Perception of the location of objects in the environment determines many other perceived characteristics. The example of crossing the road, described in the first chapter, illustrated some of the ways in which the perception of location is necessary before other aspects of a scene, like motion or recognition, can be determined. Our perceptual-motor coordination relies upon localising objects accurately, so that we can step over them, reach for them, avoid them or orient appropriately to them. Perception is a platform for action, and actions take place in a three-dimensional environment. It is necessary, therefore, for spatial perception to share the three-dimensional coordinate system in which behaviour occurs. The aspect of visual perception that will be examined in this chapter is location. Specifying the location of an object requires information for its direction and distance. Direction and distance will be considered separately, although they almost always function in tandem: Information that an object is a given distance away is of little assistance for guiding behaviour if its direction is not also detected, and vice versa.
Any statement about the locations of objects requires the specification of a frame of reference relative to which the objects can be assigned. No better description of such relativities can be given than that by the empiricist philosopher, John Locke (1690/1975, pp. 76-77):
Thus a Company of Chess-men, standing on the same squares of the Chess-board, where we left them, we say are all in the same Place, or unmoved; though, perhaps, the Chess-board hath been in the mean time carried out of one Room into another, because we compared them only to the Parts of the Chess-board, which keep the same distance one with another. The Chess-board, we also say, is in the same Place it was, if it remains in the same part of the Cabin, though, perhaps, the Ship it is in, sails all the while: and the Ship is said to be in the same Place, supposing it kept the same distance with the Parts of the neighbouring Land; though, perhaps, the Earth hath turned round; and so both Chess-man, and Board, and Ship, have every one changed Place in respect of remoter Bodies.