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Target Luminance for 50% ST 2084 PQ?

 
Author CFC
User - Mono 
#1 | Posted: 3 Jul 2020 00:46 
I'm jumping back to using LightSpace ZRO for HDR10 calibration and noticed that the luminance target with ST 2084 for 50% is 90.450 nits.
I was under the impression that at 50% that target should be around 94.378.
Is the difference due to the way LightSpace converts down to 8-bit values?
Just hoping to learn and understand.

Thanks,
CFC

Author Steve

Mod - Scene 
#2 | Posted: 3 Jul 2020 09:40 | Edited by: Steve 
The target values for ST2084 depend on the signal range.
As ST2084 is an absolute standard, using the full 10 bit range, regardless of the signal range, we get:

Full Data range (both Full Data and SMPTE) have 512 bit input as being 92.7 nits (slight variation between the two, but irrelevant)
And if we re-scale the range between the actual signal range min/max for TV Legal we get 50% at 502 bit, and 92.2 nits

But, the sliders on LightSpace are always full Range, so 92.7 is the 10 bit target value.
(Any scaling is done post the sliders setting)

And as 8 bit cannot round directly to 10 bit, or vice-versa, there will obviously be slight differences, as seen.
(And ST2084 is actually specified in 12 bit, so even the 10 bit values are not 'accurate', for the same reason.)

Steve
Edited to make it easier to understand
Steve Shaw
Mob Boss at Light Illusion

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 Target Luminance for 50% ST 2084 PQ?

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