This is mostly for me so I can look it up again later:
JPEG XL is a new (for appropriate definitions of "new") picture compression algorithm (and much more). One very interesting option is to losslessly recode JPEG files to JXL and back. There is a reference implementation with sources and binary releases on GitHub (including prebuild binaries for Windows 32 and 64 bit).
My first tests show an impressive size reduction about 15% for JPEG quality 85 files. Converting them back to JPEG restored the original file (I compared them with BeyondCompare).
Being a lazy basterd™ I searched for and found two PowerShell commands that allow to batch convert all files in a directory on Reddit.
gci *.jpg,*.jpeg -File | % { cjxl $_ "$($_.Basename).jxl" --quiet; Write-Host "." -NoNewLine }
gci *.png -File | % { cjxl $_ "$($_.Basename).jxl" -m --quiet; Write-Host "." -NoNewLine }
I added the –lossless_jpeg=1 option to the first one to suppress an annoying output.
IrfanView has got a plugin for viewing JXL files.
The reference implementation of the library is apparently written in C++.
There is a commercial Delphi component called ImageEn that claims to support JPEG XL (haven’t checked it out yet).
Unfortunately Google has removed experimental support for JPEG XL in the Chrome sources a while ago. That doesn’t bode well for its future as Google used to be a major backer of it.