The progressive mode in Jpeg and JPEG XL is quite different, because the quality is so much better your perception of it changes. Where Progressive Jpeg are literally useless before it finish loading, JPEG XL provides decent quality. - Imagecon 2019 - Jon Sneyers / HN / FLIF / browser support?

Lossless conversion from JPG/PNG to JXL

• JPEG XL and the Pareto Front - compare encoder taking account result compression size & encoding speed.
• How does lossless JPEG-XL compared to PNG? - to clarify

using libjxl

Install

see libjxl for installation

# or double click on package in Nemo
$ sudo apt-get install ./jxl_0.8.2_amd64.deb

Compression / decompression

$ cjxl input.jpg output.jxl			# compress/convert

$ djxl input.jxl output.jpg			# convert back

Then can be converted back to jpeg and assert that there is no loss.

using FFmpeg

$ ffmpeg -i image.png -c:v libjxl image.jxl

Batch conversion - TBD

see also

• Which apps support to open and convert JPEG XL (.jxl) pictures?
• libjxl / info - JPEG XL reference implementation

• Existing JPEG files can be losslessly transcoded to JPEG XL / 2

• JPEG XL, the New Image Format Nobody Wanted or Needed
• browser support - mostly unsupported by default as of 2021