Thus, in this context, the changing size serves as a distance cue. The dynamic stimulus change enables the observer not only to see the object as moving, but to perceive the distance of the moving object. Another name for this phenomenon is depth from optical expansion. When an object moves toward the observer, the retinal projection of an object expands over a period of time, which leads to the perception of movement in a line toward the observer. Some animals that lack binocular vision due to their eyes having little common field-of-view employ motion 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 Nearby things pass quickly, while far off objects appear stationary. This effect can be seen clearly when driving in a car. If information about the direction and velocity of movement is known, motion parallax can provide absolute depth information. When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance. Monocular cues include size: distant objects subtend smaller visual angles than near objects, grain, size, and motion parallax. Binocular cues include stereopsis, eye convergence, disparity, and yielding depth from binocular vision through exploitation of parallax. These are typically classified into binocular cues that are based on the receipt of sensory information in three dimensions from both eyes and monocular cues that can be represented in just two dimensions and observed with just one eye. Depth sensation is the corresponding term for animals, since although it is known that animals can sense the distance of an object (because of their ability to move accurately or to respond consistently, according to that distance), it is not known whether they "perceive" it in the same subjective way that humans doĭepth perception arises from a variety of depth cues. Depth perception is the visual ability to perceive the world in three dimensions (3D) and the distance of an object.
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