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Meeting #145, March 27th, 1pm PT

Attendees

Alex Fry
Scott Dyer
Nick Shaw

Remi Achard
Daniel Brylka
Alex Forsythe
Christopher Jerome
Jeffrey D Mathias
Willem Nagtglas
Pekka Riikonen
JP Zambrano

Meeting Notes

  • Alex Fry: No Kevin this week. We're going to discuss the blue issues and Scott's CTL work.
  • Pekka Riikonen: I pushed v56. It only changes the LMS primaries. The Blink is identical to v55. I posted comparisons with ARRI Reveal. I think both are reasonable renderings of these images. With the hue wheel the blue issue is still there but less. I plotted the result of a blue to magenta ramp. It's in the corner so sudden changes are expected. And I showed that ARRI Reveal doesn't reach the corners, so it's easier for it to be smoother. You can see they compress blue a lot.
  • Alex Fry: They have different requirements. We want to hit all screen colors. Some people said smoothness should be prioritized, but others pointed out different people have different priorities.
  • Nick Shaw: And an LMT can add smoothness.
  • Pekka Riikonen: Plotting all the stars there is clipping at every edge, which is the sharp kinks. Our gamut mapping isn't precise, so there is always some clipping.
  • Alex Fry: Cusp smoothing puffing out adds to that.
  • Pekka Riikonen: ZP Zambrano posted and interesting experiment with ring input. But you have to remember the ring is outside AP1, so is clamped on input which produces skews. So interpreting the rings as ACEScg, I could still see distortions due to the hue dependent chroma compression.
  • Alex Fry: It's interesting but it's not clear what a circle in chromaticity space means.
  • Nick Shaw: I did an experiment which was just a proof of principle that you can improve smoothness with LMT that's a hue qualified M compression in JMh space.
  • Alex Fry: That means you can't reach the corner any more, but it's smoother. Are you changing hues as well?
  • Nick Shaw: I am just compressing M, but I think it pulls stuff out of the region that would have got skewed, so it changes hue as well. It defaults to compressing hue of 250 which is the hue of the AP1 primary, but there are hue center and width controls. It doesn't seem to break any of our sample images.
  • Alex Fry: It presumably isn't specific to our transform.
  • Nick Shaw: It uses the same JMh space, so may be slightly better at preconditioning the data for that, but it could be used with any DRT.
[Nick showed the Fabian Matas nightclub image from the gamut mapping set with the K1S1 and DRT v55]
  • Nick Shaw: That image is so extreme there is blue fringing even with K1S1, and my LMT improves that.
  • Pekka Riikonen: Do you have compress mode enabled.
  • Nick Shaw: I don't currently. But maybe I should, because it doesn't need to match the exact JMh space in the DRT. And I don't have an AP1 clamp.
[Pekka showed a 3D plot of his variation of JP's ring image turning on each stage of the DRT]
  • Pekka Riikonen: The path to white from the chroma compression creates a strange shape. When we add the gamut mapper you can see the shape of the Rec.709 gamut. I assume the discontinuity comes from the fact that we don't touch anything outside the limit. We just clip.
  • Alex Fry: JP, you showed a rendering of your own that was much smoother with the rings.
  • JP Zambrano: I't a mix of a bunch of things to get smooth results, but it doesn't reach the corners, and it isn't invertible. It's based on AGX and Jed's OpenDRT plus some of my own stuff.
  • Christopher Jerome: Pekka, how much control do you have over the direction of the 'hook' with the star image? Blue skewing towards green is the opposite direction than I would expect. All hue lines I've seen curve in the opposite direction near blue.
  • Alex Fry: Straight lines in xy are curved in JMh and vice versa. The constant hue line we are compressing along curves one way in xy, so compressing back along that bends a straight xy line the other way. And clamping always skews towards the secondary, unless it's perfectly on the primary axis.
  • Christopher Jerome: Could something line a matrix in Jab control that curve in blue?
  • JP Zambrano: I think something may be doubling up. My guess is that because a straight line in xy is not straight in JMh, when it goes out to display it keeps that color. If it's too purple in JMh, which would be blue in xy it stays purple. In my DRT I engineer it to remove that, where here it's keeping it or even making it stronger.
  • Christopher Jerome: I've been experimenting with  known hue consistent gradients from primaries, and then inverting them in the model to see where they land.
  • Alex Fry: The only way to eliminate the kink at the end is to perfectly compress to the boundary.
  • Nick Shaw: I see very little difference from the final clip. It seems to be the gamut compression that is bending the line at the end.