cjxl input.png output.jxl
cjxl input.jpg output.jxl
cjxl input.gif output.jxl

Displays the options that cjxl supports. On its own, it will only show basic options. It can be combined with -v or -v -v to show increasingly advanced options as well.

Increases verbosity. Can be repeated to increase it further, and also applies to --help.

The preferred way to specify quality. It is specified in multiples of a just-noticeable difference. That is, -d 0 is mathematically lossless, -d 1 should be visually lossless, and higher distances yield denser and denser files with lower and lower fidelity. Lossy sources such as JPEG and GIF files are compressed losslessly by default, and in the case of JPEG files specifically, the original JPEG can then be reconstructed bit-for-bit. For lossless sources, -d 1 is the default.

Alternative way to indicate the desired quality. 100 is lossless and lower values yield smaller files. There is no lower bound to this quality parameter, but positive values should approximately match the quality setting of libjpeg.

Controls the amount of effort that goes into producing an “optimal” file in terms of quality/size. That is to say, all other parameters being equal, a higher effort should yield a file that is at least as dense and possibly denser, and with at least as high and possibly higher quality.

Recognized effort settings, from fastest to slowest, are:

# Compress a PNG file to a high-quality JPEG XL version.
$ cjxl input.png output.jxl
# Compress it at a slightly lower quality, appropriate for web use.
$ cjxl -d 2 input.png output.jxl
# Compress it losslessly. These are equivalent.
$ cjxl -d 0   input.png lossless.jxl
$ cjxl -q 100 input.png lossless.jxl
# Compress a JPEG file losslessly.
$ cjxl input.jpeg lossless-jpeg.jxl