Overview
ISO/IEC 18477-2:2016 defines JPEG XT Part 2 - a codestream and coding format for high dynamic range (HDR) continuous‑tone still images. Designed as a scalable extension of legacy JPEG, it enables storage and interchange of HDR and wide‑gamut photographic content while remaining backward compatible with Rec. ITU‑T T.81 | ISO/IEC 10918‑1 (baseline JPEG). JPEG XT provides a migration path so existing JPEG toolchains can decode a low‑dynamic‑range, 8‑bit version of images encoded with this standard.
Key technical topics and requirements
- Scalable compression and codestream structure: Specifies a codestream composition that carries both legacy-compatible LDR data and HDR residual data (residual codestream).
- HDR coding and reconstruction: Supports multiple‑component images in floating‑point (optimized for 32‑bit float HDR and wide colour gamut).
- Backward compatibility: Legacy decoders reconstruct an 8‑bit per component, lower-quality image; full HDR requires a JPEG XT‑aware decoder.
- Decoder and encoder requirements: Defines high‑level functional blocks for decoding (e.g., inverse decorrelation, base mapping, residual decode, HDR reconstruction) and sets conformance language and operator conventions.
- Codestream syntax: Details Main Profile header structure, parameter ASCII and binary segments, and residual codestream segment layout.
- Terminology and normative rules: Includes terms/definitions, symbols, mathematical operator rules, and checksum computation (Annex A).
- References and colour management: Normative references include ISO/IEC 10918‑1, ISO/IEC 18477‑1 (core system), and IEC 61966‑2‑1 (sRGB).
Practical applications
- Archival and digital photography workflows that require preservation of HDR and wide‑gamut imagery.
- Imaging software, photo editors, and transcoding tools that need to produce files compatible with legacy JPEG decoders while offering HDR fidelity to capable clients.
- Camera firmware and capture pipelines that output compact, interoperable HDR imagery.
- Printing, broadcast, and visual effects pipelines where high precision colour and luminance are required.
Who should use this standard
- Codec implementers and multimedia software developers building JPEG XT encoders/decoders.
- Digital asset managers and archivists planning long‑term storage of HDR photographic content.
- Camera and imaging device manufacturers seeking an interoperable HDR file format.
- Standards and QA teams verifying conformance, compatibility, and migration strategies from legacy JPEG.
Related standards
Keywords: ISO/IEC 18477‑2, JPEG XT, HDR images, scalable compression, backward compatible JPEG, residual codestream, wide colour gamut.
Open preview (PDF)Information technology — Scalable compression and coding of continuous-tone still images — Part 2: Coding of high dynamic range images
