What Is an HDR Gain Map?
An HDR gain map is an auxiliary image that describes how to adjust a base image between standard dynamic range (SDR) and high dynamic range (HDR) renditions. It works with accompanying metadata to help compatible software adapt the image to a display’s capabilities.
A gain-map image file contains three main components: a base rendition, the gain map, and information needed to interpret the map. The base can be SDR or HDR, depending on the encoding.
For media teams, this provides a way to deliver an image that supports different viewing conditions while retaining more control over its appearance.
How an HDR Gain Map Works
Store the Base Image and Gain Information
In an SDR-based implementation, the base image supplies a conventional rendition. The gain map records spatially varying adjustments that allow supporting software to reconstruct the HDR rendition.
The map represents relative gains derived from the relationship between the SDR and HDR images. Metadata describes how its encoded values should be interpreted.
Adapt the Result During Display
Compatible software applies the gain information according to the image metadata and available HDR capacity. This capacity, often called HDR headroom, describes the brightness available above SDR white. A display with limited headroom may show an intermediate rendition rather than the full HDR effect.
The result can therefore adapt to the viewing environment instead of requiring every screen to reproduce the same brightness range.
HDR Gain Maps and Tone Mapping
Tone mapping adjusts an image’s tonal range to fit a target range. For example, it can compress an HDR rendition into an SDR rendition while retaining useful visual detail.
A gain map stores information relating to the renditions. Tone mapping can therefore be part of creating the SDR base used in a gain-map workflow.
The two concepts serve different roles:
| Concept | Main function |
|---|---|
| Tone mapping | Adjust tonal values for a target dynamic range |
| HDR gain map | Store gain information used to reconstruct or adapt between renditions |
| SDR base image | Provide the conventional rendition in an SDR-based gain-map file |
A gain map does not automatically recover scene detail that was never captured or retained. Its usefulness depends on the source material and the renditions used to create it.
Preserving Gain Maps During Image Processing
Image transformations must maintain the relationship between the base image and its gain map. Cropping or rotating the base requires corresponding treatment of the map so its adjustments remain aligned.
More substantial edits can require updating or regenerating the gain information. An old map may no longer suit an image after compositing or tonal changes.
Android’s editing documentation explicitly distinguishes basic geometric transformations from content edits that can make existing gain information inappropriate.
