1. A method for obtaining, from a retinal image of an eye, data characterizing the cup region of the optic disc of the eye, the method comprising: (a) using a database of N reference retinal images, labelled by an index i=1,. .. N, each reference image being associated with corresponding cup data characterizing a corresponding cup region of the reference image, to obtain, for each reference retinal image i, a corresponding weight value w i , the weight values minimizing a cost function comprising: a construction error term indicating a difference between a portion of the retinal image and a sum of the reference retinal images weighted by the corresponding weight value w i , and a cost term; and (b) using the set of weight values and the corresponding cup data, to obtain the data characterizing the cup region of the optic disc of the retinal image.