1. In geometric perspective, all straight lines in space project to straight lines (or points, if end on) in the picture plane. Rotating the line within the plane of projection will not introduce any curvature, just a change in its extent within the line of projection. In the limit when the line is viewed head on, the projected line will contract to a point in the picture plane.

Humans actually view scenes with two eyes, but the straight-line projections for each eye are both straight lines (Fig. 2 L & R). The average, or binocularly-fused, projection is the combination of two straight lines and is therefore also a straight line (Fig. 2B). No curvature is introduced by the geometry of binocular combination.

Fig. 2. Binocular perspective. The different left (L) and right (R) eye views of the three parallel lines projecting onto the picture plane of Fig. 2. The two straight lines from each eye are combined in the brain combine by binocular fusion into the straight lines of fused image, B, whatever their geometric relationships.