1. A computer implemented method comprising: obtaining one or more first image regions of a first image of an eye, wherein each of the first image regions includes a view of a respective portion of vasculature of the eye external to a corneal limbus boundary of the eye; until a score exceeds a threshold, progressively increasing a filter resolution and calculating the score based on the filter resolution as follows: applying a plurality of filters having the filter resolution to the first image regions to generate filter output; deriving a plurality of first descriptors from the filter output; and calculating the score by comparing the first descriptors to second descriptors derived from second image regions of a second image of an eye.