调整jpeg图像大小会影响他们的压缩吗？ [英] Does resizing jpeg images affect their compression?

问题描述

我正在使用Graphics.DrawImage方法调整jpegs的大小（见下面的代码片段）。任何人都可以确认这不会影响新图像的压缩？
我看过这个帖子，但我在说

I'm resizing jpegs by using the Graphics.DrawImage method (see code fragment below). Can anyone confirm that this will not affect the compression of the new image? I have seen this thread, but I am talking specifically about compression of jpegs.

    private byte[] getResizedImage(String url, int newWidth)
    {
        Bitmap bmpOut = null;
        System.IO.MemoryStream outStream = new System.IO.MemoryStream();

        //input image is disposable
        using (Bitmap inputImage = LoadImageFromURL(url))
        {
            ImageFormat format = inputImage.RawFormat;
            decimal ratio;  //ratio old width:new width
            int newHeight = 0;

            //*** If the image is smaller than a thumbnail just return it
            if (inputImage.Width < newWidth)
                return null;

            ratio = (decimal)newWidth / inputImage.Width;
            decimal h = inputImage.Height * ratio;
            newHeight = (int)h;

            bmpOut = new Bitmap(newWidth, newHeight);
            Graphics g = Graphics.FromImage(bmpOut);
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            // try testing with following options:
            //g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
            //g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
            //g.PixelOffsetMode = System.Drawing.Drawing2D.PixelOffsetMode.HighQuality;
            //g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;

            g.FillRectangle(Brushes.White, 0, 0, newWidth, newHeight);
            g.DrawImage(inputImage, 0, 0, newWidth, newHeight);
            bmpOut.Save(outStream, getImageFormat(url));
        }

        return outStream.ToArray();

    }

推荐答案

调用bmpOut.Save你必须传递一些EncoderParameters来告诉方法你要保存它的质量水平。

When you call "bmpOut.Save" you have to pass in some EncoderParameters to tell the method what quality level you would like to save it with.

http://msdn.microsoft.com/en-us/ library / system.drawing.imaging.encoderparameters（VS.80）.aspx

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