oldWidth * newHeight / oldHeight);
            }
            else if (oldWidth == oldHeight && oldWidth > (int)imageenum)
            {
                newHeight = (int)imageenum;
                newWidth = Convert.ToInt32(oldWidth * newHeight / oldHeight);
            }
            else
            {
                newWidth = Convert.ToInt32(oldWidth);
                newHeight = Convert.ToInt32(oldHeight);
            }
            
            //新建图片
            Image bitmap = new Bitmap(newWidth, newHeight);
            //新建画板
            Graphics g = Graphics.FromImage(bitmap);
            //清空画布并设置背景色
            g.Clear(Color.Transparent);
            //设置画布的绘画质量
            g.CompositingQuality = CompositingQuality.HighQuality;
            //设置高质量,低速度呈现平滑程度
            g.SmoothingMode = SmoothingMode.HighQuality;
            //设置高质量插值法
            g.InterpolationMode = InterpolationMode.HighQualityBilinear;
            //显示图像的位置
            Rectangle destRect = new Rectangle(0, 0, newWidth, newHeight);
            //显示图像那一部分
            Rectangle srcRect = new Rectangle(0, 0, image.Width, image.Height);
            g.DrawImage(image, destRect, srcRect, GraphicsUnit.Pixel);
            g.Dispose();

            //以下代码为保存图片时,设置压缩质量
            EncoderParameters encoderParams = new EncoderParameters();            
            EncoderParameter encoderParam = new EncoderParameter(Encoder.Quality, qualityNum);
            encoderParams.Param[0] = encoderParam;
            string imgname = _context.Server.MapPath("/") + directoryPath + "/" + imageenum.ToString() + ".jpeg";
            bitmap.Save(imgname, GetEncoderInfo("image/jpeg"), encoderParams);
            bitmap.Dispose();
            
            return bitmap;
        }

        private static ImageCodecInfo GetEncoderInfo(string mimeType)
        {
            int j;
            ImageCodecInfo[] encoders;
            encoders = ImageCodecInfo.GetImageEncoders();
            for (j = 0; j < encoders.Length; ++j)
            {
                if (encoders[j].MimeType == mimeType)
                    return encoders[j];
            }
            return null;
        }
        /// <summary>
        /// 图片格式
        /// </summary>
        public enum ImageFormatEnum
        {
            _60 = 60,
            _90 = 90,
            _160 = 160,
            _220 = 220,
            _m = 400,
            _660 = 660,
            _800 = 800,
            _1200 = 1200,
            _b = 0      //原图
        }