A contrario patch matching, with an application to keypoint matches validation

We describe a simple metric for image patches similarity, together with a robust criterion for unsupervised patch matching. The gradient orientations at corresponding positions in the two patches are compared and the normalized errors are accumulated. Based on the a contrario framework, the matching criterion validates a match between two patches when this cumulative error is too small to have occurred as the result of an accidental agreement. The method is illustrated in the validation of keypoint matches.