While developing a new variant of chroma compression that wouldn’t need the saturation boost step, I’m coming to a conclusion that saturation boost is needed in order to be able to create the appearance match between different displays.
The reason is that normal range of colors actually end up less saturated as a result of the differences in the tonescale, namely how much the middle gray is lifted. Following video shows the effect the tonescale has to J and M. This shows the tonescale step alone; there is no chroma compression, no scaling, no roll-off, or gamut mapping happening. The video shows me switching between 100 nits, 1000 nits and 4000 nits tonescales and back. The widest one (most saturated one) is the 100 nits and the narrowest (least saturated one) is the 4000 nits. What the video shows is how higher nits tonescale will result into desaturation in the region shown. Only way to recover the lost saturation is to boost it (reducing chroma compression wouldn’t be enough to compensate. Even disabling the chroma compression wouldn’t be enough for the 4000 nits curve).
In testing I have found that by lowering how much middle gray is lifted would be one way to address this. Another way is to do the saturation boost. And yet another would be to lower the saturation of the 100 nits rendering to create better match.
What the video of course doesn’t show is that how much higher up 1000 and 4000 nits curves extend compared to the 100 nits. But in this case the region of interest is the normal range and middle gray region, and to show how the tonescales affect it.