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Relationship between Gamma / Perceived Contrast / Display illumination / Surround Illumination

Author finnjaeger
#1 | Posted: 17 Jan 2021 23:12 
Trying to wrap my head around if there is any nice mathematical construct/ theory out there that explains how the following things work together to create the 'rendering intend' or whatever you call that.

-Surround Illumination
- system Gamma
- Display Illumination and black-illumination , or 'contrast'

what I am basically trying to do is get a model going that would give me a mathematical basis for adjusting aces ODTs in VFX to adjust for different viewing environments. T-CAM from Baselight has some nifty surround compensation, but nobody talks about what happens when we deal with high brightness monitors unless we dive into the world of HDR, there are plenty of 200Nit + monitors out there and they obviously look "more contrasty" compared to 100NIT displays , as there is more contrast .

For 100NIT Displays with the only difference being the surround illumination the general wisdom is/was that we deal with this with the mismatch in gamma from 2.4 to 2.2 which is a system gamma of approx 1.09 , ok fine, that seems to work but what about cranking the display illumination?

Given example displays that are calibrated for 200NIT, gamma 2.2(pure power law). D65, Rec709 Gamut in a "office surround", how should we the transform from scene-reffered images to display reffered in that case? given the ACES Rec709 RRT+ODT (pure 2.4 powerlaw gamma) viewed on a proper 100Nit gamma 2.4 Monitor , a simple approx 1.09 added system gamma seems to be pretty far off from that.

Just from a subjective view it seems like I have to introduce a "lower system gamma" something like gamma 0.9 to account for the added contrast, basically making the image "flatter" ,or adding a logarithmic section to the high end,
which seems to be sort of what HLG is trying to do if I understood that correctly, HLG content looks really good on SDR monitors its just really flat up top, increasing display illumination almost looks HDR-Ish without really beign HDR. (of course when accounting for the Rec2020/708 gamut mismatch). But now how does this play into surround compensation?
Surround mostly affects the darker portion of the image, visually I would say, again subjective, higher surround brightness gives us a higher visual contrast, up to a point where it completely overpowers the screen.
( https://www.koreascience.or.kr/article/CFKO200923557662490.pdf )

I hope it makes sense, i might have confused some words as I am not a calibrator I just make the stuff that gets displayed by monitors

Author Steve

#2 | Posted: 18 Jan 2021 11:34 
The HLG standard includes a 'surround illumination' compensation.
It is build into ColourSpace/LightSpace.

But HLG is the only standard that has any such attempt at compensation included...

Steve Shaw
Mob Boss at Light Illusion

Display Calibration Light Illusion Forums / Display Calibration /
 Relationship between Gamma / Perceived Contrast / Display illumination / Surround Illumination

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