Adaptive Spherical Transformation method

I would like to perform (global) thresholding to apple images so as to find out the defected parts of them. Because of the spherical shape of the apples,the reflectance effect on the sides on the apple is low-vignetting effect. To deal with this, I encountered the term adaptive spherical transformation, in the literature.

I suppose a transformation of coordinates is done and then a correction based on the new coordinates.

Does anyone have any further knowledge on this method or any MATLAB implementation?

Regards, Panos