Systems and methods for tracking a head position of a user include obtaining digital images of the user's head, processing the images to locate anatomical structures beneath the visible surface, and using those determined locations as inputs to a computing device. In an embodiment, images of a user's face are processed to identify the irises of the eyes, identify pixels along a boundary between each iris and the surrounding sclera, determine an ellipse defined by the identified iris-sclera boundary pixels, determine a distance-to-pixel ratio based on a pixel length of a long axis of such an ellipse compared to a known or presumed diameter of the iris, locating the iris in a three-axis coordinate system, determining an optical axis vector of the eye in the three-axis coordinate system, and calculating a center of the eyeball based on the optical axis vector and a known or presumed eyeball radius.
