cjxl

Convert an image to JPEG XL

$ cjxl [path/to/image.ext] [path/to/output.jxl]

$ cjxl --distance 0 --effort 9 [path/to/image.ext] [path/to/output.jxl]

$ cjxl [[-h -v -v -v -v|--help --verbose --verbose --verbose --verbose]]

cjxl [options] input_file output_file

-d, --distance NUM
    Sets the target Butteraugli distance for lossy compression. 0 for lossless. Higher values mean more compression, lower quality. (Default: 1.0)

-q, --quality NUM
    Sets a JPEG-like quality setting (0-100). Correlates with --distance but is more user-friendly. Higher values mean better quality.

-e, --effort NUM
    Controls the encoding effort/speed (1=fastest, 9=slowest/best compression). Higher effort yields smaller files but takes longer. (Default: 7)

-s, --speed NUM
    Alias for --effort.

--modular
    Forces modular mode (lossless encoding). Useful for synthetic images or when --distance is 0.

--container
    Forces the use of the JXL container format, which enables features like animations or embedded JPEG bitstreams.

--jpeg_recompress
    Losslessly recompress a JPEG file to JXL. The original JPEG bitstream is optionally embedded for backward compatibility.

--num_threads NUM
    Specifies the number of threads to use for encoding. (Default: based on CPU cores)

-h, --help
    Displays help message and exits.

--version
    Displays version information and exits.

The cjxl command is the official command-line encoder for the new JPEG XL (JXL) image format. JPEG XL is a royalty-free, next-generation image coding system designed for superior compression efficiency and rich functionality. cjxl converts various input image formats (e.g., PNG, JPEG, GIF, PPM, TIFF) into highly optimized JXL files. It supports both lossy and lossless compression modes, allowing users to balance file size with image fidelity. Key features include support for animated images, progressive decoding, and the ability to losslessly recompress existing JPEG files into JXL, potentially reducing their size without quality loss while maintaining the original JPEG data within the JXL stream for backward compatibility. Its versatility makes it suitable for web delivery, photography, and archival purposes.

Encoding with cjxl, especially at higher --effort settings, can be computationally intensive and time-consuming. While JPEG XL generally offers superior compression at high qualities, for very low-detail images at extremely low target sizes, older formats might occasionally yield smaller files or faster encoding. Browser and software support for JPEG XL is growing but might not be universal yet.

cjxl accepts a wide range of input image formats including PNG, JPEG, GIF, PPM, and TIFF. The output is always a JPEG XL file with the .jxl extension. It can also handle multi-frame inputs for animation encoding.

By default, cjxl operates in a lossy mode optimized for perceptual quality using the Butteraugli metric (controlled by --distance). Setting --distance 0 or using the --modular option forces lossless compression, ensuring no data is discarded.

JPEG XL (JXL) is a modern, royalty-free image format developed by the Joint Photographic Experts Group (JPEG) committee. Its development began in 2017, and it was officially standardized as ISO/IEC 18181 in October 2021. The cjxl encoder is part of the reference implementation library, libjxl, which was designed to be highly optimized and serve as a successor to legacy image formats like JPEG, PNG, and GIF, aiming to provide better compression and more features.

djxl(1), ffmpeg(1), convert(1) (ImageMagick), cjpeg(1)