Mode without preserving structure of the original JPEG file

[vrubleg]

As far as I can see, JXL created by Brunsli can be extracted exactly to the same (byte-by-byte) JPEG file. Could compression ratio be a bit better if we recompress just JPEG image data losslessly, without storing additional information how original JPEG was encoded?

[eustas]

Yes, that is also possible: just carefully retouch JPEGData object after ReadJpeg, but before BrunsliEncodeJpeg. Later we will provide utility function that will do exactly that.

[vrubleg]

If this additional information is removed, will it be possible to convert this image data back to JPEG without losing quality? It is clear that the JPEG data will be encoded in some different way in this case, and the JPEG file won't be equal to the original JPEG, but it is OK until the image data is still the same.

[eustas]

There are 2 parts (modulo ICC profiles) that contain the complete information about image (pixels) - DCT coefficients and quantisation table. Everything else (Huffman tables, scan scheme, reset points) are details of encoding. So if we drop / replace those and serialise JPEG we will get different file, but same image.