Alex Fry: Nick and I have both made tools to make Delta E ITP heat-maps to compare implementations.
Nick Shaw: Alex's is better commented and probably more robust than mine. We were just discussing whether Colour's RGB_to_ICtCp function expects linear or PQ input. Looking at ITU-R BT.2100, the input to ICtCp encoding has RGB without prime symbols.
Alex Fry: Neither BT.2100 nor the Colour documentation are completely clear.
Nick Shaw: The function includes applying a PQ curve, so I think the input must be linear.
Alex Fry: So I should be passing in absolute nits.
Nick Shaw: My code takes non-linear input and linearizes it.
Alex Fry: Now I've changed my code to use absolute nits, I'm seeing a lot of Delta E values around 1.2 which seems a bit high.
Nick Shaw: I'm seeing similar values. What are you comparing?
Alex Fry: CTL vs my LUT implementation. I can't see differences visually. Over 1 I feel I should see something.
Nick Shaw: Between Blink v60 and CTL I got values of around 1.3 maximum. But comparing Kevin's version with the CTL they were under 1 for SDR but maximum of ~9 for HDR. We should ask Thomas what the intend of his code is. But also we could do a test where we grade an image really subtly until we can just see differences, and then see what size Delta E ITP we get.
[Nick showed BT.2100 and his code running on SDR and HDR images]
Scott Dyer: OCIO have been working on making the algorithm faster. They will have questions for us about the intent of the algorithm. Rémi also found some NaN values from inverse compression and when he changed a > to >= it removed them. I think we should do the same in the CTL. Call it 2.0.1 or something.
Nick Shaw: Because I see Delta E values over 1, using 1 is too tight a tolerance. Maybe 2 is more reasonable.
Alex Fry: I see 10 with HDR
Nick Shaw: That isn't unreasonable with a LUT implementation of HDR. Can you see differences.
Alex Fry: Yes, even in the raw PQ.
Nick Shaw: Kevin's optimizations that OCIO are looking at are producing similar values of ~10 which doesn't seem acceptable. So if those are optimizations get used they should be rolled back into the CTL as a change to the render.
Alex Fry: I should compare the Blink too.
Nick Shaw: I am comparing v60 Blink, Kevin's Blink and the CTL. Between v60 and the CTL the Delta E is less than 1. Kevin's Blink you can see differences, particularly in HDR. It's not a different looking image, but you can see when you A/B them.
Alex Fry: If OCIO are the only implementers, it feels what they do should get rolled in.
Nick Shaw: I believe FilmLight are waiting to see what OCIO do. If people get forced to use LUTs, because it's not performant enough, we should change the reference to be more performant, because the difference will probably still the less than the difference with a LUT.
Alex Fry: This is a natural back and forth.
Nick Shaw: I haven't done any testing with Rémi's OCIO implementation. I'll email Thomas and check what the intended input of RGB_to_ICtCp is.
Alex Fry: Presumably 1 is a JND in Delta E ITP.
Nick Shaw: I can ask Dolby. Jaclyn Pytlarz is behind Delta E ITP, I think.
ACES Output Transforms VWG
Meeting #171, October 30th, 1pm PT
[Meeting Recording]
Attendees
Meeting Notes
Meeting #170, October 16th, 1pm PT
[Meeting Recording]
Attendees