1. A computational method of obtaining a similarity value indicative of a similarity between a first retina image and a second retina image, the method comprising: (i) a computer processor deriving a plurality of transform invariant points in the first retina image, each derived point being associated with a transform invariant score; (ii) the computer processor using an over-segmentation algorithm to segment the first retina image into a set of non-overlapping super-pixel regions; (iii) for each super-pixel region, the computer processor selecting a sub-set of said points having the highest transform invariant scores; (iv) the computer processor generating a first graph, the first graph having vertices and edges, the vertices corresponding to respective ones of the selected sub-sets of transform invariant points in the super-pixel regions of the first retina image, and each edge connecting a respective pair of the vertices, each of the vertices being associated with a descriptor vector which is informative about features of a part of the first retina image proximate the corresponding point in the respective super-pixel region of the first retina image; and (v) the computer processor comparing the first graph with a second graph derived from the second retina image, to generate said similarity value.