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To estimate the optical axis, it is sufficient to estimate two points that
belong to it. The easiest points to estimate are the center of curvature of
the cornea, c, and pupil center,
p.
The cornea center, c, is estimated by tracing rays of light from light sources to
the cameras and employing the law of reflection.
The pupil center, p, is estimated by back-projecting, in both cameras, an image
of the pupil center through the nodal point and triangulating these two
imaginary rays in 3-D space..
The estimation algorithm is described in more
details in this paper.
It is important to note, that the estimation of
the optical axis is done without assuming anything about eye dimensions or
distance of the eye from the camera. Therefore, estimation of the optical
axis does not require subject calibration. In addition, free head movements
are tolerated.
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