I repeat myself saying that the choice doesn’t have to be between not doing lightness mapping at all (preserve original lightness == horizontal projection) and the current projection. There’s a whole world between those two. Doing horizontal projection below the cusp I think is perfectly reasonable. Less so above the cusp as it can desaturate colors too much and make them appear clipping when there’s no reason to do so. I think @bottosson says it well in:
One challenge with preserving lightness though is that all colors … get projected to a single point, and that highly saturated colors can become almost completely desaturated if light or dark enough, even though sacrificing just a small amount of lightness would have resulted in a much closer match.
Björn’s page is a good read obviously. I’ve implemented most of those mappers previously and the one I like the best is Adaptive L hue dependent one. I have a blink version of that somewhere but the behavior with dark colors doesn’t make it suitable for this DRT, was my conclusion.
Here’s few images comparing v035, v035 horizontal projection and ARRI Reveal (for state of the art third party comparison), in that order. I’m showing here what I would consider over desaturation that happens with horizontal projection above the cusp:
Here’s just an example with RED christmas of v035 and v035 with an LMT that changes contrast. One thing to keep in mind is that the rendering out of the box is less contrasty than ACES1, so if we assume people are going to increase contrast it’s good to check how it looks (I’ve been doing this throughout the DRT development). This is still less contrasty than ACES1 but much closer. Make what you will of it.
But, the problem at hand was that the the ARRI bar cyan-green appearance match was less than perfect between Rec.709 and P3. My feeling is we should primarily address that issue rather than revamp the whole rendering. The Rec.709 rendering, with its darker mapping, is a result of developing the SDR/HDR appearance match, and I can take the blame for it. Obviously we could do it the other way around and match HDR to SDR instead. Horizontal projection would create more of a mismatch for SDR/HDR.
So, I think we should first do what we did in the meeting, which is to bring the projection on the same trajectory in different gamuts. That should make it a little bit better. If that’s still not good enough appearance match for Rec.709 and P3 (SDR), then we could think about matching the lightness also between the gamuts. And that then opens that whole world of possibilities between the horizontal projection and the current one. I assume the biggest discrepancy in the match between Rec.709 and P3 comes from the brightness of the color, so it would then make sense to test how they look when the lightness mapping is identical for all the gamuts.
But, personally, I consider the less than perfect appearance match between different gamuts less of an issue compared to following:
Correct. The mapping is not smooth. I’ve tried to bring this up a few times. It seems doing the compression along the constant hue lines results into discontinuities in the final mapping. This is what we should address - somehow.
Here’s those three example DRTs again in same order.
Anyway, these are my thoughts on this particular issue…