Comparing first and second images taken on different imaging systems and
under different exposure conditions is achieved by the steps of: applying
a model to the data representing the first image, which model simulates
at least one process step in the creation of said first image, to derive
a representation of the first image at a level of processing common to
the creation of the second image; and applying a further model to said
representation, which model simulates at least one process step in the
creation of said second image, to derive a further representation of the
first image which corresponds to the same level of processing as said
second image.