ACES Output Transforms VWG
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Meeting #38, January 19th, 1pm PT
Francesco Luigi Giardiello
- Kevin Wheatley: We should recap some discussions from ACES Central, and discuss how to prepare things for testing. To summarize the discussions, there's a tension between gamut volume coverage and the preferred path to white.
- Björn Ottosson: To summarize . If you do a simple unwrapping of the RGB cube you see very obvious harsh edges. Those are still visible if you use a perceptual model. It's particularly obvious in blue because between pure blue and greenish blue, there's a huge drop in saturation. I only see a few possible solutions. One is to not reach the extents of the RGB cube. Or you can accept the problem, although you can hide it a bit with a mapping that affects the interior of the cube less. The third option is to accept hue distortions, at least in very saturated colors. The next part of the post shows mathematically why the derivative of the tone curve drives desaturation in an RGB curve approach. You also don't want your path to white to become fluorescent on the way. The last thing is about the input gamut. If you want to reach the corners of the RGB cube, and have a natural transform for normal colors, you have to go quite far out into imaginary yellows, for example, to get full yellow output.
- Kevin Wheatley: That all matches the things I had imagined we need to consider.
- Jean-Michel Gilbert: ACEScg may be a suitable input gamut. It's close to Rec.2020 which is the target for HDR.
- Björn Ottosson: That's an example of what I was saying. ACEScg yellow is close to natural real world yellows. So if you map that to the corner, you won't have a natural mapping for saturated yellows.
- Daniele Siragusano: That's why most working spaces have imaginary primaries.
- Björn Ottosson: It's useful to have that headroom available in grading.
- Daniele Siragusano: One point that's missed is that if the slope when you hit clipping is shallow enough, you may not notice clipping artifacts. We have that with tone-mapping, where a tone mapper can overshoot peak white, and get clipped, but if the slope is shallow at that point it's not noticeable. It's just at the end of the day, so I'm not afraid of the sharp edges of the hull. But I agree with most of what you said. I'm not a big fan of ACEScg, because it's too close to the target gamuts, so it's hard to make a good mapping, particularly for reds and yellows. Lightness is not a good metric to use for me, because it's too complex for a per pixel model. Energy conservation metrics may be a good thing, and if all reflective colors have a certain limit that may be a good place to start the path to white.
- Alex Fry: Do people have strong opinions of the input gamut? Currently we chop everything outside AP1.
- Kevin Wheatley: It ties to what I said last week about targeting a real or virtual intermediate device and then down-mapping to the real display looking for the best representation of that. Or at least do that conceptually. And it seems a set of virtual primaries might be what's needed. But not AP0, as the directions are inappropriate.
- Thomas Mansencal: I did some tests and putting AP0 primaries into the ZCAM model it blows up. , and it's obvious it's not a good choice.
- Björn Ottosson: No perceptual model has been designed to behave outside the visible range.
- Thomas Mansencal: Which is why I think you shouldn't use a perceptual model with non-realizable colors.
- Daniele Siragusano: But if you start with XYZ and take Y, you break down as soon as you're near the spectral locus.
- Björn Ottosson: So there would be an argument for making your working space use perceptual correlates, if that were feasible.
- Jean-Michel Gilbert: sRGB will always clip if you have real yellow.
- J. Schulte: What is real yellow? A particular wavelength?
- Kevin Wheatley: We don't want to be constrained by Rec.709's limitations.
- Nick Shaw: That's the logic for having a bigger target, so what you show in sRGB is just the best approximation you can manage of that.
- Matthias Scharfenberg: Isn't choosing a virtual display with wide primaries just shifting the problem into the final display mapping. What does that get us?
- Kevin Wheatley: I thought originally it would be better to target a real device, because you just get one and measure it. But you are then constrained by the limitations of that device.
- Matthias Scharfenberg: My ZCAM DRT is input gamut agnostic. The first step is conversion to XYZ.