JPEG XL

JPEG XL is a royalty-free raster-graphics file format that supports both lossy and lossless compression. It is designed to outperform existing raster formats and thus to become their universal replacement.^[2]

Name

• JPEG is the Joint Photographic Experts Group, which is the committee that designed the format.
• X is part of the name of several JPEG standards since 2000: JPEG XT, JPEG XR, JPEG XS.
• L means Long-term because the authors' intention for the format is to replace the legacy JPEG and last as long too.^[3]

History

In 2017, JTC1/SC29/WG1 (JPEG) issued a Call for proposals for JPEG XL – the next generation image coding standard.^[4]

The file format (bitstream) was frozen on December 25, 2020, meaning that the format is now guaranteed to be decodable by future releases.^[5]

Features

The main features are:^[6][7]

• Improved functionality and efficiency compared to traditional image formats (e.g. JPEG, GIF and PNG);
• Progressive decoding (by resolution and precision);
• Lossless JPEG transcoding with ~20% size reduction;
• Lossless encoding and lossless alpha encoding;
• Support for both photographic and synthetic imagery;
• Graceful quality degradation across a large range of bitrates;
• Perceptually optimized reference encoder;
• Support for wide color gamut and HDR;
• Support for animated content,
• Efficient encoding and decoding without requiring specialized hardware
  • In particular, JPEG XL is about as fast to encode and decode as old JPEG using libjpeg-turbo and an order of magnitude faster to encode and decode compared to HEIC with x265.^[8]
• Royalty-free format with an open-source reference implementation.^[9]

Technical details

JPEG XL codec architecture diagram

JPEG XL is based on ideas from Google's Pik format and Cloudinary's FUIF format (which was in turn based on FLIF).^[10]

The format has a variety of encoding modes. On the legacy side, it has a mode that transcodes legacy JPEG in a more compact way for storage. On the more modern side, it has a lossy mode called VarDCT (variable-blocksize DCT) and a lossless/near-lossless/responsive mode called Modular which optionally uses a modified Haar transform (called "squeeze") and which is also used to encode the DC (1:8 scale) image in VarDCT mode as well as various auxiliary images such as adaptive quantization fields or additional channels like alpha. Both modes can use separate modeling of specific image features: splines, repeating "patches" like text or dots, and noise synthesis. Lossy modes typically use the XYB color space derived from LMS.^[11]

Prediction is run using a pixel-by-pixel decorrelator without side information, including a parametrized self-correcting weighted ensemble of predictors. Context modeling includes specialized static models and powerful meta-adaptive models that take local error into account, with a signalled tree structure and predictor selection per context. Entropy coding is LZ77-enabled and can use both Asymmetric Numeral Systems and Huffman coding (for low complexity encoders or for reducing overhead of short streams).^{[citation needed]}

It defaults to a visually near-lossless setting that still provides good compression.^[8]

Animated (multi-frame) images do not perform advanced inter-frame prediction, though some rudimentary inter-frame coding tools are available:

• a frame can only update part of the canvas;
• a frame can not just replace the contents on the canvas, but also be blended, added or multiplied to it;^[12]
• up to four frames can be 'remembered' and referenced using the "patches" coding tool in later frames.^[13]

Software

Codec implementation

• JPEG XL Reference Software (libjxl)
  • license: Apache License 2.0
  • contains (among others):
    • coder cjxl
    • decoder djxl
    • tool for benchmarking speed and quality of image codecs benchmark_xl
    • GIMP and Gtk pixbuf plugin file-jxl

Official support

• ImageMagick^[14] – toolkit for raster graphics processing
• XnView MP^[15] – viewer and editor of raster graphics
• MConverter^[16] – online media converter
• Squoosh^[17] – WebAssembly-based image converter (online media converter available)
• gThumb^[18] – free Linux image viewer
• ImageGlass^[19] – free and open-source Windows image viewer

Unofficial support

• Qt / KDE apps^[20] – via plugin
• Microsoft Windows^[21] – via WIC plugin, i.e. for viewing in Photo Viewer, File Explorer etc.
• macOS^[22] – via standalone app and Quick Look plugin
• GIMP^[3] – raster graphics editor; plugin for GIMP 2.10 available

Preliminary support

• Chromium^[23] – web browser; in testing (as Chrome Beta and Edge Canary)
• Firefox^[24] – web browser; in testing (as Firefox Nightly)

Standardization status

References

External links

• Official website
• Reference implementation
  • builds for Windows:
    • Windows Bin Builds (the page also contains many other programs)
    • AppVeyor
• Author's personal page

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