1. A method for tracking motion of an object image, comprising: receiving an input image containing plural subimages; in a computing system, computing based on differences between the input image and a reference image, a first motion vector for a portion of the input image containing the plural subimages; in the computing system, computing a respective subimage motion vector for at least one of the plural subimages of the input image, each subimage motion vector being based on differences between an image data subset of the input image and an image data subset of the reference image, the image data subset of the input image corresponding to the respective subimage of the input image; storing the subimage motion vector on a non-transitory computer readable medium; and outputting, from the computing system, a tracking assessment of object image motion based on the computed first motion vector and the computed at least one subimage motion vector.