C# System.Drawing.Graphics 画图后，如何保存一个低质量的图片，一个占用空间较小的图片？

首先要控制图片保存后硬盘后的大小（即占用硬盘的空间，而非尺寸），真正要处理的是控制 System.Drawing.Bitmap.Save 方法的参数。

具体实现如下：

private void ThumbPicture(Image SourceImage, int TargetWidth,string savePath)
        {
            int IntWidth; //新的图片宽  
            int IntHeight; //新的图片高  
            try
            {
                int TargetHeight = (int)Math.Round(TargetWidth / (SourceImage.Width * 1.0f / SourceImage.Height));
                if (TargetWidth * TargetHeight >= SourceImage.Width * SourceImage.Height)
                {
                    SourceImage.Save(savePath);
                    SourceImage.Dispose();
                    return;
                }
                //计算缩放图片的大小  
                if (SourceImage.Width > TargetWidth && SourceImage.Height <= TargetHeight)//宽度比目的图片宽度大，长度比目的图片长度小  
                {
                    IntWidth = TargetWidth;
                    IntHeight = (IntWidth * SourceImage.Height) / SourceImage.Width;
                }
                else if (SourceImage.Width <= TargetWidth && SourceImage.Height > TargetHeight)//宽度比目的图片宽度小，长度比目的图片长度大  
                {
                    IntHeight = TargetHeight;
                    IntWidth = (IntHeight * SourceImage.Width) / SourceImage.Height;
                }
                else if (SourceImage.Width <= TargetWidth && SourceImage.Height <= TargetHeight) //长宽比目的图片长宽都小  
                {
                    IntHeight = SourceImage.Width;
                    IntWidth = SourceImage.Height;
                }
                else//长宽比目的图片的长宽都大  
                {
                    IntWidth = TargetWidth;
                    IntHeight = (IntWidth * SourceImage.Height) / SourceImage.Width;
                    if (IntHeight > TargetHeight)//重新计算  
                    {
                        IntHeight = TargetHeight;
                        IntWidth = (IntHeight * SourceImage.Width) / SourceImage.Height;
                    }
                }

                var x = (TargetWidth - IntWidth) / 2;
                var y = (TargetHeight - IntHeight) / 2;

                using (var newImage = new Bitmap(TargetWidth, TargetHeight))
                {
                    using (var graphic = Graphics.FromImage(newImage))
                    {
// 核心参数啊，感觉相当于PS保存时间的质量设置参数
                        Int64 qualityLevel = 80L;

                        // 高质量
                        graphic.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                        graphic.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
                        graphic.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;
                        qualityLevel = 100L;

                        // 低质量
                        //graphic.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
                        //graphic.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.Low;
                        //graphic.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighSpeed;
                        //qualityLevel = 60L;

                        System.Drawing.Imaging.ImageCodecInfo codec = System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders()[1];
                        System.Drawing.Imaging.EncoderParameters eParams = new System.Drawing.Imaging.EncoderParameters(1);
                        eParams.Param[0]=new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, qualityLevel);

                        graphic.DrawImage(SourceImage, 0, 0, TargetWidth, TargetHeight);
                        SourceImage.Dispose();

                        // 使用控制图片质量的保存方式
                        //newImage.Save(savePath);
                        newImage.Save(savePath, codec, eParams);
                    }
                }
            }
            catch (Exception ex)
            {

            }
        }