ISO 21496-1:2025

International
Standard
ISO 21496-1
First edition
Digital photography — Gain map
2025-07
metadata for image conversion —
Part 1:
Dynamic range conversion
Photographie numérique — Carte de gain pour la conversion
d’images —
Partie 1: Conversion de plage de dynamique
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, and acronyms . 1
4 Gain map requirements . 2
4.1 General .2
4.2 Gain map dimensions.3
4.3 Gain map colour components .3
4.4 Gain map quantization .3
4.5 Orientation .4
5 Metadata . 4
5.1 General .4
5.2 Gain map metadata .4
5.2.1 General .4
5.2.2 Dimensions .4
5.2.3 Quantization .4
5.2.4 Number of gain map components .4
5.2.5 Per-component metadata .4
5.2.6 Baseline high dynamic range headroom.5
5.2.7 Alternate HDR headroom .5
5.2.8 Version tag .5
5.3 Colorimetry metadata .5
5.3.1 General .5
5.3.2 Baseline image colorimetry metadata .5
5.3.3 Alternate image colorimetry metadata .6
5.3.4 Gain map application space colour primaries metadata .6
6 Gain map application . . 6
6.1 General .6
6.2 Processing the gain map .6
6.2.1 Unnormalizing the gain map .6
6.2.2 Resampling the gain map .7
6.3 Applying the gain map .7
Annex A (informative) Computing the gain map . 8
Annex B (normative) Colour conversion .10
Annex C (normative) Storing the gain map .11
Bibliography .15

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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This document was prepared by Technical Committee ISO/TC 42, Photography.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
High dynamic range (HDR) images, that have been produced for HDR displays, can provide a better user
experience. One challenge is ensuring that there is no image quality regression when displaying an HDR
image on a display with limited or no headroom, such as a standard dynamic range (SDR) display.
This can be solved by creating a standardized solution that includes storing in the same file a baseline image
and a gain map, to convert between the baseline and the alternate representations with different dynamic
ranges. Compared with storing both representations, the benefit of this solution is the optimization of the
file size, obtained by minimizing the amount of redundant data stored. It also avoids the logistical issues
caused by having separate files.

v
International Standard ISO 21496-1:2025(en)
Digital photography — Gain map metadata for image
conversion —
Part 1:
Dynamic range conversion
1 Scope
This document defines a gain map used in HDR digital photography applications, for dynamic range
conversion between two image representations.
This includes the definition of the gain map metadata and its attributes, how to specify the gain map and
associated metadata, and how to apply the gain map using this metadata.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
CIPA DC-007-Translation-2025 Multi-Picture Format
CIPA DC-008-Translation-2024 Exchangeable image file format for digital still cameras : Exif Version 3.0
ISO/IEC 10918-5, Information technology — Digital compression and coding of continuous-tone still images:
JPEG File Interchange Format (JFIF) — Part 5:
ISO/IEC 23091-2, Information technology — Coding-independent code points — Part 2: Video
3 Terms, definitions, and acronyms
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
alternate image
data structure that contains pixels and obtained by combining the baseline image and the gain map in the file
Note 1 to entry: The data is structured in accordance with the requirements of this document.
3.2
baseline image
data structure that contains pixels and image-related data, and is stored in the file as the main image
3.3
gain map
data structure that contains local quotients, and is used for conversion between two image representations,
the baseline image (3.2) and the alternate image (3.1)

3.4
gain map application space
linear RGB colour space in which the gain map (3.3) is applied
Note 1 to entry: This space is scaled such that the value of each of the 3 {R,G,B} colour components of the HDR reference
white is 1,0. Its colour primaries are either the colour primaries of the baseline image, or the colour primaries of the
alternate image (see 5.3.4 and B.2).
3.5
high dynamic range and wide colour gamut encoding
HDR/WCG encoding
colour image encoding that can contain wider variations in brightness, with a dynamic range significantly
higher than that of a standard dynamic range encoding, and a wider range of colours with more saturated
colours than standard colour gamut encodings
[SOURCE: ISO/TS 22028-5:2023, 3.12]
3.6
high dynamic range headroom
HDR headroom
ratio of nominal peak luminance (3.10) to HDR reference white luminance expressed as a log base 2 value
Note 1 to entry: For example, in an HDR image with the HDR reference white luminance at 203 nits, and the nominal
peak luminance at 1 624 nits, the HDR headroom would be log2 (1 624/203), or 3. This means 3 stops of HDR headroom
beyond HDR reference white.
3.7
high dynamic range image
HDR image
image that contains wider variations in brightness, with a dynamic range significantly higher than that of a
standard dynamic range image
[SOURCE: ISO/TS 22028-5:2023, 3.13]
3.8
HDR reference white
HDR signal level that would typically result from a 100 % Lambertian reflector placed at the centre of
interest within a scene under controlled lighting
Note 1 to entry: This is commonly referred to as diffuse white in the HDR content.
Note 2 to entry: The HDR reference white can be used as an anchor for HDR and SDR content compositing. The HDR
reference white is the HDR signal level typically reached by nominal peak SDR.
3.9
image
data structure that contains pixels and image-related data
[SOURCE: ISO/IEC 12087-1:1995, 3.1.3]
3.10
nominal peak luminance
luminance resulting on a display from the specified encoding peak white signal level
[SOURCE: ISO/TS 22028-5:2023, 3.18]
4 Gain map requirements
4.1 General
The gain maps specified in this document conform to the requirements specified in Clause 4.

These gain maps are used in HDR digital photography applications, for dynamic range conversion between
two image representations, the baseline image and the alternate image. The alternate image is obtained by
combining the baseline image and the gain map in the file, as shown in Figure 1, in accordance with Clause 6.
Figure 1 — Simplified view of the gain map application workflow
An algorithm described in Annex A may be used to compute a gain map from two image representations. If
the gain map is stored in an image file, it shall be stored as specified in Annex C.
NOTE Software applications conforming to this document can ignore a gain map that does not meet these
requirements.
4.2 Gain map dimensions
The gain map contains gain information related to the baseline image. For maximum accuracy, the length
and width of the gain map should be equal to the length and width of the baseline image.
Alternatively, the dimensions of the gain map can be modified by resampling the gain map (see 6.2.2 and
A.3.1), for example by a resampling factor of 2 (1⁄2 width and 1⁄2 height).
The length and width of the gain map shall be indicated using metadata, as defined in 5.2.2.
NOTE Applying resampling, lossy compression, chroma subsampling and other storage optimization algorithms
to the gain map can reduce its size but might also impact the image quality of the alternate image.
4.3 Gain map colour components
For maximum accuracy, the number of colour components in the gain map should match the number of
colour components in the baseline image: a gain map for a 3 colour components RGB image should have 3
{R,G,B} colour components.
Alternatively, the gain map can store a single achromatic component. In this case, the same achromatic gain
map will be applied to each of the 3 {R,G,B} colour components of the RGB image (see 6.3).
The number of components shall be indicated using metadata, as defined in 5.2.4.
NOTE Reducing the number of colour components in the gain map can impact the accuracy of the colour
reproduction in the alternate image.
4.4 Gain map quantization
The bit depth of the gain map should be at least 8 bits per component. The bit depth of the gain map does not
need to match the bit depth of the baseline image.

The bit depth of the gain map shall be indicated using metadata, as defined in 5.2.3.
NOTE The bit depth of the baseline image and the bit depth of the gain map impact the effective bit depth of the
alternate image representation.
4.5 Orientation
The orientation of the gain map shall match the orientation of the baseline image.
5 Metadata
5.1 General
This clause introduces the metadata associated with the gain map. The metadata provides information
regarding the gain map, and the colorimetry of the two image representations. The metadata shall be stored
in files as specified in Annex C.
5.2 Gain map metadata
5.2.1 General
The following metadata shall be stored along with the gain map.
5.2.2 Dimensions
The gain map dimensions metadata stores the length and width of the gain map (see 4.2).
NOTE The gain map dimensions can be derived from the dimension information of the corresponding image
file format.
5.2.3 Quantization
The quantization metadata stores the bit depth of the gain map (see 4.4).
NOTE The gain map quantization can be derived from the codec bit depth of the corresponding image file format.
5.2.4 Number of gain map components
The number of components metadata stores the number of colour components of the gain map (see 4.3). Its
value shall be either 1 for an achromatic gain map, or 3 for an RGB gain map.
5.2.5 Per-component metadata
5.2.5.1 Number of metadata components
The number of components stored for the following per-component metadata can either be 1 or 3.
If the number of components is 1, the same per-component value will be used for each of the 3 {R,G,B} colour
components of the RGB image (see 6.3).
If the number of components is 3, a separate value will be used for each of the 3 {R,G,B} colour components
of the RGB image, regardless of the number of components of the gain map.
5.2.5.2 Per-component min values
The per-component min gain map values metadata stores min(G), the minimum value for each component of
the gain map, in log base 2 space. It is used to normalize and unnormalize the gain map (see 6.2.1 and A.3.2).

5.2.5.3 Per-component max values
The per-component max gain map values metadata stores max(G), the maximum value for each component of
the gain map, in log base 2 space. It is used to normalize and unnormalize the gain map (see 6.2.1 and A.3.2).
For each component, max(G) shall be greater than or equal to the min(G) value (see 5.2.4) for that component.
5.2.5.4 Per-component baseline offset constants
The per-component baseline offset metadata stores the baseline offset for each component of the gain map
k . It is used to avoid numerical issues when computing and applying the gain map (see 6.3 and A.2),
baseline
and to enable raising low values in the baseline image.
5.2.5.5 Per-component alternate offset constants
The per-component alternate offset metadata stores the alternate offset for each component of the gain map
k . It is used to avoid numerical issues when computing and applying the gain map (see 6.3 and A.2),
alternate
and to enable raising low values in the alternate image.
5.2.5.6 Per-component gamma values
The per-component gamma values metadata stores the γ applied to each component of the gain map as a
pre-compression step (see 6.3 and A.3.3). Its value shall be greater than zero.
5.2.6 Baseline high dynamic range headroom
The baseline HDR headroom metadata stores H , the HDR headroom of the baseline image.
baseline
NOTE H is the value of HDR headroom for which the gain map is not applied (see 6.3).
baseline
5.2.7 Alternate HDR headroom
The alternate HDR headroom metadata stores H , the HDR headroom of the alternate image. H
alternate alternate
shall not be equal to H (see 5.2.5).
baseline
NOTE H is the value of HDR headroom for which the gain map is fully applied (see 6.3).
alternate
5.2.8 Version tag
The version tag metadata stores the version of the gain map. See minimum_version and writer_version in
C.2.2 and C.2.3 for details.
For the first edition of this document, the minimum_version shall be 0, and the writer_version shall be >= 0.
5.3 Colorimetry metadata
5.3.1 General
The colorimetry metadata provides information regarding the colorimetry of the two image representations,
along with the colour space in which the gain map shall be applied. The colour conversion shall be
implemented as specified in Annex B.
5.3.2 Baseline image colorimetry metadata
Colorimetric metadata specifying the colour encoding of the baseline image shall be included, such
as an Exif ColorSpace tag, an ICC profile containing a CICP tag, in accordanc
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