|Internet media type|
|Type of format||Lossy/lossless bitmap image format|
|Open format?||Yes (royalty-free)|
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.
The name consists of JPEG (for the Joint Photographic Experts Group, which is the committee which designed the format), X (part of the name of several JPEG standards since 2000: JPEG XT, JPEG XR, JPEG XS) and L (for long-term). The L was included as the authors' intention for the format is to replace the legacy JPEG and last as long too.
Since August 2017, JTC1/SC29/WG1 issued a series of draft calls for proposals on JPEG XL – the next generation image compression standard with substantially better compression efficiency (60% improvement) comparing to JPEG. The standard is expected to exceed the still image compression performance shown by HEVC HM, Daala and WebP and to outperform JPEG unlike previous attempts to replace it for the web eg. (JPEG 2000) and also providing more efficient recompression lossless transport options for storage of images in the traditional/legacy JPEG format. The core requirements include support for ultra-high resolution images (at least 40 MP), 8-10 bits per component. RGB / YCbCr / ICtCp color encoding, animated images, alpha channel coding, Rec.709 (sRGB) and gamma (2.4-power) color space, Rec.2100 wide color gamut color space (Rec.2020), and high dynamic range transfer functions (PQ and HLG), high-quality compression of synthetic images, such as bitmap fonts and gradients.
The standard should also offer higher bit depths (12–16 integer bits and floating point): additional color spaces with transfer functions such as Arri's Log C. Embedded preview images, lossless alpha channel encoding, image region encoding and complexity encoding. Any patented technology would be licensed on a royalty-free basis. The proposals were submitted by September 2018, leading to a committee draft in July 2019, with file format and core coding system were formally standardized on 13 October 2021 and 30 March 2022 respectively.
The main features are:
JPEG XL is based on ideas from Google's PIK format and Cloudinary's FUIF format (which was in turn based on FLIF).
Modular mode based on FUIF, born after FLIF. It contains elements of PIK lossless, lossless WebP and new ideas that have been developed during development of Jxl from its starting point PIK + FUIF.
The format is mainly based on two encoding modes, included in the finalization of January 2021.
VarDCT (variable-blocksize DCT) – it utilizes same DCT algorithm as legacy JPEG, but blocks instead of being restricted to 8×8 come in various sizes (2×2 up to 256×256), non-square shapes (e.g. 16×8, 8×32, 32×64) or can use another transforms (AFV, Hornuss). The VarDCT mode is based on (lossy) PIK.
Modular, which covers lossless, near-lossless/palette delta) - responsible, among other things, for efficient lossless content encoding. This is the mode exploited internally in VarDCT to save a whole series of auxiliary 2D data such as the fields/weights of adaptive quantization and any additional/extra channels (e.g. alpha, depth, thermal, spot colors etc.) and DC image, 1:8 subsampled image (DC coefficients) of the VarDCT mode.
Modular, allows lossy compression with the help of modified Haar transform (called squeeze), which has progressive properties: the quality of the image increases with the amount of data loaded. One of the ways VarDCT-based images can be loaded progressively is by saving VarDCT DC coefficients with modular "squeeze", making both modes work in tandem. They can be assisted by separate modeling of specific image features, unknown in other codecs at the time of creating the format:[a]
Lossy modes typically use the XYB color space derived from LMS. JPEG XL can also losslessly reencode existing JPEG files by directly copying JPEG's DCT block coefficients to 8×8 VarDCT blocks, making smaller file sizes possible due to better entropy coding, with JPEG reconstruction data allowing transcoding back to original JPEG file, although constraints limit support for some files.
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 signaled tree structure and predictor selection per context. Entropy coding is LZ77-enabled and can use both asymmetric numeral systems (ANS) and Huffman coding (for low complexity encoders or for reducing overhead of short streams).
JPEG XL defaults to a visually near-lossless setting that still provides good compression.
Animated (multi-frame) images do not perform advanced inter-frame prediction, though some rudimentary inter-frame coding tools are available:
|Initial release||December 27, 2020|
0.7 / September 21, 2022
|License||New BSD License (previously Apache License 2.0)|
In 2017, JTC1/SC29/WG1 (JPEG) issued a call for proposals for JPEG XL – the next generation image coding standard. The main authors of the specification are Jyrki Alakuijala, Jon Sneyers and Luca Versari. Other collaborators are Sami Boukortt, Alex Deymo, Moritz Firsching, Thomas Fischbacher, Eugene Kliuchnikov, Robert Obryk, Alexander Rhatushnyak, Zoltan Szabadka, Lode Vandevenne and Jan Wassenberg.
The file format (bitstream) was frozen on 25 December 2020 and formally standardized on 13 October 2021. Any file compliant to this standard is guaranteed to be decodable by all future releases.
|Common Name||Part||First public release date (First edition)||ISO/IEC Number||Formal Title|
|JPEG XL||Part 1||30 March 2022||ISO/IEC 18181-1||JPEG XL Image Coding System — Part 1: Core coding system|
|Part 2||13 October 2021||ISO/IEC 18181-2||JPEG XL Image Coding System — Part 2: File format|
|Part 3||planned for October 2022||ISO/IEC PRF 18181-3||JPEG XL Image Coding System — Part 3: Conformance testing|
|Part 4||5 August 2022||ISO/IEC 18181-4||JPEG XL Image Coding System — Part 4: Reference software|
The current contributors have committed to releasing it publicly under a royalty-free and open source license.