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Convert To Linear Light: On for 32 bpc

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Filip Stillerska
Convert To Linear Light: On for 32 bpc
on Aug 8, 2018 at 3:09:16 pm

The default setting for the "Convert To Linear Light:"-option (during export) is "On for 32 bpc".
Can someone please give me a scenario in which it is a good idea to set this option to "Off"? (For example, I'm working in a 32 bpc linearized Rec.709 Gamma 2.4-project and am about to export two version of the project: one compressed H.264 for YouTube and one Apple ProRes 4444 - should this option be set to "On for 32 bpc" in both of these scenarios?)



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Andrew Somers
Re: Convert To Linear Light: On for 32 bpc
on Aug 8, 2018 at 5:33:37 pm

Hi Filip,

First of all, linearized means a gamma of 1.0. It does not matter what the profile gamma is, if you are working in linearized space, your gamma is 1.0

Second, Adobe leaves a lot to be desired in the way they handle linear working spaces, in particular the settings are worded in a way that often leads to confusion.

I see from the above screenshot that you are working in 32 bit scene linear, gamma 1.0, colorspace. I see you are outputting to the DISPLAY REFERRED (gamma 2.4) version of Rec709 - not something I recommend but you may have your reasons for doing so.

Here's where Adobe confuses all: The switch for "convert TO linear". Well, you are ALREADY in linear, and you need to convert TO gamma encoded. But that's okay because that switch does nothing in this position since you are not going out to a 32 bit output file format.

So you can leave it as is, "on for 32 bit" works for most everything. Note that it ALSO keeps 16 bit EXR in linear, as it should. In fact, it really should NOT be labeled as "on for 32 bit" — it should be labeled as "on for floating point outputs".

So, in YOUR case, going out to 8 bit H264 and 12 bit ProRes 4444, both of which are NOT linear as you are going to encode a 2.4 gamma curve in them, you definitely do not want to output in linear. So the proper setting is either OFF or ON FOR 32 BIT. Do not set it to "ON".

Personally, for color critical applications I no longer use Adobe CMM in the output module, and instead use adjustment layers on the output with OpenColorIO or other LUT or Profile conversion, then "Preserve RGB" on the output module. I discuss more in depth, including a tutorial, here.

The main thing you need to be concerned about when working in scene linear space is having your monitor properly calibrated AND PROFILED, so that using "Display Color Management" gives you an accurate view of the work.

Andrew Somers
VFX & Title Supervisor
http://GeneralTitles.com


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Filip Stillerska
Re: Convert To Linear Light: On for 32 bpc
on Aug 8, 2018 at 6:07:11 pm

Hi Andrew,
read through your article and learned a lot (thank you!), but yes - Ae's color management is definitely a headache. I've been trying to really figure it out for months now, but it's really hard to completely wrap my head around it.

"I see from the above screenshot that you are working in 32 bit scene linear, gamma 1.0, colorspace. I see you are outputting to the DISPLAY REFERRED (gamma 2.4) version of Rec709 - not something I recommend but you may have your reasons for doing so."

Why are you not recommending me outputting a non-linearized file? Should I export the 8 bit H264 and 12 bit ProRes 4444 as linearized instead?


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Andrew Somers
Re: Convert To Linear Light: On for 32 bpc
on Aug 8, 2018 at 9:52:15 pm
Last Edited By Andrew Somers on Aug 8, 2018 at 10:04:30 pm

No no - I AM recommending that you output a gamma encoded (non linear) file. Just not necessarrly with Rec709 2.4 gamma.

Why? Because it's wrong - Unless you did everything in a specific monitoring environment and now all your corrections are based on a 2.4 gamma monitor simulation.

Here's the thing: Rec709 is NOT 2.4 gamma, and it never has been. Rec 1886 is.

Rec 1886 defines the MONITOR, Rec709 defines the SCENE CAPTURE, i.e. the camera. The gamma curve for Rec709 is an offset 2.22 curve, that results in an effective gamma of 1/1.9-2.0 (i.e. about 0.53)

This was originally intended to feed into a CRT monitor (circa 1990) with a gamma of around 2.4, adjusted for the viewing environment. This resulted in a total system gain of gamma 1.2, in order to present *perceptually* on a monitor in a darkened living room the scene as it was shot.

And keep in mind that there is NO "color management" in this system. The camera is defined and the output is defined, but not color "managed" with profiles or LUTs like After Effects is.

The upshot of all this is: the FILE that gets gamma encoded and written to the ProRes files is expected to be Rec709 "source gamma" (for want of a better term, scene referred isn't really accurate either), and that is a gamma of about 0.53 (but again Adobe calls it wrong as "1.9" which is the inverse).

The way this should work in a color managed environment is:
1) You work in LINEAR (scene referred) space.

2) You UNWIND Gamma encoded Rec709 files into linear space using an accurate Rec709 profile. (I recommend Rec709-elle-V4-rec709.icc as the correct profile for reading the source gamma curve to unwind into linear, and also to encode the correct transfer curve out of linear into a Rec709 ProRes.)

3) You need to setup your Display Color Management such that you go from linear INTO the Rec709-elle profile, THEN into a Rec1886 profile (whcih people have labeled as Rec709 Gamma 2.4, or alternately Rec709 Display.)

4) Now you are SEEING what a correctly encoded Rec709 output file (Gamma 1/1.9) looks like played back on a reference rec709 MONITOR (gamma 2.4).

5) You output to a video files using the source gamma encoding (~0.53 ish) and it gets played back on a display as defined in Rec 1886, (but commonly called a Rec709 display.)

This has caused so many people so many problems (including myself) I actually have three separate articles on the issue that I have not yet had a chance to combine:

The CORRECT Rec709 Profile with Download link
More on Rec709 gamma
More on linear to Rec709 encoding.

ADDITIONAL READING:
From Charles Poynton, the only human being on the planet that really understands this stuff:
His classic GAMMA FAQ
And the accompanying COLOR FAQ
And his web site of even more useful info.

ON THE DRYER SIDE, the actual specs:
The actual specification for Rec709 Notice that the Rec709 spec does NOT specify the display charactaristics, except that finally in 2015 they mentioned Rec1886 as the reference display device (over 20 years after the original Rec709 spec was written, they finally defined the display !!!) and even so, it's only a FOOTNOTE on page 3.
(1)In typical production practice the encoding function of image sources is adjusted so that the final picture has the desired look, as viewed on a reference monitor having the reference decoding function of Recommendation ITU-R BT.1886, in the reference viewing environment defined in Recommendation ITU-R BT.2035.
LOL. Better late than never.
The specification for the complementary Rec1886 DISPLAY for viewing Rec709 ...
The specification for the VIEWING ENVIRONMENT in which you are supposed to view the reference monitor

-----
This is still such a problem for everyone (and Adobe is not helping by shipping incorrect profiles), I am working on a tutorial. Someday I'll finish it.....


EDIT TO ADD: If you did all your color adjustments and work looking at a display color management setup where you went straight from the linearized gamma 1.0 environment converted to a gamma 2.4 profile and then a monitor simulation using the 2.4 profile, THEN I suppose you should output using that 2.4 profile. But if you find your results are inconsistent (i.e. wrong contrast, wrong saturation) then I'd suggest outputting with the Elle profile I mention above.

Best of luck

Andy

Andrew Somers
VFX & Title Supervisor
http://GeneralTitles.com


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