The least conspicuous virtue of an Ultra HDR JPEG is that it still behaves like a JPEG. Open it in software that does not understand HDR and you see an SDR image. Open it in a compatible viewer on an HDR display and the highlights can reach luminance that an SDR file alone cannot describe.
Two renditions in one file
The file contains an SDR base image and a smaller secondary image called a gain map. The map describes how much the HDR rendition should differ from the base across the photograph. Metadata tells the reader how to apply that map and what luminance headroom is available. Android's published format description explains the structure and the fallback behaviour of compatible and older readers.
This is not a filter over the whole photograph. A street lamp can grow brighter while a face remains gentle; a reflection can sparkle without turning the sky into a white band. Both renditions still need thoughtful editing. A gain map cannot rescue a poor SDR base.
ISO 21496-1:2025 defines gain-map metadata and its application for dynamic-range conversion. Its purpose is to help different formats and devices interpret the relationship between two representations of the same image.
What different readers see
An SDR device displays the JPEG base. A compatible HDR reader combines the base and map, adjusting the expansion to the light its screen can produce. A display whose peak is lower than the file's target cannot show every encoded nit; the outcome depends on the monitor, operating system and viewer.
This is why Acutance keeps SDR and HDR development profiles separate. You can judge the photograph in the conditions your audience will actually use. The SDR rendition is an intentional part of the export. Greg Benz offers practical examples of the value of this dual rendition.
A practical precaution
Many editors and online services recompress JPEGs. Open an Ultra HDR file and save it again as an ordinary JPEG, and you may lose the gain map while keeping the .jpg extension. Preserve the original export and inspect the final distributed file in its intended viewer. Acutance Photo-View lets you switch between the two renditions with H.