Figure A2.

The visual rays aa and bb correspond to fiducial points in the picture. Let the observed slants be sa and sb. Then the depth difference dZ is determined by a unique parabolic arc, here drawn in red. Since AB corresponds to a distance in the picture plane, it is measured in pixels, and, consequently, so is the depth difference dZ. This illustrates the equation used in the text. The point C lies halfway between A and B in the picture plane. This easily shows dZ = ½AB [tan (sa) + tan (sb)], thus fitting the parabolic arc is equivalent to averaging the slopes (tangents of the slant angles).