Optics model of the affine transformation of parallel lines implemented by vision []. The optical axis of the vision points to a distant focus, the fixation point. Straight parallel lines converge at the focus. The focus and its vanishing line are projected on the retina or imaging plane through the vanishing line and point in the retina. The visual system then perceives a distant intersection in the scene of the external world. Figure 2 shows the Cartesian coordinate system in which the Z axis and the optical axis are consistent.