Q.A driver has temporarily lost the use of one eye due to an injury and can now only use monocular vision. Explain any three monocular cues that can still help this driver judge how far away an approaching vehicle or a stationary obstacle is.

Depth perception -- our ability to judge distance and three-dimensionality even though the retinal image itself is flat -- relies on two broad families of cues. Monocular cues are those that remain effective even when an object is viewed with only one eye; because artists frequently use exactly these cues to create a convincing sense of depth on a flat, two-dimensional canvas or page, monocular cues are also referred to as pictorial cues. Several distinct monocular cues have been identified. Relative size draws on the fact that an object's retinal image shrinks as the object moves farther away, so an object that looks smaller (relative to our experience of similar objects) is judged as farther, and one that looks bigger is judged as nearer. Interposition (or overlapping) occurs whenever one object partly covers another -- the object that is partly hidden from view is judged to be the farther of the two. Linear perspective refers to the way that parallel lines receding into the distance, such as railway tracks, appear to converge toward a single vanishing point on the horizon, with greater apparent convergence signalling greater distance. Aerial perspective arises because the air itself contains tiny particles of dust and moisture that make very distant objects look hazy or bluish, while the same objects look sharper and closer when the air is especially clear. Light and shade refers to the highlights and shadows cast across an object's surface by a light source, which supply information about its three-dimensional form and its distance. Relative height uses the general expectation that when two objects are assumed to be the same actual size, the object that looks smaller is judged to be farther away. Texture gradient relies on the fact that a surface covered in a repeating fine texture (such as individual stones, blades of grass, or pebbles) shows a steadily increasing density of that texture as the surface recedes into the distance. Motion parallax, unlike the other cues listed here, is a kinetic (movement-based) cue rather than a purely pictorial one -- it depends on the fact that, as we ourselves move, nearby objects appear to sweep past rapidly in the opposite direction to our own motion, while distant objects appear to drift by slowly in the same direction as our motion. Binocular cues, in contrast, depend on the coord …