FAQ: NLC tags 1-1-1 vs 1-2-1 and PrPro's actions and practices

Adobe Community Professional ,
May 04, 2021 May 04, 2021

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I got this response from Adobe DVA Chief Scientist Lars Borg today, over on the public beta forum ... with specific permission to reprint. So here is a long-sought answer to Premiere Pro's treatment of the NLC tags used to tell equipment what primaries/gamma to use for display.

 

Neil

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From Lars Borg, 4 May 2021

 

1-1-1 is standard Rec. 709. Only 1-1-1 is standard 709.
The middle 1 does not specify a display gamma.
AVC and HEVC declares 1 as the transfer function of a fictitious Rec. 709 camera, approx gamma 2.  But camera vendors say no camera uses that TF in production.  Definitely not for display.
More recent Rec.709 specs say that when viewed on a broadcast monitor (which historically is based on a CRT TV in a dim room), you should use gamma 2.4 (see ITU-R BT.1886:2011).
On the other hand, sRGB (approx. gamma 2.2) was developed in 1996 by Microsoft and HP specifically to make a computer monitor in an average-lit office (500 lux?) match the color appearance of a TV in a dim room, so that no color conversion would be required. Blacks need to appear a bit lighter in the office than in the living room. Thus, 709 video should be viewed on an sRGB display when in the office, for an equivalent appearance.
Then we have the deviant, Apple’s QuickTime Player, which, long before BT.1886, decided to use gamma 2 for color conversions. Apple’s choice seems to be a compromise between the 1987 Apple RGB monitors (gamma 1.8) and 1996 sRGB monitors (gamma 2.2). Earlier, Web designers had made the same gamma compromise between Apple and Windows displays. Or maybe Apple just picked the Rec. 709 camera gamma 2 (going scene-referred). Unfortunately this has been adopted by other video players, giving us the QT gamma bug, where video looks too light compared to an sRGB display.Premiere uses 1-1-1 for Rec. 709 video (import, timeline and export)
Right now, Premiere defaults viewing to broadcast monitor viewing conditions, gamma 2.4 (but we’re tweaking that).
DCM off would be useful for viewing 709 on an sRGB monitor.

 

1-2-1 is a hack. And it’s a hack to fix a hack.
AVC and HEVC declares 2 as undefined transfer function.
In many devices (QTP?!) this disables TF color conversions and color management, as there is no defined curve to apply and you get Preserve RGB. OK for 709 to an sRGB display or 709 TV, but not elsewhere where color management is needed.
In other devices, 2 signals a private, non-standard TF, including some HDR TF cases.
Given the inconsistent results, 1-2-1 should not be used generally.
1-2-1 won’t pass broadcast QC as it’s not standards compliant.

 

Premiere imports 1-2-1 as 709 (1-1-1). This is because Premiere ignores the invalid 1-2-1, and uses the default which is 709. It doesn’t matter anyhow as import to a 709 timeline preserves the 709 media values.
Premiere exports compliant 709 (1-1-1).

 

There are 3rd-party tools that can change the media’s tag setting from 1-1-1 to 1-2-1 or vice versa.
These tools usually only hack the top level NLC record, and not the VUI records inside the video frames, so as a result the video file is inconsistent, with different color spaces in different parts of the media file. This can lead to unpredictable results when playing or transcoding depending on which part the app looks at. So the hack to fix a hack would need more hacks.

 

A trivia question: Why did Apple select gamma 1.8 for Mac displays?
No points given for asking Charles Poynton!

TOPICS
Editing, FAQ, Formats, Hardware or GPU

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