1. A computer-implemented method for automatic image registration comprising: providing a previously aligned image pair; extracting a first number of samples of features from the previously aligned image pair; extracting a second number of the samples of features from an observed image pair; determining a Euclidean minimum spanning tree of a union set of the samples of features from the previously aligned image pair and the observed image pair; determining a similarity measure of the observed image pair based on the Euclidean minimum spanning tree; estimating a gradient of the similarity measure, wherein a gradient estimate is used to update transformation parameters applied to register the observed image pair; and outputting a registration of the observed image pair, wherein the registration of the observed image pair is one of displayed and stored on a storage media.