libultrahdr is an image compression library that uses gain map technology
to store and distribute HDR images. Conceptually on the encoding side, the
library accepts SDR and HDR rendition of an image and from these a Gain Map
(quotient between the two renditions) is computed. The library then uses
backward compatible means to store the base image (SDR), gain map image and
some associated metadata. Legacy readers that do not support handling the
gain map image and/or metadata, will display the base image. Readers that
support the format combine the base image with the gain map and render a
high dynamic range image on compatible displays.

For additional information, see android hdr-image-format
https://developer.android.com/guide/topics/media/platform/hdr-image-format.