Monocular depth cues
Monocular cues provide depth information when viewing a scene with one eye.
Motion parallax – When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance. If information about the direction and velocity of movement is known, motion parallax can provide absolute depth information.[5] This effect can be seen clearly when driving in a car. Nearby things pass quickly, while far off objects appear stationary. Some animals that lack binocular vision due to wide placement of the eyes employ parallax more explicitly than humans for depth cueing (e.g. some types of birds, which bob their heads to achieve motion parallax, and squirrels, which move in lines orthogonal to an object of interest to do the same).[note 1]
Depth from motion – One form of depth from motion, kinetic depth perception, is determined by dynamically changing object size. As objects in motion become smaller, they appear to recede into the distance or move farther away; objects in motion that appear to be getting larger seem to be coming closer. Using kinetic depth perception enables the brain to calculate time to crash distance (aka time to collision or time to contact - TTC) at a particular velocity. When driving, we are constantly judging the dynamically changing headway (TTC) by kinetic depth perception.