matlab object recognition

Phase one: From a rectangular aperture search area, detect the object ( circular object for now, but can be non-circular one). Determine the edge of the object. Add ~2.42 pixels to the edges of the object to form a new search aperture (circular for now) I need to get the coordiates of this new search aperture. (Phase two involves tracking the object motion, which will be described once phase one is completed).