With the use of a 3D model of the eye and a pinhole model of the camera based on Swirski’s work in “A fully-automatic, temporal approach to single camera, glint-free 3D eye model fitting" (PETMEI, 2013), you can model the eyeball as a sphere and the pupil as a disk on that sphere. The sphere used is based on an average human eyeball diameter of 24 mm. The state of the model is the position of the sphere in the eye camera space and two rotation vectors that describe the location of the pupil on the sphere.
Comments