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Meeting #20, June 9th 2021, 1pm PT

Attendees

Alex Fry
Kevin Wheatley
Scott Dyer
Nick Shaw

Rémi Achard
Valentin Alt
Chris Brejon
Daniel Brylka
Sean Cooper
Thomas Mansencal
Daniel Mulligan
Michael Parsons
Carol Payne
James Pickett
Joshua Pines
Matthias Scharfenberg
Daniele Siragusano
Jed Smith
Shane Smith
Mike Whipple

Meeting Notes

  • Alex Fry: Following on from the thread on ACES Central, I've been experimenting with an LMT to reproduce the look of the older RRT under the Open DRT, without using a full invert. It uses a truncated version of the ODT and copies the chromaticities across to the scene values. There's also a curve to match the tone scale, which is accurate for a black and white image, but a bit less with color. It still needs more work on highlight appearance for some images. This is similar to something I did at my last company to keep the feel of the SDR in the HDR, so HDR skies desaturated the same but had HDR dynamic range, which masked variation.
  • Kevin Wheatley: I've done more with the LUT curves using Principal Component Analysis to look at properties contributing to variations from the average. Mid grey hit almost exactly 10cd/m2 (10% of peak). It produced a number of curves, smooth at first, but later ones getting noisier, which can create all the variations by being added in different proportions to the  average.
[See recording from 12:35 for detail]
  • Kevin Wheatley: DNEG did the same with LUTs they had. They had more variation, but their process didn't quite replicate mine. I built something in Nuke where I could add the first five components to the average. The first component affects thw position of the highlight roll-off.
  • Daniele Siragusano: HDR10+ uses a similar approach to build curves using multiple fixed curves in different proportions.
  • Kevin Wheatley: I want to see if the main components are replicable with standard grading operators.
  • Daniele Siragusano: Typically you only have about five DRTs, mutated by further color grading. If you took your curves and plotted them in 3D I think you would see repeating behaviors. I'm often asked to make scene looks from DRTs and find it's a modified version of something I've done before. It's like virus evolution! The Adobe PFE is the basis of a lot of what I see. If you average all the images in the world you get a Gaussian distribution, and to compress to a Gaussian, where most of the distribution is in the middle you use a sigmoid, which is the fist derivative of a Gaussian. The visual system is fitted to nature, and sigmoids are ubiquitous in nature. And in film, percolation effects cause the toe, and saturation effects the highlight roll-off, so that creates a sigmoid. There's an experiment showing saturation throwing darts at a wall of balloons, and the more balloons have already been hit, the smaller the chances are of hitting a balloon.
  • Joshua Pines: We're often asked specifically to create variants of one show LUT. And those different CDLs plus the same LUT end up being passed around. The S-curve comes also fro the history of painting, and what artist had to do to represent high dynamic range scenes on low dynamic range media.
  • Daniele Siragusano: Natural systems, valves, audio etc. all have this saturation effect which digital doesn't, so we need to engineer it.
  • Sean Cooper: Even single photon avalanche detectors naturally arrive at sigmoid behavior. Kevin, earlier you said the greatest variation in the log-log plot was at the low end, but you say the primary component affects highlights.
  • Joshua Pines: On a true log plot, any DRT that goes to zero will shoot down to minus infinity at some point.
  • Kevin Wheatley: I trimmed the bottom ends for that reason. But we will need to factor in something for dealing with where black goes. Maybe something like what Gary and Doug dit at the top and bottom end, where it doesn't roll off completely. The average curve that you add things to could specify things like, where is black? what's the slope at mid grey? Where does diffuse white go, etc.
  • Sean Cooper: What domain were the components analysed in?
  • Kevin Wheatley: The y-axis is linear cd/m2, and the x-axis is linear in a range of stops, so logarithmic.
  • Joshua Pines: These LUTs were all SDR, yes? 10% middle grey is almost a law. It's always that in SDR. Lars got an intern to analyze average pixel value relative to peak in some varied ASC samples, graded to "look nice". It came to [0.11, 0.09, 0.11] and that slight magenta shift was due to neutralizing DCI white. LAD density on print was 1.0 and density is log10 of 1/transmitance, so 0.1 becomes 1.0. The CTL for the ODTs explicitly say 0.18 maps to 10%.
  • Nick Shaw: Some variation in LUT exposure could be down to whether they were intended to be used with "print down" CDLs, or if that was baked into the LUT, because often there is a print down somewhere to force DPs to use more light.
  • Joshua Pines: Our show LUTs are always biased to make DPs overexpose a bit.
  • Daniele Siragusano: Isn't it also important to have a function which translates in a continuum between many different viewing conditions? If you take tone curves designed for simulating biological processes, like the Michaelis–Menten curve I proposed, you end up with the same thing.
  • Nick Shaw: It would be interesting to plot differences from that curve, rather than the average.
  • Daniele Siragusano: Or see if you could find parameters for that equation to match the average curve.