ACEScct w/ LogCamera Transform y-intercept Discrepancy

Discussions - Using ACES
1

Hello,
I’m creating a custom OETF using from_scene_reference:!<LogCameraTransform> to recreate the log curve of ACEScct for a linear sRGB scene-linear space to experiment, and noticed a discrepancy-- According to ACEScct Specification - ACES Technical Documentation, the CCT piecewise function is

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, but the linear portion of the piecewise function for when linAP1 <= 0.007 has the form y=mx+b, whereas when using the !<LogCameraTransform> there is only support for the form y=mx with the key linear_slope (where the key lin_side_break is the 0.007 value).

https://opencolorio.readthedocs.io/en/latest/guides/authoring/authoring.html#logcameratransform
https://opencolorio.readthedocs.io/en/latest/api/transforms.html#logcameratransform

This doesn’t make sense to me, as the LogCameraTransform formula says that it is used for ACEScct, but at least according to the acescct specification this isn’t possible unless I’m missing a way to offset the y intercept value by the required ~0.07291 with !<LogCameraTransform>.

2

Good question. The linear offset is automatically derived, so you don’t need to provide it.

See CLF Specification - ACES Technical Documentation, “Solving for LogParams”