ISO 21496-1:2025

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:

• CIPA DC-007 and CIPA DC-008 (MPF / Exif metadata mappings)
• ISO/IEC 10918-5 (JPEG/JFIF related parts)
• ISO/IEC 23091-2 (Coding-independent code points)
• ISO/TS 22028-5 (HDR/WCG definitions and terminology)

Keywords: ISO 21496-1:2025, gain map, HDR, dynamic range conversion, digital photography, gain map metadata, HDR headroom, colorimetry, image conversion.