Evaluation of invariant models for dolphin photo-identification

Individual identification of dolphins is of much interest to marine mammologists. Natural markings and notches on the trailing edge of the dorsal fin provide an almost unique signature within a study population for individual recognition. The dorsal fin is an almost rigid, planar object, and, as a result, the problem boils down to planar curve matching. Invariants under three groups of similarity, affine and projective transformations are examined in this paper. Three families of differential, algebraic and semi-differential invariants are studied and evaluated for their applicability and performance in dolphin identification from photographs. It is observed that, under practical conditions, for smooth quasi-rigid curves like the trailing edge of the dorsal fin, curvature-based matching methods outperform almost all of the other techniques which offer a higher level of invariance.