HDR Gamma Curve in Lightroom?

[sobhhi]

When working with images in HDR mode in Lightroom, the export dialogue allows for the selection of color gamut. For example, sRGB or Rec 2020. However, what is the gamma curve embedded in HDR exports? PQ? HLG?

Do the luminance limiter settings also get embedded into the image (e.g. limited at 800 nits or 3 stops over SDR)?

And lastly, does this vary based on exported format? Such as JPEG vs JPEG XL?

[MadManChan2000 · Accepted solution]

This depends on the file format. When using 10-bit or 16-bit export for file formats AVIF, JPEG XL, PNG, and TIFF, the transfer curve is always PQ, with the reference white assumed at 203 nits, or 58% of the PQ maximum encoding level (per the ISO 22028-5 standard).

When using 32-bit floating-point export for file formats such as TIFF, the transfer curve is linear and 1.0 means reference white.

When using JPEG or other formats in the "Maximum Compatibility" mode, we instead use a HDR Gain Map per ISO 21496-1. The base image is SDR and the gain map is then used to derive the alternate (HDR) image.

See https://www.iso.org/standard/86775.html and https://helpx.adobe.com/camera-raw/using/gain-map.html for more technical details on the gain map.

[sobhhi]

Thank you so much for this informative answer, especially highlighting the white point.

May I ask as a follow up why the luminance limiter in the latest Lightroom update extends to +8 stops above SDR, corresponding to 25,600 nits, if PQ's maximum theoretical luminance corresponds to 10,000 nits?