Some precision about Rec709

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#1

Hi,
I’m currently and recently in charge of the color pipeline in our Studio. I’m reading the forum for a long now to figure out what ACES workflow could improve for us. Did a lot of tests with some CG DCC, and the results look really great.
Here’s why I’ posting here : I read a lot about Rec709/BT1886 TRC, not an easy thing ! From what I read, the final Rec709 TRC inside ACES is gamma 2.4. Right ? I heard that on a BT1886 calibrated display, the resulting gamma is about 2.2, due to black level correction (assuming it’s not perfect). So, is the ACES-Rec709 output transform the correct choice in this case (at lighting/compositing stage, I mean) ?
Another thing that’s not clear for me, if I decide to create a digital media content for HDTV, what’s the best output transform to choose before encoding ?

Best regards

#2

Hello @paillecedric and welcome to acescentral !

I would say Rec.709 (ACES) is the correct Output Transform for both questions. From the ACES GitHub repo :

This transform is intended for mapping OCES onto a Rec.709 broadcast monitor that is calibrated to a D65 white point at 100 cd/m^2. The assumed observer adapted white is D65, and the viewing environment is a dim surround.

I think it uses a Gamma 2.4 by seeing this line in the CTL reference code :
const float DISPGAMMA = 2.4;

We had a similar question at our studio since we were using a P3D65 (ACES) ODT with a gamma of 2.6 on a monitor with a gamma of 2.2. I found fascinating the answer I had been given at the time :

There is a general consensus in the industry that working in an office (normal/dim surround) with a Gamma 2.2 display (100 nits) would be “equivalent” to looking at imagery with a video projector (48 nits) in a dark surround. But most importantly you should try to compare between your office’s monitor and a theater to make sure renders are faithful to what you originally intended.

We do not have any theater on site, so it was difficult for us to do so. But we did not get any bad surprises during our DI tests. In the end, the surround is a just a power function if I recall correctly. I hope this helps (a bit), it is good to see some Full CG artists/supervisors on this forum !

Chris

PS : I will not go into the debate of encoding profile VS display profile as it is way beyond my skills. :wink:

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#3

The specified gamma for Rec709 is 2.4 (which would be on a 100 nit display).

From what I understand this visually tracks closely with a gamma of 2.2 on a display at 200-300 nits (office or home environment), and likewise a gamma of 2.6 on a projector or display at 48 nits (P3).

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#4

Thanks @garrett.strudler ! I did understand something similar to what you just explained, although you did explain it much better than me. :wink:

Much appreciated,

Chris

#5

Thank you for your reply, so if I understand correctly, I should set my gamma (TRC) depending on the viewing condition ?

  • 48 nits -> 2.6
  • 80-100 nits -> 2.4
  • > 200 nits -> 2.2

I guess it’s to preserve details on shadows tones.

#6

Yes, it is my understanding those gamma values (at the display) should yield a similar “looking” image (assuming an appropriate surround illuminance as well), at least in SDR.