Scott Dyer: We are well into implementation. I have heard form some vendors about their status, but some are waiting for OCIO. I know there are issues with speed.
Doug Walker: OCIO 2.4 shipped at the end of September with preview ACES 2 support by Rémi. It is at least 3x slower than ACES 1. People want it to be comparable as they need some processing for other operations. We are on hold waiting on Kevin's optimizations. Our goal is to ship a more performant OCIO 2.4.1 in December.
Nick Shaw: Is comparable performance realistic, given 2.0 is much more complex?
Doug Walker: Maybe with the current algorithm. We want to avoid 3D LUTs. 2x or 1.5x would be ok.
Scott Dyer: Do Kevin's changes make it that much faster?
Rémi Achard: He was optimizing the cusp table sampling and reduce the search to two iterations.
Nick Shaw: He now doesn't iterate because it if hard coded to two fixed iterations.
Rémi Achard: This is mainly for the GPU. For CPU and GPU the number of power functions are a concern.
Alex Fry: Is even 1.5x optimistic? Or could the original RRT/ODT have been more optimized.
Doug Walker: The red modifier and glow added complexity for minimal effect.
Rémi Achard: Real-time issues are for the GPU, correct? Game engines etc?
Nick Shaw: Don't game engines already take short cuts and not exactly match ACES?
Doug Walker: And that's an issue if something is called ACES but doesn't match it. They drop the red modifier and glow, and maybe the surround compensation. That could get worse with ACES 2.
Nick Shaw: If your source is known and bounded you could even drop the whole gamut compression.
Scott Dyer: Kevin is traveling for work in a different time zone. I'm not sure when he is back.
Doug Walker: We need him for the table optimizations, but we could look at the tone curve. We are doing unnecessary power functions to convert back and forth. We could look at that and maybe do a 1D LUT or spline.
Nick Shaw: It is a purely 1D function in J. We only convert back and forth because the tone scale is defined in luminance.
Alex Fry: The tone scale was always independent of the CAM back when we had ZCAM. Is a 1D LUT an option?
Doug Walker: IN OCIO we are trying to avoid LUTs, for various reasons, but it's an option in the short term.
Nick Shaw: We already use lookups for the cusps. It's not like one 3D LUT but we use LUTs. 1D LUTs are pretty accurate.
Doug Walker: What range do we need to cover and in what space?
Nick Shaw: J is already perceptual, so a reasonable space to apply it in. Does the tone curve go below zero? I think it clamps at zero. It goes at a shallow slope through limitJmax.
Doug Walker: So maybe zero to 10% above limitJmax?
Nick Shaw: It would be whatever the input J is that maps to limitJmax. That would be easy to calculate. Maybe add a bit and then invert the tone curve. I can investigate that using my Y_to_J and J_to_Y.
Rémi Achard: I was also wondering if we could use a 1D LUT in the cusp lookup, making an inverse 1D LUT at initialization. Maybe wait for Kevin.
Nick Shaw: His samples are evenly spaced in each sextant between each primary and secondary, so the initial guess is more accurate, and one more step can find the right interval to lerp. And he only needs one lerp t value because all the values are sampled at the same hues. It should be a significant speed improvement.
Rémi Achard: There is also the possibility of using the GPU's built in interpolator. It reduces precision, so we need a metric for acceptability.
Nick Shaw: If it's not visible according to e.g. Delta E ITP, it's probably ok.
Alex Fry: Did you find out what an acceptable tolerance is?
Nick Shaw: A value of one, according to the Dolby documentation 1.0 means a difference **may** be noticeable, not will be. So I think we need a threshold higher than 1.0, particularly for HDR.
Scott Dyer: We put a lot of effort into making it not do things ACES 1 did, but now some people want those things! Especially fire going yellow in ACES 1. So we need an LMT which undoes hue preservation. I know people made black body images for testing. If anybody has those samples please send them.
ACES Output Transforms VWG
Meeting #172, November 13th, 1pm PT
[Meeting Recording]
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Meeting Notes