Overview
ISO 21496-1:2025 - "Digital photography - Gain map metadata for image conversion - Part 1: Dynamic range conversion" specifies a standardized gain map format and associated metadata for HDR digital photography. The standard defines how to describe, store and apply a gain map that converts between a stored baseline image and an alternate image with a different dynamic range. Its goal is efficient, reliable HDR/SDR conversions while minimizing file size and avoiding multiple full-image files.
Key topics and technical requirements
- Scope: Defines a gain map data structure and the metadata needed to convert between two image representations (baseline and alternate).
- Gain map structure and requirements:
- Dimensions: gain map width/height should ideally match the baseline image; resampling is permitted and must be indicated in metadata.
- Colour components: prefer matching the baseline (e.g., 3-component RGB); a single achromatic component is allowed (applied to all RGB channels).
- Quantization: bit depth should be at least 8 bits per component; actual bit depth is recorded in metadata.
- Orientation: gain map orientation must match the baseline image.
- Metadata (required fields):
- Dimensions, quantization (bit depth), number of gain map components, per-component metadata.
- HDR parameters: baseline HDR headroom and alternate HDR headroom.
- Version tag and colorimetry metadata (baseline, alternate, and gain map application space primaries).
- Application workflow (Clause 6):
- Processing steps such as unnormalizing, resampling, and applying the gain map are defined; Annex A provides an algorithm for computing a gain map.
- Storage rules for embedding the gain map in image files are given in Annex C.
- Quality caveats: Resampling, lossy compression or chroma subsampling of the gain map can reduce file size but may degrade the alternate image quality.
Practical applications and who uses this standard
ISO 21496-1:2025 is intended for:
- Camera manufacturers and firmware teams implementing HDR capture workflows that store a baseline plus a compact gain map rather than duplicate full-resolution images.
- Image processing software, mobile apps and post-production tools that need to reconstruct HDR or alternate dynamic-range images from baseline images plus gain maps.
- Media asset management, streaming and delivery systems that want to support efficient HDR/SDR conversions while minimizing storage and distribution overhead.
- Standards engineers and integrators ensuring interoperability between capture devices, file formats and display pipelines.
Practical benefits include reduced file size compared with storing two full images, simpler asset management (single-file storage of baseline + gain map), and standardized metadata for predictable conversion across devices and software.
Related standards
The document references and interoperates with existing imaging and metadata standards, including:
Keywords: ISO 21496-1:2025, gain map, HDR, dynamic range conversion, digital photography, gain map metadata, HDR headroom, colorimetry, image conversion.
Open preview (PDF)Digital photography — Gain map metadata for image conversion — Part 1: Dynamic range conversion
