Convert JXL to JPG

JPEG XL and JPEG: the same name, thirty years apart

The two formats share a committee and nothing else. JPEG (1992) splits a picture into 8×8 blocks and throws away the frequencies the eye is bad at; JPEG XL (2022) uses variable block sizes, a perceptual colour space and an entropy coder a generation newer, and lands at roughly 60 per cent of the bytes for the same visible quality — plus alpha, animation, HDR and lossless, none of which JPEG has. Converting one into the other is therefore a step down, taken because the destination insists.

The encoder here is MozJPEG rather than the browser's own: trellis quantisation, progressive scans, and full-resolution colour above quality 90, which is what keeps red text and fine detail from smearing. Where the JXL was lossy, the JPG inherits its artefacts and adds a smaller set of its own — the usual price of a second lossy pass, and why a quality above 80 is the sensible default.

Colour is converted to sRGB, which is what every JPG viewer assumes. A wide-gamut or HDR capture loses its extremes in the conversion — the most saturated reds and the brightest highlights are clipped — and the EXIF block of the original does not come across, so the JPG carries no camera data or GPS position.

When you want something else

If your .jxl files are repacked JPEGs — ones made with cjxl from a JPEG without the -d flag — djxl in.jxl out.jpg on the command line restores the original byte for byte, with no second lossy pass and no size growth. That is strictly better than anything a decoder-and-re-encoder can do.

For a camera's worth of files, djxl in a loop or ImageMagick (built with libjxl) converts a directory in one command and keeps the EXIF, which this page does not.