ISO/IEC 18477-1:2024

Overview

ISO/IEC 18477-1:2024 specifies the core coding system for JPEG XT - a scalable image coding format optimized for continuous‑tone photographic content. Known as the JPEG XT core, this third edition defines the base codestream format, encoder/decoder requirements and the functional building blocks that extend legacy JPEG (Rec. ITU‑T T.81 | ISO/IEC 10918‑1) in a backward‑compatible, low‑complexity way. The standard forms the foundation for the wider ISO/IEC 18477 series.

Key topics and requirements

• Codestream format and syntax: Defines the compressed representation (codestream) that carries all data needed to reconstruct image samples; normative codestream syntax is specified (see Annex B).
• Backward compatibility: Builds on legacy JPEG constructs (markers, entropy‑coded segments, 8×8 DCT blocks) so existing JPEG decoders can interoperate with minimal changes.
• Encoder and decoder roles: Formalizes encoder/decoder behavior - encoders create codestreams from source image data (including obtaining low dynamic‑range representations) and decoders reconstruct sample grids per the standard.
• Sampling and transform processing:
  • Component subsampling / expansion rules (Annex A) and horizontal/vertical sampling indicators (H/V).
  • Use of 8×8 blocks and DCT-based processing consistent with legacy JPEG.
  • Upsampling / downsampling conventions and sample grid definition.
• Multi‑component decorrelation: Defines inverse decorrelation (and markers to select it) needed to map component data into display color spaces (Annex C); includes ICT handling when converting to RGB.
• Conventions & conformance: Symbols, precision, operator semantics, marker and marker‑segment definitions, and conformance language for implementers.
• Low complexity & scalability: Designed to enable efficient encoders/decoders and scalable image representations while improving quality and compression efficiency.

Applications and users

• Camera and imaging device manufacturers wanting backward‑compatible advanced photo formats.
• Codec and library implementers (image codecs, libjpeg derivatives) integrating scalable or extended JPEG features.
• Image processing and editing software that needs interoperable codestream handling and higher‑quality compression.
• Digital asset management, archiving and publishing systems requiring efficient storage/transmission of photographic images with compatibility to existing JPEG workflows.
• Standards bodies and profile registries using the baseline core as the foundation for extended parts of the ISO/IEC 18477 series.

Related standards

• Rec. ITU‑T T.81 | ISO/IEC 10918‑1 (JPEG core)
• Rec. ITU‑T T.86 | ISO/IEC 10918‑4 and Rec. ITU‑T T.871 | ISO/IEC 10918‑5 (profile/format registration and JFIF)
• Other parts of the ISO/IEC 18477 (JPEG XT) series extend features beyond this core.

Keywords: ISO/IEC 18477-1:2024, JPEG XT, scalable compression, continuous‑tone still images, codestream, backward compatible JPEG, DCT, subsampling, decorrelation.