Inconsistent ACES results

resolve
Tags: #<Tag:0x00007f3c5d2d7058>

(Cary Knoop) #1

I took a video levels HLG (with Rec.2020) test clip, converted this clip to V-Log/V-gamut and saved the converted video using data levels.

Comparing the original clip and the converted clip using YRGB Color Managed Science with the proper conversions gave a satisfactory result, however under ACES there was a noticeable shift, predominately in the shadows. Also as an effect of these changed levels some noise shows up as saturated pixels

Unless I made a mistake something does not add up, it could be Resolve or some ACES definitions.

For a shift in the levels see here: https://forum.blackmagicdesign.com/viewtopic.php?f=21&t=79972


(Nick Shaw) #2

I do not know what the Resolve implementation does, but there is no official ACES Input Transform for HLG as far as I am aware. Are you sure that what you are using is not an inverse HLG Output Transform? That would not give the same result as an RCM transform.


(Cary Knoop) #3

My understanding is that ACES 1.1 supports an HLG IDT


(Scott Dyer) #4

That is incorrect. ACES 1.1 does not have an Input Transform for HLG.

ACES 1.1 simply added some additional ODTs, including an HLG Output Transform (which really just re-encodes 1000-nit ST.2084 (PQ) output to HLG using the method specified in Section 7 of ITU-R BT.2390-0).


(Cary Knoop) #5

Interesting!

So then what is Resolve using as IDT?

Is it the inverse of the RTT+ODT?

Would that give scene v.s. display referred issues perhaps explaining the gamma shifts and out of gamut issues in the shadows?


(Nick Shaw) #6

Quite possibly. I don’t have a Resolve 15 system here to test. But if you choose the HLG Input and Output Transforms, does the whole Resolve ACES pipeline become “transparent”? That is the test for whether it is an inverse Output Transform.


(Nick Shaw) #7

I had a quick look and the only HLG Input Transform I see in Resolve is the Rec.2020 HLG 1000 nits one, which is indeed the inverse of the HLG Output Transform. It is therefore not intended for scene-referred HLG recordings from a camera.

Bear in mind that even without the RRT, the HLG OETF and EOTF are not the inverse of one another. That’s why the Resolve Colour Space Transform includes HLG and HLG (Scene). I suspect, but haven’t tested, that if you set the Resolve Input Transform to ACEScct (i.e. do nothing) and then use a Colour Space Transform as the first node, going from Rec.2020/HLG (Scene) to AP1/ACEScct, you would get a match.