Support HDR gain maps

[kmilos]

The storage of SDR image + gain map is currently being standardized as ISO/CD 21496-1.

This is already used by Adobe in "enhanced" DNGs, in Google's UltraHDR JPEGs (see also libultrahdr), and by Apple in HDR iPhone HEICs.

libavif and libjxl also added preliminary support for it.

Ideally, both import and export should be supported.

This approach seems to be gathering more momentum in the market compared to e.g. "true" 10-bit HDR only containers.

Related:

https://gregbenzphotography.com/hdr-photos/jpg-hdr-gain-maps-in-adobe-camera-raw/

https://www.reddit.com/r/DarkTable/comments/1f421d0/how_to_export_an_hdr_image_taken_on_iphone/

[wpferguson]

There is a PR for a lua script that can export UltraHDR, darktable-org/lua-scripts#502.

The PR also includes instructions on how to create the gainmap.

[ralfbrown]

darktable already has some support for gain maps on import (mostly in DNGs, e.g. #10289)

[kmilos]

@ralfbrown Not the same thing, that one is for flat field correction in raw domain.

[JLTastet]

Some relevant discussion on pixls.us (where the lua plugin came from)
https://discuss.pixls.us/t/manual-creation-of-ultrahdr-images/45004