Sustainability of Digital Formats: Planning for Library of Congress Collections

JPEG XL is a raster-graphics image file format, designed to support lossy and lossless compression, particularly for display on the web and the needs for the web environment on a multitude of devices. JPEG XL is being developed to outperform other popular web image formats such as PNG, JPEG 2000, GIF, and WebP with higher quality images and improved compression ratios.

The JPEG XL file format has two forms:

• A 'naked' codestream. In this case, only the image/animation data itself is stored, and no additional metadata can be included. Such a file starts with the bytes 0xFF0A (the JPEG marker for "start of JPEG XL codestream").
• An ISO_BMFF-based container, a box-based container that includes a JPEG XL codestream box (jxlc), and can optionally include other boxes with additional information, such as Exif metadata. In this case, the file starts with the bytes "0x0000000C 4A584C20 0D0A870A"

The JPEG XL container can include three different types of metadata:

• Exif
• XMP
• JUMBF

This metadata can include a host of information such as camera settings or GPS coordinates. Any image rendering-impacting information such as Exif orientation is ignored with information in the codestream taking precedence. The file format container allows this type of metadata to be stored either uncompressed or by Brotli-compression. This metadata can also be essential for JPEG XL to losslessly recompress existing JPEG files. The "jbrd" box, the JPEG Bitstream Reconstruction Data, contains the information needed to reconstruct the bit-identical file. The JPEG bitstream reconstruction data uses the image data along with any other metadata present sch as Exif, XMP, or JUMBF, the original JPEG file can be reconstructed. This inherent compatibility allows for an efficient transition from JPEG formats to JPEG XL without the need to store two files.

The "X" of the JPEG XL file name is based in part on the JPEG group's naming convention for many of its new standards since 2000 including, JPEG XT, JPEG XR, and JPEG XS. The authors used the "L" as an indication of the hopeful longevity of the file format as it will replace the legacy JPEG. Comments welcome.

See JPEG XL pages on the JPEG website and JPEG_XL for information on the JPEG XL Image Encoding.

As of March 2022, the file format specification of this standard is in published status. ISO/IEC 18181-2 documented format specifications are broken into four parts. Part 1 of the format specifications was fully published in March 2022, while Part 2 was published in October 2021. Parts 3 and 4 remain in development.

Part 1 carries the running title Part 1: Core coding system. As stated on the JPEG group's own description of the standards, Part 1: "Defines the JPEG XL codestream and decoder, which can be used for lossy encoding, lossless encoding, and lossless re-compression of existing JPEG images."

Part 2 carries the running title Part 2: File Format. Part 2, "specifies an extensible box-based file format which adds support for metadata (e.g. Exif and JUMBF) and legacy JPEG bitstream reconstruction data."

Part 3 carries the running title: Conformance testing. As stated by the JPEG group, this part, "provides test material and procedures to validate proprietary solutions against the standard specification."

Part 4 carries the running title Part 4: Reference software. Part 4 "provides a free and open source, royalty-free JPEG XL reference implementation."

Adoption of the JPEG XL encoding and file format appears limited at this time. Few active web browsers support JPEG XL file display with a currently monitored list found on GitHub. This list includes various types of JPEG XL Software Support and its stated purpose is to, "point end-users to software that can read/write jxl and keep track of the adoption status of jxl."

JPEGXL.info maintains a list of supported software and browsers including

• ImageMagick
• libvips
• Imlib2
• ImageGlass
• GIMP (with associated plugin)
• XnView
• KolourPaint
• KPhotoAlbum
• VookiImageViewer
• For additional software, see this JPEG XL overview slide deck.

Sample JPEG XL files and conformance testing can be found at https://github.com/libjxl/conformance.

Comments welcome.

JPEG XL files can contain three types of metadata including Exif, XMP, and JUMBF. See Identification and Description. The JPEG XL container allows the listed metadata to be stored via Brotli-compression or uncompressed. If using Brotli-compression, the box type is defined as "brob" and the first four bytes of the box contents define the actual box type that it represents.

Per Jon Sneyers' JPEG XL documentation on GitLab, there is a clear separation between metadata and image data. Whatever is needed to correctly display the image is considered to be image data including elements such as ICC profiles and Exif orientation. The developers goal is to reduce the ambiguity and potential for incorrect implementations by different applications. Remaining metadata such as Exif or XMP, can be stored in the container format but has no impact on image rendering. Exif orientation for example, is a field ignored by applications since the orientation defined in the codestream takes precedence. This metadata can be done without affecting the image display.

Accessibility Features

JPEG XL has some support for enhanced accessibility with progressive decoding which allows users to see an image before it's fully downloaded, which is especially useful when bandwidth is low. Its metadata options also provide options for image description.

See JPEG_XL

See JPEG_XL