JPEG 压缩产生更大的尺寸

Question

I want to compress image quality of a jpg.

I am using the sample code from the Microsoft documentation below:
https://docs.microsoft.com/en-us/dotnet/framework/winforms/advanced/how-to-set-jpeg-compression-level?redirectedfrom=MSDN

Whatever image I choose for testing purposes, the new image size is becoming bigger than the original one when choosing the quality between 85L and 100L. Starting down from 85L, it is getting less.

I don't understand, why the image size is getting bigger using that code for the range 85L-100L.

    void SaveImage(Image image, string destPath, long quality)
    {
        if (quality < 0 || quality > 100)
            throw new ArgumentException("quality has to be between 0 and 100 !!!");

        ImageCodecInfo jpgEncoder = GetEncoder(ImageFormat.Jpeg);

        // An EncoderParameters object has an array of EncoderParameter objects. 
        // In this case, there is only one EncoderParameter object in the array.  
        var myEncoderParameters = new EncoderParameters(1);

        var myEncoderParameter = new EncoderParameter(Encoder.Quality, quality);
        myEncoderParameters.Param[0] = myEncoderParameter;
        image.Save(destPath, jpgEncoder, myEncoderParameters);
    }

    ImageCodecInfo GetEncoder(ImageFormat format)
    {
        ImageCodecInfo[] codecs = ImageCodecInfo.GetImageDecoders();
        foreach (ImageCodecInfo codec in codecs)
        {
            if (codec.FormatID == format.Guid)
            {
                return codec;
            }
        }
        return null;
    }

    SaveImage(new Bitmap(srcPath), destPath, 85L);

Answer 1

What is happening is that your JPEGs are compressed. When you load them into memory they are uncompressed. This uncompressed size is definitely larger than the original JPG.

When you save the image, you are not compressing from the original size but from the larger uncompressed image in memory. The recompression process is not aware of the fact that the image it is compressing came from a compressed image originally. It just runs the algorithm on the image in memory at the compression setting you give.

This is why lossy formats like JPEG show compression artifacts after a few rounds of open/save.