I should think that if the quantization matrix (which is stored in the JPEG file) is known, then it should be possible to re-encode the JPEG file losslessly aft

I should think that if the quantization matrix (which is stored in the JPEG file) is known, then it should be possible to re-encode the JPEG file losslessly after it has been converted from JPEG into a lossless format so that the DCT is lost. However, I do not know if any program actually does this, or how to write such a program. Another thing I considered is if a decoder could be made to somehow try to improve the quality of the output by producing a picture which could have been the input to the encoder with the specified quantization matrix. (Of course, all of this is not what the article is about (it is about progressive and multi-scan JPEG), but still it is my questions/comments anyways.)