Overview
ISO/IEC 21122-2:2024 specifies profiles, levels, sublevels and buffer models for JPEG XS - the low-latency, lightweight image coding system. Building on the core syntax and coding tools defined in ISO/IEC 21122-1, this part narrows and constrains the codestream and runtime behaviour so that implementers can produce interoperable, cost-efficient devices and systems that meet strict latency requirements.
Key points:
- Defines profiles as subsets of the ISO/IEC 21122-1 codestream syntax for targeted applications.
- Defines levels (decoded-domain throughput) and sublevels (encoded-domain throughput) that conforming decoders must support.
- Specifies a buffer model (decoder model + channel model) and smoothing buffer semantics to guarantee interoperability under latency constraints.
- Contains normative and informative annexes that describe packet-based decoder and buffer models, frame buffer bandwidth levels, encoder models and latency analysis.
Key Topics and Requirements
- Profiles: selectable subsets of coding tools to reduce complexity and ensure consistent behaviour across implementations (new profiles added in this edition include TDC 444.12, MLS 444.12, CHigh 444.12, MLS.16).
- Levels and Sublevels: lower bounds on throughput in spatial/pixel (decoded) and codestream (encoded) domains; used to dimension implementations and ensure performance conformance.
- Buffer Model: precise definition of a decoder smoothing buffer, encoder smoothing buffer, and a constant‑bit‑rate channel model to prevent overflows/underflows and bound latency.
- Conformance: rules and conformance points that codestreams and implementations must meet; includes packet-based constant bit rate and decoder models.
- Practical considerations: interpretation of Bayer data, frame buffer bandwidth levels, and methods to estimate minimum achievable latency.
Applications and Who Should Use It
ISO/IEC 21122-2 is intended for:
- Encoder/decoder implementers (hardware and software) designing JPEG XS products.
- SoC and IP designers integrating low-latency image codecs.
- Broadcast and live-production engineers building real-time video transport (e.g., contribution, live switching).
- AV-over-IP and streaming system architects requiring deterministic latency and interoperability.
- Standards and compliance teams verifying conformance to JPEG XS profiles and buffer constraints.
Using this standard helps ensure interoperability, predictable end-to-end latency, and cost-effective implementations tailored to specific application profiles.
Related Standards
- ISO/IEC 21122-1 - JPEG XS Part 1: Core coding system (normative reference for syntax and coding tools).
- The ISO/IEC 21122 series (other parts) address complementary aspects of JPEG XS implementation and conformance.