Given parameters alpha and beta, return an RDD of tiles upon-which the sigmoidal contrast operation has been performed.
Given parameters alpha and beta, return an RDD of tiles upon-which the sigmoidal contrast operation has been performed.
The approach used is described here: https://www.imagemagick.org/Usage/color_mods/#sigmoidal
The center around-which the stretch is performed (given as a fraction)
The standard deviation in the computation, used to avoid saturating the upper and lower parts of the gamut
An RDD of output tiles