/// <summary>/// 压缩图片至n Kb以下/// </summary>/// <param name="img">图片</param>/// <param name="format">图片格式</param>/// <param name="targetLen">压缩后大小</param>/// <param name="srcLen">原始大小</param>/// <returns>压缩后的图片内存流</returns>public static MemoryStream Zip(Image img, ImageFormat format, long targetLen, long srcLen = 0){//设置允许大小偏差幅度 默认10kbconst long nearlyLen = 10240;//返回内存流  如果参数中原图大小没有传递 则使用内存流读取var ms = new MemoryStream();if (0 == srcLen){img.Save(ms, format);srcLen = ms.Length;}//单位 由Kb转为byte 若目标大小高于原图大小，则满足条件退出targetLen *= 1024;if (targetLen >= srcLen){ms.SetLength(0);ms.Position = 0;img.Save(ms, format);return ms;}//获取目标大小最低值var exitLen = targetLen - nearlyLen;//初始化质量压缩参数 图像 内存流等var quality = (long)Math.Floor(100.00 * targetLen / srcLen);var parms = new EncoderParameters(1);//获取编码器信息ImageCodecInfo formatInfo = null;var encoders = ImageCodecInfo.GetImageEncoders();foreach (ImageCodecInfo icf in encoders){if (icf.FormatID == format.Guid){formatInfo = icf;break;}}//使用二分法进行查找 最接近的质量参数long startQuality = quality;long endQuality = 100;quality = (startQuality + endQuality) / 2;while (true){//设置质量parms.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);//清空内存流 然后保存图片ms.SetLength(0);ms.Position = 0;img.Save(ms, formatInfo, parms);//若压缩后大小低于目标大小，则满足条件退出if (ms.Length >= exitLen && ms.Length <= targetLen){break;}else if (startQuality >= endQuality) //区间相等无需再次计算
                {break;}else if (ms.Length < exitLen) //压缩过小,起始质量右移
                {startQuality = quality;}else //压缩过大 终止质量左移
                {endQuality = quality;}//重新设置质量参数 如果计算出来的质量没有发生变化，则终止查找。这样是为了避免重复计算情况{start:16,end:18} 和 {start:16,endQuality:17}var newQuality = (startQuality + endQuality) / 2;if (newQuality == quality){break;}quality = newQuality;Console.WriteLine("start:{0} end:{1} current:{2}", startQuality, endQuality, quality);}return ms;}