Two histograms, say H and H', can be compared using a χ ! distance metric (Eq. 3). This permits analysis of the histograms as distributions, compared to others like Euclidean or taxi-cab distances which would treat the histograms as vectors. This value, assuming H and H' are normalized, range between 0 and 1. The approach has been shown to be more robust than comparison of local vector field features . Randomly generated distributions H and H' produce a χ ! value of around 0.18 (Supplementary Notes). This provides a useful reference point from which to compare the similarities of any two distributions.