Overview
ISO/IEC 21122-2:2022 specifies profiles, levels, sublevels and buffer models for the JPEG XS low-latency, lightweight image coding system. Building on the core syntax defined in ISO/IEC 21122-1 (Part 1), this part defines interoperable subsets of the codestream (profiles), minimum throughput bounds in the decoded and encoded domains (levels and sublevels), and a reference buffer model to guarantee decoder/encoder interoperability under strict latency constraints.
This second edition adds profiles for colour filter array (CFA) images, mathematically lossless compression, and 4:2:0 sampled images, enabling new use cases while preserving low complexity and low end‑to‑end latency.
Key topics and requirements
- Profiles: Defined subsets of the Part 1 codestream syntax and admissible parameter ranges to reduce decoder/encoder complexity and ensure interoperability for targeted applications.
- Levels and sublevels: Constraints that set lower bounds on throughput in the decoded (spatial/pixel) and encoded (codestream) domains respectively; used to dimension implementations and guarantee performance.
- Buffer model: A combined decoder model and channel model describing interactions between a reference decoder (including a decoder smoothing buffer) and a constant‑bitrate channel. The buffer model:
- Specifies parameters (fill level, buffer capacity, cycle timing, blanking periods, codestream fragments) to prevent underflow/overflow.
- Enables encoders to form codestreams that any conforming decoder can decode within latency constraints.
- Decoder/encoder models and units: Definitions for decoder/encoder units, smoothing buffers and timing (cycles), including packet‑based models and a constant bit‑rate channel.
- Normative and informative annexes: Annex A–C (normative) describe profiles, packet‑based decoder model and packet‑based CBR buffer model; Annex D–E (informative) provide encoder model, latency bounds, codestream conformance properties and latency analysis.
Applications and who uses it
ISO/IEC 21122-2:2022 is valuable for:
- Codec implementers (hardware and software) designing JPEG XS encoders/decoders to meet specific throughput and latency targets.
- System architects and integrators building real‑time video systems (live production, contribution links, broadcast, cloud rendering, AR/VR, automotive vision) that require deterministic low latency and lightweight compression.
- Silicon vendors and FPGA developers who need reference buffer models and level/sublevel constraints for resource and memory budgeting.
- Broadcasters and streaming platforms seeking interoperable, low-latency image transport.
Related standards
- ISO/IEC 21122-1 - JPEG XS Part 1: Core coding system (normative reference).
Together, Parts 1 and 2 provide the coding tools and practical interoperability constraints for low-latency JPEG XS deployments.