Sub pixel image alignment includes mapping first pixels from a first image and second pixels from a second image to a coordinate system and applying one or more sub-pixel shifts to the mapped first pixels. For each sub-pixel shift, an overall energy is calculated and is based on a plurality of gradients...http://www.google.com.au/patents/US8055101?utm_source=gb-gplus-sharePatent US8055101 - Subpixel registration
