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ACES Output Transforms VWG 

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Meeting #147, April 10, 1pm PT


Alex Fry
Scott Dyer
Nick Shaw

Chris Clark
Christopher Jerome
Jeffrey D Mathias
Willem Nagtglas
Pekka Riikonen
JP Zambrano

Meeting Notes

  • Alex Fry: We talked last week about using the surround condition only at the output stage.
  • Nick Shaw: Scott, you exposed it in your CT, but is it more complex than that?
  • Scott Dyer: It's easy because it's there, but is it a good way to do it? Or should we just do what we do in v1, a gamma in luminance? The effect of changing the surround in the model is much larger than that. I only exposed the surround value used for the final JMh to XYZ.
  • Nick Shaw: I think it does the right thing inside the gamut, but the curvature of the display gamut hull in JMh is affected by the surround parameter, it's shape changes, and just using a different surround in the final JMh to XYZ, when we've gamut mapped to a hull with the original surround value, will cause either more clipping or not being able to reach the boundary. I've not worked out which.
  • Alex Fry: The output surround parameter in the Blink affects the hull shape and the final conversion.
  • Nick Shaw: We'd need to test if out gamut approximation still works when we do that.
  • Pekka Riikonen: I tested with adjusted numbers to better match ACES 1, and going to dark increases clipping a bit, and going to average reduces clipping. Does it matter if it clips a bit or doesn't quite reach the boundary?
  • Scott Dyer: It would need a lot more testing.
  • Nick Shaw: Given we are in XYZ on the way to output, it's easy to bounce to Yxy and apply gamma, which seems safer.
  • AD: We can ship something with defaults now and add a function later.
  • Alex Fry: Dim vs dark was not a complaint area for the original.
  • Nick Shaw: If anything some people complained the dark to dim messed up their expectations. I suspect changes people might make in a trim pass would be larger anyway, so it's a confusion. If we leave it in, I think it should be hooked up to nothing, with a "to do" comment, rather than hooking it up to the output surround condition where people could use it.
  • Christopher Jerome: Was there a plan to provide an LMT to match the ACES 1 tone curve? Could that work for this.
  • Scott Dyer: When we have a final version we will definitely make LMTs like that which will match ACES 1 contrast.
  • Nick Shaw: But an LMT affects all outputs, and the dark to dim creates a difference between targets.
  • Pekka Riikonen: Scott, have you found the inversion issues you had in your CTL?
  • Scott Dyer: I'm making progress. I'm using Rémi's NumPy version as a reference. But that is v55, so I'm updating it. I think my issue is in my gamut compression code. I obviously need to make it match the Blink before we can release.
  • Alex Fry: I need to bake some v57 LUTs.
  • Nick Shaw: Pekka, you mentioned a v58.
  • Pekka Riikonen: That would just be parameter changes.
  • Scott Dyer: Are we switching to Reinhard from powerP?
  • Pekka Riikonen: You can simulate that by changing the exponent in powerP to 1.0, which I could do in v58 so people can see the difference.
  • Nick Shaw: I don't know if powerP is a published curve, or something Jed invented for the RGC. It's an extra layer on top of Reinhard, and without that layer the maths is simpler.
  • Pekka Riikonen: It makes the compression a little more aggressive. It doesn't help the blue. The change is very small. If I change the powerP exponent to 1.0 in v58 and people are happy with it we can simplify the final code. I'll push v58.
  • Nick Shaw: Then I'll aim to make a DCTL v58.
  • Scott Dyer: Most of this week's forum discussion has been about the naming of sRGB and gamma.
  • Alex Fry: I have made a visualization with constant M rings to look at the kinks we were discussing last week.
[Alex showed his Nuke 3D JMh visualization with different chroma compression stages on and off]
  • Pekka Riikonen: The effect comes from the reach compression in the chroma compression – the in gamut compression. The limit is AP1, so there is a discontinuity there.
  • Nick Shaw: But we clamp to AP1, so there shouldn't be anything outside AP1. Or at least anything that was outside there is already skewed into AP1. Is there something that will occur in real images that will create the hue sweeps that show this effect.