C# 图片压缩

        /// 

        /// 压缩图片至200 Kb以下，最小可压到30KB
        /// 
        /// 图片
        /// 图片格式
        /// 压缩后大小
        /// 原始大小
        /// 压缩后的图片
        public Image ZipImage(Image img, ImageFormat format, long targetLen, long srcLen = 0)
        {
            //设置大小偏差幅度 10kb
            const long nearlyLen = 10240;
            //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);
                img = Image.FromStream(ms);
                return img;
            }

            //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);
            }
            img = Image.FromStream(ms);
            return img;
        }

        /// 

        ///获取图片格式
        /// 
        /// 图片
        /// 默认返回JPEG
        public ImageFormat GetImageFormat(Image img)
        {
            if (img.RawFormat.Equals(ImageFormat.Jpeg))
            {
                return ImageFormat.Jpeg;
            }
            if (img.RawFormat.Equals(ImageFormat.Gif))
            {
                return ImageFormat.Gif;
            }
            if (img.RawFormat.Equals(ImageFormat.Png))
            {
                return ImageFormat.Png;
            }
            if (img.RawFormat.Equals(ImageFormat.Bmp))
            {
                return ImageFormat.Bmp;
            }
            return ImageFormat.Jpeg;//根据实际情况选择返回指定格式还是null
        }

绝对路径直接压缩 (调用几次就有问题)

/// 

        /// 生成缩略图
        /// 
        /// 源图路径（物理路径）
        /// 缩略图路径（物理路径）
        /// 缩略图宽度
        /// 缩略图高度
        /// 生成缩略图的方式    
        public static bool MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode= "HW")
        {
            bool f = false;
            Image originalImage = Image.FromFile(originalImagePath);

            int towidth = width;
            int toheight = height;

            int x = 0;
            int y = 0;
            int ow = originalImage.Width;
            int oh = originalImage.Height;

            switch (mode)
            {
                case "HW"://指定高宽缩放（可能变形）                
                    break;
                case "W"://指定宽，高按比例                    
                    toheight = originalImage.Height * width / originalImage.Width;
                    break;
                case "H"://指定高，宽按比例
                    towidth = originalImage.Width * height / originalImage.Height;
                    break;
                case "Cut"://指定高宽裁减（不变形）                
                    if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
                    {
                        oh = originalImage.Height;
                        ow = originalImage.Height * towidth / toheight;
                        y = 0;
                        x = (originalImage.Width - ow) / 2;
                    }
                    else
                    {
                        ow = originalImage.Width;
                        oh = originalImage.Width * height / towidth;
                        x = 0;
                        y = (originalImage.Height - oh) / 2;
                    }
                    break;
                default:
                    break;
            }
            //新建一个bmp图片
            Image bitmap = new System.Drawing.Bitmap(towidth, toheight);
            //新建一个画板
            Graphics g = System.Drawing.Graphics.FromImage(bitmap);
            //设置高质量插值法
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
            //设置高质量,低速度呈现平滑程度
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            //清空画布并以透明背景色填充
            g.Clear(Color.White);
            //在指定位置并且按指定大小绘制原图片的指定部分
            g.DrawImage(originalImage, new Rectangle(0, 0, towidth, toheight),
                new Rectangle(x, y, ow, oh),
                GraphicsUnit.Pixel);
            try
            {
                //以jpg格式保存缩略图
                bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
                f = true;
            }
            catch (System.Exception e)
            {
                f = false;
            }
            finally
            {
                originalImage.Dispose();
                bitmap.Dispose();
                g.Dispose();
                File.Delete(originalImagePath);        
            }
            return f;
        }

引用：https://blog.csdn.net/weixin_42953003/article/details/119751529