Ultra HDR is a true HDR image format, and is backcompatible. libultrahdr is the reference codec for the Ultra HDR format. The codecs that support the format can render the HDR intent of the image on HDR displays; other codecs can still decode and display the SDR intent of the image.
I'm getting the following test failures in the current git master:
A git bisect shows the offending commit as: c0c184568d38c8b1cb0ad79a5a86f02a69bc522f.
System Information: