Here’s another take on a video showing this. This time it’s showing one hue slice with a horizontal line included so that it’s easier to see where the points along that line end up as J is compressed less with higher nits tonescales. I left the mouse cursor on the spot on the 100 nits curve I was following. I’m switching between 100, 250, 500, 1000 and 4000 nits tonescales, and back.
The M of course doesn’t change, only J changes, and the ratio of those two. Same as with previous video, this shows only the tonescale step. There’s no chroma compression or gamut mapping applied.
The chroma compression creates the appearance match of course, but the problem is that higher nits rendering without chroma compression applied may look less saturated than fully chroma compressed 100 nits rendering for certain colors. So increasing saturation would then be only way in that case to match them, or to lower the saturation of the 100 nits rendering. The tonescaledJ / J ratio can’t recover it (unless we let the ratio to go >1.0, which then would increase the saturation).
Not sure how relevant it is but it’s also clear looking at post-transform display RGB saturation values that they’re getting lower as the tonescale peak is increased (for certain colors). And, that by increasing the saturation, I feel the appearance match gets better.