ACES Output Transforms VWG
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Meeting #23, July 22nd, 1pm PT
Francesco Luigi Giardello
Scott Dyer expanded on his explanation of the SSTS from the previous meeting, and showed the interactive Python plots that were used during development of the 1.1 OTs. He showed both an , and then a , which also included presets for various display classes. He demonstrated with this how the mid grey luminance adjustment is achieved using a scene side exposure adjustment, which effectively moves the curve side to side, not up and down, sliding the window of scene exposure mapped to the display. Open to debate if this is the right way to do it. It was done to keep the shape of the curve consistent.
- Scott Dyer: The SSTS enabled me to adjust the curve in any way I needed. I feel we could use it to look at B&W images and find what we want the tone curve to do, and then add in things like the color stuff Jed has been working on. It's just one proposal. We could replace it with the Michaelis–Menten based curve Daniele proposed, or something else. I'm going to make a proposal for a changed SSTS curve, with different end point slopes and mid-tone contrast, and compare it to other popular renderings.
- Daniele Siragusano: How are the values for the presets derived?
- Scott Dyer: They are just the values used in the current OTs. They are the modifiable values in the Output Transform module in the current CTL.
- Daniele Siragusano: How is mid grey chosen for e.g. an 800 nit monitor?
- Scott Dyer: There are currently inconsistencies, and no definition for e.g. 800 nit mid grey. The 1000, 2000 and 4000 nit OTs all use 15 nits for grey, and Dolby Cinema uses 7.2. There's nothing in between, and no formula.
- Daniele Siragusano: So the formula gives you freedom to change things, but doesn't tell you what the values should be. Do you need dynamic branching in the formula. No simple y = f(x) equation?
- Scott Dyer: Yes. It's piecewise in 4 parts – linear extensions at the ends, and two b-splines, with smarts to make the join smooth.
- Daniele Siragusano: I'm thinking of computational efficiency. Dynamic branching on a GPU is not efficient, and it needs to have benefits to justify it. We need to think about how much complexity we need at each point, and what it gives us.
- Kevin Wheatley: Is that an implementation discussion? Back to the parameters, it doesn't seem right to me to use scene exposure to move the mid grey level. It's related to lack of understanding of why the values are what they are. We should have a curve that defines where grey comes out at any level, not just particular instances.
- Scott Dyer: The SSTS doesn't have to be the final form, but I think it is useful for intuitive control over the curve, to find the behavior we want.
- Lars Borg: I notice the curve is not symmetrical in the low and high end. I assume there is a reason for that. We need to find what is the overall shape of the curve, and what do we need to keep tweakable?
- Scott Dyer: The asymmetry comes from needing to match the original RRT, which targets a theoretical 0.0001-10000nit display, with +18 stops mapping to 10000 nits, -15 stops to 0.0001, and 18% grey at 4.8 nits. We need to focus on deciding what the desired behavior is for displays with different capabilities. B&W helps us focus only on the tone scale and not be distracted with other things.
- Daniele Siragusano: You set black luminance to the capability of the reference monitor, yes? SO ignoring PQ, if you have a relative EOTF, you map that value to zero before applying the inverse EOTF, so it's at zero in the encoding?
- Lars Borg: In cinema, doesn't zero represent an unachievable absolute black, so you have to clip at 64 for what the projector can actually do?
- Daniele Siragusano: No, the DCI spec is very explicit that black is relative, and zero maps to whatever is the darkest the projector can produce.
- Scott Dyer: Absolute encodings complicate things, because if black is defined absolutely, you rarely actually get that output. Really you want it to float with natural flare etc.
- Daniele Siragusano: So wouldn't it make sense to remove the black luminance parameter? Put it to zero and make it relative.
- Scott Dyer: It's definitely confusing and inconsistent in the current version.
- Lars Borg: But if you set black to zero, on two devices with different black levels, you get a different appearance of the shadow detail.
- Joshua Pines: We have a canonical reference display for each standard, and that has a black level. What Scott is trying to answer is what range from the scene should be mapped to that display range. And he's doing that in nits. How you then go out to an encoding is a separate issue.