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ACES Output Transforms VWG 

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Meeting #161, July 17th, 1pm PT

Attendees

Alex Fry
Kevin Wheatley
Scott Dyer
Nick Shaw

Rémi Achard
Daniel Brylka
Jeffrey D Mathias
Willem Nagtglas
Pekka Riikonen
Doug Walker

Meeting Notes

  • Kevin Wheatley: Hopefully a straightforward discussion around wrapping up. Pekka posted a follow up to last week.
  • Pekka Riikonen: I made pex2 version which fits tighter and a Python script to generate the parameters and plots. I based my scaling fit on AP1 green which works best. We talked last week about comparing cube size differences, so I compared 65 cube 33 cube and pex2. You can see the approximation has less difference than a 33 cube, except maybe skin tones, because chroma compression affects less saturated colors more. But you can still only see the differences when you gamma up a diff. It's visually identical so could be a good optimization. I used ACEScct as the shaper.
  • Alex Fry: ACEScct is a commonly used shaper.
  • Kevin Wheatley: Nick and I looked into optimized shapers, and how  much range it should cover. We already clamp AP1 at zero, but where could the upper clamp be? It's related to the peak value, and we found 8x r_hit worked well.
  • Pekka Riikonen: Isn't r_hit sufficient?
  • Nick Shaw: No. That would be for achromatic. The 8x makes sure you cover everything up to clipping at 10,000 nits even for the works case scenario, which is pure AP1 blue. Blue's contribution to J is low.
  • Kevin Wheatley: The shaper could be a straight log with offset to pin zero. I have tested the varying clamp in my code, but not tested LUTs. We could go a bit lower and find the exact value for blue, but 8 is a nice easy number, and a power of 2.
  • Nick Shaw: These would cover a much higher range than ACEScct. The LUTs in OCIOv1 were problematic for high values from CG. Out r_hit values are quite high. We need to test how precision is affected by such a wide range.
  • Kevin Wheatley: You have to ask if a LUT is suitable for a 10,000 nit display?
  • Alex Fry: Even if we had a 10,000 nit display, the issues would probably be more visible in the first 1000 nits, so we can look at that.
  • Kevin Wheatley: These LUT shapers would only be implementation guidance. But we might as well use it to set clipping.
  • Pekka Riikonen: When I fitted the gain for my approximation I only used high nit values. Trying to fit a range including smaller ones didn't work well.
  • Kevin Wheatley: Red and green are the points that fit worst, which may be the reason for the skin tones.
  • Pekka Riikonen: Hand tuning for skin tones means you don't hit the red.
  • Nick Shaw: But the skin tones aren't "wrong", are they? They are just slightly different if you A/B them.
  • Pekka Riikonen: And not even visually for images. Are we considering using this?
  • Doug Walker: Anything that simplifies the implementation with minimal impact on look is very interesting to OCIO.
  • Kevin Wheatley: We can't change the reference, but can suggest it as an optimization.
  • Nick Shaw: I thought we were considering making this the new ground truth.
  • Kevin Wheatley: It's up to the TAC if we can change the official version.
  • Nick Shaw: I thought we talked last week about making RC2 the bug fixes we have now, and maybe putting this and your hue table optimizations in an RC3.
  • Kevin Wheatley: What do you think, Scott?
  • Scott Dyer: If it sets us up better in the long term, I think we should do it.
  • Nick Shaw: Ideally it would already be locked, but it isn't. If this becomes the new ground truth it's what everybody will use.
  • Kevin Wheatley: Pekka's code is a simple swap out.
  • Rémi Achard: I tested implementing this today, and it was very quick to do. It's maybe 10% faster on the GPU, but the CPU is slower. I'm still having to use textures for the lookups, to maintian compatibility with a range of systems. Reducing the number of lookups is a good step.
  • Kevin Wheatley: My changes are more complex. But would reduce it to 5 columns with this.
  • Pekka Riikonen: We could maybe fit a function to the upper gamma too. It has a similar shape.
  • Kevin Wheatley: It may be simpler, as it has no secondary cusps.