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; (ii) the computer processor selecting a sub-set of said points in the first retina image; (iii) the computer processor generating a first graph, the first graph having vertices and edges, the vertices corresponding to respective ones of the selected points in the first retina image, and each edge connecting respective pair of the vertices, each of the vertices being associated with a descriptor value which is informative about features of a part of the first retina image proximate the corresponding point in the first retina image; and (iv) the computer processor comparing the first graph with a second graph derived from the second retina image, to generate said similarity value.