C# 图片按比例进行压缩

1、对图片进行压缩，保存在本地

对于一个200k的png文件按0.6的缩放比例进行压缩，压缩后的大小为20k左右

对于一个80k的jpg文件按0.6的缩放比例压缩，压缩后为13k左右

public void imageZoom(string name, Double zoomScale)
        {
            Bitmap btImage = new Bitmap(name);
            Image serverImage = btImage;
            int width = (int)(serverImage.Width * zoomScale);
            int height = (int)(serverImage.Height * zoomScale);
            //画板大小
            int finalWidth = width, finalHeight = height;
            int srcImageWidth = serverImage.Width;
            int srcImageHeight = serverImage.Height;
            if (srcImageWidth > srcImageHeight)
            {
                finalHeight = srcImageHeight * width / srcImageWidth;
            }
            else
            {
                finalWidth = srcImageWidth * height / srcImageHeight;
            }


            //新建一个bmp图片
            Image newImage = new Bitmap(width, height);
            //新建一个画板
            Graphics g = Graphics.FromImage(newImage);
            //设置高质量插值法
            g.InterpolationMode = InterpolationMode.High;
            //设置高质量,低速度呈现平滑程度
            g.SmoothingMode = 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(serverImage, new Rectangle((width - finalWidth) / 2, (height - finalHeight) / 2, finalWidth, finalHeight), 0, 0, srcImageWidth, srcImageHeight, GraphicsUnit.Pixel);
            //以jpg格式保存缩略图
            MemoryStream msSaveImage = new MemoryStream();
            newImage.Save(@"D:\1.png",ImageFormat.Jpeg);
            serverImage.Dispose();
            newImage.Dispose();
            g.Dispose();
        }

 private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.ShowDialog();
            imageZoom(ofd.FileName,0.6);
           
            
        }

2、对图片进行压缩，转换成Base64后进行传输，可测量字符串长度来对比

对一张图片转换成base64后，测量base64字符串的长度，对于一个80k的图片，压缩前长度为110800多，压缩后长度不到20000，减小了很多，便于传输。

网上有关于对字符串进行压缩的，那是针对纯字符串，对于已经转换成base64的字符串，使用GZipStream类来压缩数据基本上没有效果。

public  string ImageToBase64(string name,Double zoomScale)
        {

            Bitmap btImage = new Bitmap(name);
            Image serverImage = btImage;
            int width = (int)(serverImage.Width * zoomScale);
            int height = (int)(serverImage.Height * zoomScale);
            //画板大小
            int finalWidth = width, finalHeight = height;
            int srcImageWidth = serverImage.Width;
            int srcImageHeight = serverImage.Height;
            if (srcImageWidth > srcImageHeight)
            {
                finalHeight = srcImageHeight * width / srcImageWidth;
            }
            else
            {
                finalWidth = srcImageWidth * height / srcImageHeight;
            }
           

            //新建一个bmp图片
            Image newImage = new Bitmap(width, height);
            //新建一个画板
            Graphics g = Graphics.FromImage(newImage);
            //设置高质量插值法
            g.InterpolationMode = InterpolationMode.High;
            //设置高质量,低速度呈现平滑程度
            g.SmoothingMode = 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(serverImage, new Rectangle((width - finalWidth) / 2, (height - finalHeight) / 2, finalWidth, finalHeight), 0, 0, srcImageWidth, srcImageHeight, GraphicsUnit.Pixel);
            //以jpg格式保存缩略图
            MemoryStream msSaveImage = new MemoryStream();
            newImage.Save(msSaveImage, ImageFormat.Jpeg);
            serverImage.Dispose();
            newImage.Dispose();
            g.Dispose();
            byte[] imageBytes = msSaveImage.ToArray();
            msSaveImage.Close();
            return Convert.ToBase64String(imageBytes);
        }