高质量图片缩略图生成（编程笔记）

        /// <summary>
        /// 生成缩略图
        /// </summary>
        /// <param name="maxWidth">生成的缩略图最大宽度</param>
        /// <param name="maxHeight">生成的缩略图最大高度</param>
        /// <param name="imgFileStream">图片文件流对象</param>
        /// <param name="savePath">文件保存路径</param>
        /// <param name="imgFormat">生成图片的格式</param>
        public void GenerateThumbnail(int maxWidth, int maxHeight, System.IO.Stream imgFileStream, string savePath,
                                         System.Drawing.Imaging.ImageFormat imgFormat)
        {
            using (var img = System.Drawing.Image.FromStream(imgFileStream))
            {
                var sourceWidth = img.Width;
                var sourceHeight = img.Height;

                var thumbWidth = sourceWidth; //要生成的缩略图的宽度
                var thumbHeight = sourceHeight; //要生成的缩略图的高度

                //计算生成图片的高度和宽度
                if (sourceWidth > maxWidth || sourceHeight > maxHeight)
                {
                    var rateWidth = (double)sourceWidth / maxWidth;
                    var rateHeight = (double)sourceHeight / maxHeight;

                    if (rateWidth > rateHeight)
                    {
                        thumbWidth = maxWidth;
                        thumbHeight = (int)Math.Round(sourceHeight / rateWidth);
                    }
                    else
                    {
                        thumbWidth = (int)Math.Round(sourceWidth / rateHeight);
                        thumbHeight = maxHeight;
                    }
                }

                using (var bmp = new System.Drawing.Bitmap(thumbWidth, thumbHeight))
                {
                    //从Bitmap创建一个System.Drawing.Graphics对象，用来绘制高质量的缩小图。
                    using (var gr = System.Drawing.Graphics.FromImage(bmp))
                    {
                        //设置 System.Drawing.Graphics对象的SmoothingMode属性为HighQuality
                        gr.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                        //下面这个也设成高质量
                        gr.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                        //下面这个设成High
                        gr.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

                        //把原始图像绘制成上面所设置宽高的缩小图
                        var rectDestination = new System.Drawing.Rectangle(0, 0, thumbWidth, thumbHeight);
                        gr.DrawImage(img, rectDestination, 0, 0, sourceWidth, sourceHeight,
                                     System.Drawing.GraphicsUnit.Pixel);
                        //保存图像
                        bmp.Save(savePath, imgFormat);
                    }
                }
            }
        }        /// <summary>
        /// 生成缩略图