Not directly related to the discussion above, but I have been investigating the noticeable blue band in a colour wheel using a 3D JMh plot.
This is a crossfade between the source ACEScg colour wheel and the DRT output plotted in JMh. It would appear that the blue band comes from the way the corner of the hexagonal plane of the source is “folded in” by the gamut compression. The fold does look rather sharp, and I wondered if that was related to the use of threshold = 1 / limit in the gamut compression. However it would appear that it is related to the proximity of the AP1 reach boundary and the limiting gamut boundary around blue. Non reach compression produces a much smoother fold, and “diffuses” the blue band.
Non reach compression means it is not possible to hit that blue primary with a source inside AP1. So this seems undesirable to bake into the DRT. But it does suggest that a JMh based LMT which used a simpler M compression could smooth the output, at the expense of not being able to hit the primaries. Arguably this kind of LMT is built into other renderings, which don’t have the constraint of having to be able to fill the display cube (I don’t think any of the popular ones do) which makes it far easier for them to produce a smoother looking rendering “out of the box”. We could even have this LMT enabled by default, and it need only be switched off by people who needed to hit those corners.