We talked about this in the meeting, but I’ll try and show what we think is the issue here.
As you point out, at normal exposure, the gradient appears smooth:
But at 1 stop under, the transition starts for form a V shaped discontinuity in brightness:
If we look at the gradient in Yxy space, we can see there are a mix of in and out of gamut colours, with the cyan drifting outside of the 709 boundry:
Post gamut compressor, we can see those values have been pulled back inside the target gamut. Whilst this does seem for for a bit of a hook shape in 2D, this doesn’t seem to be what’s actually causing the issue:
If we look at this 3D visualisation in JMh space, we can see the same values represented a form that represents the pre and post gamut mapped values. Where the white the line is thin, the pre and post gamut mapped values are the same (all in gamut), but where it’s thicker, we can see the connection between the pre and post gamut mapped values.
The out of gamut cyan section is here:
If we look at it directly from the side, the issue becomes clearer.
The further the gamut compressor is pulling the value is, the more it’s boostin the J value, increasing perceptual brightness.
It also explains why the effect is exposure dependant, as the direction of the focus point varies depending on it’s position relative to the gamut cusp.
This is sweep, the outer dotted line is input JMh
The outside of the white form is post tonemapped - post chroma compression JMh
And the inside of the white form is post gamut compressor:
The same sweep can be seen from the top here:
My half considered reading of this is we perhaps need to be a bit more conservative with how far we deviate from flat with the gamut compressor J focal point. We know we need some at the extreme top nd bottom of the range, but perhaps we should be steering closer to flat in the middle.
For example, the top half this image is fully focused on the cusp
Whilst the bottom is fully focused on the SSTS midpoint, producing a much flatter projection.
