Kevin Wheatley: This week we’re mostly focusing on any OCIO implementation issues that have come up. There’ve been OCIO Slack discussions about the test images. Scott confirmed they are all up to date. Is everything as you expect, Rémi?
Rémi Achard: Thanks Scott. It all seems good. I’ve been comparing the still life and synthetic chart renders. No I need to move to the cube and CMS images for round trip testing. My comparisons were thrown off because I was using half instead of float. But I think it’s good now.
Kevin Wheatley: The round trip is cleaner in 32-bit than if you truncate the result of the inverse to half-float. But that’s what you have to do for comparison with the reference images. But there is noise then.
Nick Shaw: Do you mean visible noise, or noisy variations relative to a perfect round trip?
Kevin Wheatley: Precision issues in difference comparisons to the original, Not necessarily visible. But that‘noise’ is in the reference renders due to 16-bit quantization of the intermediate image, and shows as patterns. It’s useful to do a 32-bit round trip as well to check your implementation.
Joshua Pines: We have tested the transform on a real show, because a colorist looked at it and thought it was great. We needed to make an LMT by inversion to match a show LUT. If we invert mathematically it’s great, but when we bake a LUT we found artifacts.
[Josh showed an image illustrating the artifact.]
Joshua Pines: You can see a blue artifact in the highlights. It’s better with a 65^3 LUT, but still there. Scott tested doing it mathematically and the artifact wasn’t there. Scott had made me a D60 transform, and maybe that’s part of the issue. We’re looking into it.
Kevin Wheatley: Is that a Rec.709 BT.1886 rendering?
Joshua Pines: It’s P3-D60 gamma 2.6. Our LUT goes to that. So we went through the P3-D60 inverse and back through the forward transform.
Alex Fry: Was the inverse you used a LUT?
Joshua Pines: I built an LMT as a concatenation of the inverse and my LUT. I had the inverse as a LUT. I think I can share the image.
Alex Fry: It looks like what you might expect using LUTs. We may have to advise people they need a mathematical inverse for this kind of thing. My gut says if you made your LMT LUT by concatenation a procedural inverse with your LUT it would behave better.
Joshua Pines: I would be very happy if all color corrector manufacturers all implemented these inverses mathematically.
[Kevin showed an image of a diff with a 32-bit and 16-bit round trip]
Kevin Wheatley: This with is my own implementation, not the official one.
Nick Shaw: I imagine this is cause by the inverse taking extreme values in the display space to very extreme values in scene space, which are then quantized.
Kevin Wheatley: That’s possible. I could investigate that. Certainly these things are more likely to occur nearer the gamut boundary.
Doug Walker: Thanks Scott for preparing those test images.
Kevin Wheatley: I’m going to be away for a few weeks due to holiday then work.
Scott Dyer: I may actually be back by next Wednesday, unexpectedly.
Kevin Wheatley: To meet VFX Reference Platform requirements we have to release OCIO v2.4.0 at the end of this month. I assume Rémi’s work will be in that. It will be labelled a preview/beta. Hopefully there will be OCIO 2.4.1 at the end of the year, which will be the real release that applications will ship with.
Kevin Wheatley: So what should the meeting schedule be from now? Do OCIO need particular input from us, or can we reduce the meeting frequency or make them ad hoc?
Doug Walker: We can meet as needed, but we do need to finalize by the end of the year. So we don’t want to stall progress.
Nick Shaw: This meeting slot will still be available, and I assume most of the regular attendees are free at this time. OCIO can let his know before each Wednesday if a meeting is needed.
Kevin Wheatley: And if any other vendors want a meeting we can have one.
Nick Shaw: I know Alex and Steve are meeting people at IBC. I can also ask any developers I speak to what their plans are.
Alex Fry: I need to expand the test config and bake it using CTL instead of Blink.
Kevin Wheatley: It would be interesting to see if that affects Josh’s issue.
ACES Output Transforms VWG
Meeting #169, Septempber 11th, 1pm PT
[Meeting Recording]
Attendees
Meeting Notes