C#生成缩略图 等比例缩放

/// 获取缩略图
        public static Image GetThumbnailImage(Image image, int width, int height)
        {
            if (image == null || width < 1 || height < 1)
                return null;

            // 新建一个bmp图片
            Image bitmap = new Bitmap(width, height);

            // 新建一个画板
            using (var g = Graphics.FromImage(bitmap))
            {
                // 设置高质量插值法
                g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                // 设置高质量,低速度呈现平滑程度
                g.SmoothingMode = SmoothingMode.HighQuality;
                // 高质量、低速度复合
                g.CompositingQuality = CompositingQuality.HighQuality;
                // 清空画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绘制原图片的指定部分
                g.DrawImage(image, new Rectangle(0, 0, width, height),
                    new Rectangle(0, 0, image.Width, image.Height),
                    GraphicsUnit.Pixel);
                return bitmap;
            }
        }

        /// 

        ///     生成缩略图，并保持纵横比
        /// 
        /// 生成缩略图后对象
        public static Image GetThumbnailImageKeepRatio(Image image, int width, int height)
        {
            var imageSize = GetImageSize(image, width, height);
            return GetThumbnailImage(image, imageSize.Width, imageSize.Height);
        }

        /// 

        ///     根据百分比获取图片的尺寸
        /// 
        public static Size GetImageSize(Image picture, int percent)
        {
            if (picture == null || percent < 1)
                return Size.Empty;
            var width = picture.Width*percent/100;
            var height = picture.Height*percent/100;
            return GetImageSize(picture, width, height);
        }

        /// 

        ///     根据设定的大小返回图片的大小，考虑图片长宽的比例问题
        /// 
        public static Size GetImageSize(Image picture, int width, int height)
        {
            if (picture == null || width < 1 || height < 1)
                return Size.Empty;

            var imageSize = new Size(width, height);
            var heightRatio = (double) picture.Height/picture.Width;
            var widthRatio = (double) picture.Width/picture.Height;
            var desiredHeight = imageSize.Height;
            var desiredWidth = imageSize.Width;
            imageSize.Height = desiredHeight;

            if (widthRatio > 0)
                imageSize.Width = Convert.ToInt32(imageSize.Height*widthRatio);

            if (imageSize.Width > desiredWidth)
            {
                imageSize.Width = desiredWidth;
                imageSize.Height = Convert.ToInt32(imageSize.Width*heightRatio);
            }

            return imageSize;
        }

        /// 

        ///     获取图像编码解码器的所有相关信息
        /// 
        /// 包含编码解码器的多用途网际邮件扩充协议 (MIME) 类型的字符串
        /// 返回图像编码解码器的所有相关信息
        public static ImageCodecInfo GetCodecInfo(string mimeType)
        {
            ImageCodecInfo[] codecInfo = ImageCodecInfo.GetImageEncoders();

            return codecInfo.FirstOrDefault(ici => ici.MimeType == mimeType);
        }

        public static ImageCodecInfo GetImageCodecInfo(ImageFormat format)
        {
            ImageCodecInfo[] encoders = ImageCodecInfo.GetImageEncoders();
            return encoders.FirstOrDefault(icf => icf.FormatID == format.Guid);
        }

        /// 

        /// 保存图片
        /// 
        /// 
        /// 
        /// 
        public static void SaveImage(Image image, string savePath, ImageFormat format)
        {
            SaveImage(image, savePath, GetImageCodecInfo(format));
        }

        /// 

        ///     高质量保存图片
        /// 
        private static void SaveImage(Image image, string savePath, ImageCodecInfo ici)
        {
            // 设置 原图片 对象的 EncoderParameters 对象
            EncoderParameters parms = new EncoderParameters(1);
            EncoderParameter parm = new EncoderParameter(Encoder.Quality, ((long) 95));
            parms.Param[0] = parm;
            image.Save(savePath, ici, parms);
            parms.Dispose();
        }

转自：http://www.codefans.net/articles/234.shtml