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ACES Output Transforms VWG 

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Meeting #144, March 20th, 1pm PT

Attendees

Alex Fry
Kevin Wheatley
Scott Dyer
Nick Shaw

Remi Achard
Lars Borg
Daniel Brylka
Alex Forsythe
Luke Hellwig
Jeffrey D Mathias
Willem Nagtglas
Pekka Riikonen
Christian Wieberg-Nielsen

Meeting Notes

  • Kevin Wheatley: This week there have been a few posts on ACES Central about the blue behavior. And we want to discuss the parameter needed to create a set of deliverables.
  • Pekka Riikonen: I did som testing of the effect of the eccentricity factor.
[Pekka showed various images with and without eccentricity in the model]
  • Pekka Riikonen: Most images there is no difference. At the very top end it slightly darkens blues and lightens reds. I suggest leaving it out to keep the transform simpler. It doesn't help the blue issue we see on the color wheel. Moving the blue primary closer to Thomas's coordinates we slightly reduce the skew to cyan. But if we move it too far blues go magenta in highlights. With the star image it helps a little. It's still not as good there as ARRI Reveal. I assume the effect comes from the curvature of the hue lines from the model.
  • Alex Fry: Those should be perceptually consistent hue.
  • Nick Shaw: In the original model the lines are constant hue for colors in the source data set, but by stretching them to work for colors further out have we distorted the hue consistency for normal colors?
  • Alex Fry: Are we creating a skew at the top with our path to white?
  • Pekka Riikonen: Chroma compression is non-linear and only affects M. It doesn't change J, so doesn't preserve chromaticity. Straight lines in chromaticity space don't stay straight.
  • Nick Shaw: In Daniele's video he was talking about the importance of preserving linearity in scene space. In a display rendering we can do whatever looks right.
  • Kevin Wheatley: Moving our LMS primary happens in linear, so is no different than applying a matrix LMT, like Nick did.
  • Alex Fry: Can we visualize our output in JMh to see if we're adding skew, maybe from the final clamp?
  • Pekka Riikonen: I don't think it's that. The AP1 clamp does introduce skew.
  • Nick Shaw: I think sometimes people are fixated on images looking beautiful when untouched. There is a colorist in the loop to adjust problematic images. Images like blue bar are way outside AP1, and may need something doing to them, but it doesn't need to be in the DRT.
  • Kevin Wheatley: So there are two parts to the problem. One is the blue area that stands out, which we don't understand. Separately some blues skew cyan or purple, and a pre-grade could fix those. If we can fix the blue band by making it wider, the images will be easier to grade.
  • Pekka Riikonen: Björn Ottosson suggested the narrow blue is the result of compressing along constant hue lines. I did once suggest we might have smoothing along the hue axis. But we never looked at that.
  • Alex Fry: Smoother versions don't hit the corners.
  • Nick Shaw: Is it possible to do an LMT that rounds of the corners? It would prevent you hitting them, but you don't have to use it.
  • Alex Fry: A JMh based gamut compressor as an LMT could be useful.
  • Kevin Wheatley: There was a post about bumpy levels with a hue rotated ramp.
  • Nick Shaw: I couldn't replicate what he saw, but I think some of the jagginess came from using LUTs. If I hue rotate in Nuke the ramp goes outside the spectral locus very quickly.
[Nick showed his Nuke version of the hue rotated ramp]
  • Pekka Riikonen: There are still distortions. They are just smoother.
  • Kevin Wheatley: I think the non-linearity is combining with the gamut compression curve to produce these shapes.
  • Pekka Riikonen: Does reach mode affect it.
  • Nick Shaw: It's slightly smoother without reach mode.
  • Kevin Wheatley: Scott has made a list of suggested deliverables.