LMT Questions

[gray_marshall]

Hi all.

I’ve got a couple of “quick” LMT questions:

1. I assume that the LMT is designed to be applied in the ACES 2065-1 space (as opposed to ACEScct), correct?
2. I’m confused that LMTs are included in the “Display Refered” section of many ACES flow charts. Aren’t they properly still in ACES space?
3. Is the LMT output (ACES’) usually included in the Archived Digital Master (pre RRT)?
4. If an LMT is chosen in pre-production as a “show look”, I’m assuming it’s applied in the ACESproxy chain for viewing and Editorial. Is it usually supplied to the VFX houses as well (along with any appropriate CDL)?
5. Finally, does anyone have specific info on how LMTs would be applied in Nuke OCIO?

Thanks for your time.

[nick (Nick Shaw)]

@gray_marshall

• I assume that the LMT is designed to be applied in the ACES 2065-1 space (as opposed to ACEScct), correct?

As per spec, yes. But if you provide a show LUT designed to be applied in ACEScct, that is to all intents and purposes an LMT. Many LMTs may transform ACES2065-1 to ACEScct before performing subsequent transforms, so there may be practical efficiencies in dropping pairs of transforms which cancel each other out from the pipeline.
But if you are providing an official LMT as a CLF, it should include a transform from ACES2065-1 as the first step, and back to it as the last.

• I’m confused that LMTs are included in the “Display Refered” section of many ACES flow charts. Aren’t they properly still in ACES space?

Any chart which shows an LMT as display-referred is incorrect. Part of the purpose of an LMT is that it is a display-independent look.

• Is the LMT output (ACES’) usually included in the Archived Digital Master (pre RRT)?

Section 6.6 of TB-2014-010 states that archive masters can be made with or without LMTs baked in. I cannot say which is more commonly done.

• If an LMT is chosen in pre-production as a “show look”, I’m assuming it’s applied in the ACESproxy chain for viewing and Editorial. Is it usually supplied to the VFX houses as well (along with any appropriate CDL)?

ACESproxy is effectively deprecated. So there is not normally an “ACESproxy chain”. Or are you just meaning graded proxy files for editorial?
Common practice would be to bake the LMT, together with any per-shot adjustments, into the dailies transcodes for viewing and editorial.

• Finally, does anyone have specific info on how LMTs would be applied in Nuke OCIO?

OCIO supports looks, but OCIO 1.x does not support CLF (OCIO 2.0 will). So an LMT would need to be converted into a LUT or other suitable transform for OCIO.

[gray_marshall]

Great answers, Nick. Thank you. (So much misinformation out there).

One follow-up: you say the ACESProxy “is effectively deprecated”. What do you mean exactly? Is it not being utilized in production?

[nick (Nick Shaw)]

Correct.

ACESproxy was designed as an encoding that a camera could output, and a “naive” hardware grading system, capable of applying ASC CDL type adjustments only to a live signal, could grade the ACESproxy, with the RRT+ODT being applied downstream of that by a LUT box.

These days, all on-set live grading systems are software based, and concatenate multiple transforms, including IDT, grade, LMT, RRT and ODT into a single 3D LUT (or optimised 1D/3D pair) and push that in real time to a LUT box.