Overview
ISO/IEC 18181-2:2024 - "Information technology - JPEG XL image coding system - Part 2: File format" specifies the file and transport/container formats for JPEG XL codestreams (as defined in ISO/IEC 18181-1). It defines how to wrap a JPEG XL codestream in a box-based file, how to attach metadata and extensions, and the rules a decoder must follow. A file conforming to this document is called a JPEG XL file (media type image/jxl is registered in Annex B).
Keywords: ISO/IEC 18181-2:2024, JPEG XL file format, JPEG XL codestream, image/jxl, metadata, box structure
Key technical topics and requirements
- File forms: A JPEG XL file may be either a direct codestream (no wrapping) or a box-structured file. Decoders must accept both forms.
- Box-based organization: The file format uses a sequence of boxes (header + content) similar in syntax to other ISO image formats (see ISO/IEC 15444-1 and related standards).
- Mandatory boxes: A JPEG XL file using the box structure must begin with a JPEG XL Signature box (first) and a File Type box (second). Other boxes are optional or repeatable.
- Common box types: Level, JUMBF (JPEG Universal Metadata Box Format), Exif, XML, Brotli-compressed boxes, Frame Index, JPEG XL Codestream box, JPEG XL Partial Codestream boxes, and JPEG Bitstream Reconstruction Data box.
- Partial codestreams: The codestream may be split across multiple JPEG XL Partial Codestream boxes; the codestream is formed by concatenating their contents.
- Box binary format: Each box header contains LBox (u32 big-endian), TBox (4-byte type), optional XLBox (u64), and DBox (content). Numerical types include u32, u64 (big-endian) and Varint (variable-length integer as in ISO/IEC 18181-1).
- Extensibility and robustness: Unrecognized box types must be ignored and skipped by decoders, enabling forward compatibility.
- Metadata handling: Support for standard metadata carriers (Exif, XML, JUMBF) and compressed payloads (Brotli per RFC 7932).
Practical applications and who uses this standard
- Software and hardware codec implementers building JPEG XL encoders and decoders.
- Browser vendors, image viewers, and web platforms implementing image/jxl support.
- Digital asset management, imaging libraries, and photo editing tools that store or manipulate JPEG XL files and metadata.
- Archival and publishing systems needing a standardized container for high-quality, extensible imagery with metadata and indexing (Frame Index).
- Tooling vendors creating metadata processors (Exif/XMP), transcoders, and bitstream reconstruction tools.
Related standards
This standard provides the definitive file-level rules for packaging JPEG XL images, ensuring interoperable transport, metadata integration, and extensibility for modern imaging workflows.
