Overview
ISO/IEC 18477-9:2016 - part of the JPEG XT family - specifies alpha channel coding for scalable compression and coding of continuous‑tone still images. It defines how opacity (alpha) information associated with photographic image content can be encoded and stored, either lossy or lossless, and in ways that remain backward compatible with legacy JPEG decoders. The standard targets continuous‑tone photographic content and leverages existing JPEG XT coding tools rather than introducing new core compression technologies.
Key topics and technical requirements
- Alpha representations: Support for binary alpha (0 or 1), integer alpha with 8–16 bit resolution, and 16‑bit floating point alpha values in the range 0..1.
- Premultiplication and shading: Defines handling of premultiplied alpha and premultiplication with a matte (shade) colour M. Provides formulas for composition and for reconstructing premultiplied or shaded data so legacy decoders can still display a plausible image on a chosen background.
- Compositing equations: Explicit composition rules for non‑premultiplied and premultiplied content (e.g., C = α·A + (1−α)·B) - note: the actual rendering/compositing step is not standardized.
- Scalability and compatibility: Uses coding technology from other ISO/IEC 18477 parts (e.g., integer, floating‑point, lossless modes). The codestream syntax is fully backward compatible with Rec. ITU‑T T.81 / ISO/IEC 10918‑1 so legacy decoders ignore alpha and produce a fully opaque image.
- File/box integration: Extensions are defined as JPEG XT boxes embedded in APP marker segments (see ISO/IEC 18477‑3). Normative annexes in the standard cover encoding/decoding processes, box structures and profiles.
- Encoder/decoder roles: The standard specifies encoder and decoder requirements, conformance language, and term definitions to ensure interoperable implementations.
Applications and who should use it
- Imaging and graphics software vendors: Add robust alpha handling to JPEG XT workflows for compositing, transparency and layered graphics.
- Camera and scanner manufacturers: Store opacity channels alongside high dynamic range (HDR) or high‑precision image data.
- Digital asset management and printing: Preserve matte/alpha information for compositing, layout and prepress workflows.
- Web and multimedia developers: Offer a JPEG‑based alternative for images that require transparency while maintaining legacy decoder compatibility.
- HDR and scientific imaging workflows: Combine alpha with integer or floating‑point image components defined in other JPEG XT parts.
Related standards
Keywords: ISO/IEC 18477-9:2016, JPEG XT, alpha channel coding, scalable compression, continuous‑tone still images, HDR, lossless alpha, premultiplied alpha, compositing, backward compatible JPEG.
Open preview (PDF)Information technology — Scalable compression and coding of continuous-tone still images — Part 9: Alpha channel coding
