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Expect: see the help  at https://docs.gimp.org/2.10/en/script-fu-predator.html

Actual: Gimp message "Error: eval: unbound variable: plug-in-max-rgb "

plug-ins/common/max_rgb.c was removed in commit dd89f493 The Gimp dev wiki plugin roadmap confirms the removal was planned/intended.

I am mainly reporting this to say that: before 3.0 release, either remove Predator or restore Max RGB.

It might be possible to fix Predator filter to not use max_rgb. But I would guess that the max_rgb effect is crucial to Predator.

I suspect that the max_rgb effect could be implemented at the mask/channel/layer level instead of at the pixel level as the old implementation did. (That is, in Python or scheme, without using pixel_rgn.) But I don't understand GEGL ops enough to implement it. If there is an operation to reduce a mask from eight-bits to one, or an operation to ceil or floor a mask from eight-bits to 255 or zero, I think it could be done using difference operations on channels.

I don't have an opinion whether max_rgb is useful, but there is rare, 5 year old discussion of it:

https://www.pyimagesearch.com/2015/09/28/implementing-the-max-rgb-filter-in-opencv/