1. A computer-readable medium storing computer-executable instructions that, when executed on a computer, perform a method to estimate head motion between two images of a face, the method comprising operations of: identifying, on the computer, locations of a plurality of distinct facial features in the two images, the locations corresponding to a number of unknowns determined upon estimation of head motion, the number of unknowns being determined by a number of equations; converting, on the computer, the identified locations into a set of physical face parameters based on the symmetry of the distinct facial features, the set of physical face parameters reducing the number of unknowns as compared to the number of equations used to determine the unknowns; and estimating, on the computer, head motion from identified points in the two images, the identified points being based on the set of physical face parameters.