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ACES Output Transforms VWG 

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Meeting #78, November 24th, 1pm PT


Alex Fry
Kevin Wheatley
Scott Dyer
Nick Shaw

Rémi Achard
Lars Borg
Daniel Brylka
Michael De Caria
Alex Forsythe
Christopher Jerome
Zach Lewis
Jeffrey D Mathias
Pekka Riikonen

Meeting Notes

  • Kevin Wheatley: Alex has some updates with bug fixes.
  • Alex Fry: We were getting a slight shift in the RGB channels with a neutral ramp. There was a variable name issue that meant it wasn't being applied with Hellwig. I've also tidied the XYZ white points for consistency. Now in v24 discount illuminant brings the channels into alignment. Pekka flagged an issue where uneven input produces a discontinuity. I think it's related to the M collapse I see with Pekka's dominant wavelength image. I've updated the candidates repo to LUTs baked with v24 and added a release tag for where they were for the original tests. I've made new variations including P3-D65 and Rec.709 in a P3-D65 container, so make comparison easier. There's OCIO, Baselight and Resolve versions and the OCIO configs have multiple versions for Linux and Windows HDR and Mac EDR. Currently they don't include inverses. I need to work on that.
  • Kevin Wheatley: There are discussions in the issues section of the official repo.
  • Alex Fry: I need to move my code to that.
  • Kevin Wheatley: We need to work out the licensing and contribution agreement for that.
  • Pekka Riikonen: I opened a PR where I refactor the diagnostic modes, which improves compile speed. It highlighted that there is a load of code that could be removed. Like the MMSTC.
  • Nick Shaw: Is the "focusJbypass" checkbox something that will be needed in the final version?
  • Alex Fry: It's part of my testing to find the reason for the M collapse.
  • Nick Shaw: If you turn it off the discontinuity changes. The values are continuous either side of the break point, but there is a blip at that break like there is a zero crossing or something causing a zero division there. focusJbypass makes it snap to white at that point.
  • Pekka Riikonen: My post shows a grey ramp shifted to DCI white, and neutrals don't stay neutral. They actually expand in colorfulness. That doesn't happen with candidate A or the ARRI transforms. Anything neutral at normal exposure is neutral 5 stops over as well. Also some channels scale disproportionately, especially red making highlights warmer. I think this as related to the fact that we're squeezing large scene colorfulness values into a smaller range after the tone scale. In HDR you don't see it so much because there less compression. I can fix or adjust it with my new invertible chroma compression. I'm still working on that with Hellwig, which is harder because of the size of the space.
  • Kevin Wheatley: So are we mapping our ACES 1.0 to the right place in the model?
  • Alex Forsythe: Luke had a paper at CIC with some suggestions on how to deal with M changing as J changes.
  • Pekka Riikonen: I haven't read the paper. Are we sure the scaling by 100 for Hellwig is correct?
  • Kevin Wheatley: Maybe we should email Luke.
  • Pekka Riikonen: I have a kitchen sink ZCAM version of v24 with chroma compression. I can get a D65 curve going all the way up to where it hits display white and stays the same scaling all the way up. It makes images more neutral. Skin tones are close to candidate A. It is fully invertible in ZCAM. I am goin to try to normalize the Hellwig space to a similar range to ZCAM to see if it works for that. ZCAM is easier to work with. With Hellwig I'm still getting NaNs.
  • Kevin Wheatley: Hellwig should be simpler, which is why we looked at it. How can we focus on core bugs and tidy up of the code? It's hard for anyone except Alex and Pekka.
  • Alex Fry: I need to merge Pekka's PR, and I'll work on an inverse for the LUT bake.
  • Kevin Wheatley: How can more people assist?
  • Alex Fry: Reimplementing in Python will be useful. But with what Pekka and I are doing Blink is easier for looking at images.
  • Kevin Wheatley: We had three issues we needed work on. One is related to the compression algorithm, which we have progress on. One is the effective LMS primaries. Those two interact.
  • Alex Fry: In Hellwig we are using the stock primaries, and the compression sorts out the collapse near blue.
  • Pekka Riikonen: I've implemented compress mode for ZCAM. But in that and Hellwig it shifts the colors. So I think we need to change the matrix too. I think it's the compression that's making the blues skew magenta in Hellwig.
  • Kevin Wheatley: The third issue is the iterative solve for the gamut hull. Inverting would be much easier if we could fit a function. The other issues are bugs.
  • Nick Shaw: The discontinuity seem to disappear if I turn off HK mode.