[fold3]

Iirc, cjxl is smart enough to do it automatically.

cjxl input.jpg output.jxl

The cjxl --help and cjxl -v -v --help pages are very well done and you'll find the option to explicitly set the option on if you wish to do so.

[selcuka]

https://github.com/libjxl/libjxl#usage

> Specifically for JPEG files, the default cjxl behavior is to apply lossless recompression and the default djxl behavior is to reconstruct the original JPEG file (when the extension of the output file is .jpg).