Scott Dyer: I'll be testing in our lab where we now have the Flanders 55" OLED as well as the X300. We're inviting the ASC and some colorists, and giving some post houses a package for their own systems. I want to check I'm using the latest best version, and have the right settings in Blink.
Nick Shaw: v50 just added the D60 sim option which is not relevant for the LUTs.
Alex Fry: v49 fixed some HDR artifacts, but there are no v49 LUTs, so v50 is what to use.
Nick Shaw: Are we sticking to AP1 chroma compression primaries?
Scott Dyer: I thought that changing them made some artifacts go away, but I was changing a lot of settings.
Alex Fry: Images with extreme values do render better with a larger reach. Early versions reached right out to ARRI Wide, and so rendered some extreme images like blue bar better. But the inverse path becomes ridiculous, and you need to push to those extreme values to hit the boundary, which you can't do with a normal working space like ACEScct or ACEScg. We have to say you need the RGC for extreme values.
Nick Shaw: As I mentioned before, if reach compression is set to AP1, unless chroma compression is also set to AP1, then AP1 boundary values get double compressed and end up inside the target gamut. And likewise the target gamut boundary inverts to outside AP1.
Pekka Riikonen: Setting it to a wider space it won't invert any more. I think the two reach gamuts should match.
Nick Shaw: That makes a strong case for AP1.
Kevin Wheatley: My only concern is the hard boundary that might make.
Nick Shaw: Is there a visible band, or does it fall off smoothly to doing nothing at the boundary?
Kevin Wheatley: It's continuous but not entirely smooth.
Pekka Riikonen: That's why I think the reach gamuts should be the same, and anything beyond that is clipped at the end.
Kevin Wheatley: We should look a the effect of combining that wit the RGC.
Pekka Riikonen: I think the RGC parameters should be dialed back for ACES 2.0.
Nick Shaw: That was always the plan.
Pekka Riikonen: The v49 I pushed during the last meeting has some wrong parameters for ratio based compression. But that doesn't affect reach compression.
Nick Shaw: Version 50 is the same as v49 except I have added separated drop-downs for the primaries and white point of the limiting gamut. It affects the reference white used to convert JMh back to XYZ. Choosing ACES white with Rec.709 primaries is the same as the old D60 sim. There is also a"fit white" check box, which like the current D60 sim scales the maximum RGB channel of the simulated peak white back down to 100% so no channel is clipped. Although we talked before about this kind of tilting the neutral axis meaning some values are pushed out of one side of the encoding space, and get clipped, and a'hole' opens up on the other side.
Kevin Wheatley: It a philosophical question, depending what your intent is. And it affects inverses. So we should document that you should only use it if it does what you want.
Nick Shaw: I also started experimenting with using the same method I used to find the target gamut boundary to also find the reach gamut boundary, ti make it invertible. It's work in progress. It now round trips reach compression pretty well, but a few values shift a little. I need to investigate more. Maybe it's the fit of the hull top gamma.
Kevin Wheatley: What else is still to be done.
Alex Fry: We need a way to find those top gamma values rather than hand tuning. All the reach boundaries are calculated in the code except the locus.
Kevin Wheatley: Cusp smoothing was on the list.
Pekka Riikonen: Pushing things out helps with some boundary inaccuracies.
Alex Fry: Having a larger constant value for the top gamma that included everything is the simple solution, but would lead to lots of clipping.
Nick Shaw: And even that larger value would differ for different targets.
ACES Output Transforms VWG
Meeting #129, November 29th, 1pm PT
[Meeting Recording]
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