After watching the (excellent) EDR session at WWDC, I'm trying to understand EDR's ability to take advantage of larger headroom when display brightness is dimmed. I'm testing on a MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports) running Big Sur 11.3 (20E232), using the "Color LCD" profile that shipped with the Mac. I believe this display has a max brightness of 500 nits. At full brightness, maximumExtendedDynamicRangeColorComponentValue returns 1.25. When I turn the display brightness down, maximumExtendedDynamicRangeColorComponentValue increases, up to the maximumPotentialExtendedDynamicRangeColorComponentValue of 2.0. However, it hits this maximum surprisingly quickly, when display brightness is only slightly reduced (kIODisplayBrightnessKey returned 0.9863281). Visually testing an OpenEXR version of the Luxo image (found here http://markfairchild.org/HDRPS/Scenes/LuxoDoubleChecker.html) in Preview.app reveals that, indeed, a few more clipped highlights become unclipped at this slightly-reduced display brightness. Cool! What I don't understand is why I'm capped to 2.0. Where is this cap coming from? Intuitively, I thought maximumExtendedDynamicRangeColorComponentValue would increase with every step down in display brightness, all the way down to the minimum display brightness, but that is evidently not the case, at least on this hardware/OS/display profile/NSScreen instance.