EDR and display brightness

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Created Jun ’21

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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.

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Answer 1

darknoon OP

Sep ’21

My guess is that this is related to the LCD display in your mac.

LCD works by blocking light, but it's not perfect. Say black = 99.9% absorption or 0.001 transmittance.

Say your max brightness is 500 nits, that means full backlight + LCD at reference white -> 500 nits. Well your blacks are going to be 0.001 * 500 = 0.5 nits.

At half brightness, if you keep the backlight the same but increase the dimming on the LCD, you end up with EDR headroom. BUT, now the blacks (still 0.5 nits) got brighter relative to the reference white (250 nits).

So if you did this all the way down to minimum brightness, the laptop screen would look washed out and be consuming extra power to keep the backlight at full.

Other display types like OLED screens don't have this issue. LCDs with local-dimming can mitigate this by turning down some of the backlight zones (you still get blacks washing out in some areas, ie ghosting), so it's practical to end up with a greater dynamic range.

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