A method and apparatus determine when a subject is looking at a specific target area by estimating a
divergence angle between (1) the direction in which the subject is looking and (2) the direction from the subject directly to the target area. This technique accesses whether the subject is looking at a particular area. The invention may further condition this determination according to the subject's distance from the target area, because there is less tolerance for divergent angles when the subject is farther away. In one embodiment, the
divergence angle is estimated using the position of a glint of light in the subject's
pupil. The glint is created by a
light source located in the target area. If the glint is sufficiently central to the
pupil, with the camera and
light source being near the target area, the subject is looking at the target area. At long distances, when the glint is not sufficiently discernable from the
pupil, another technique may be employed to estimate
divergence angle. Namely, the plane of the subject's face is computed, and analyzed with respect to a vector between the subject's face and the target area. If the plane is substantially normal to the vector, the subject is looking at the target area.