Stimuli are seen in contexts. These may allow relations between an object and the scene in general to be come evident. The invariants include relations between parts of the image projected by the object and parts projected by the context of the object. 5.Helmholtz's theory of unconscious inferences argues information is obtained from a variety of sources, and inferences are made about the distal object. The information may come from non-optical sources, the theory contends, such as memory, expectations or eye movements.
Computational theories are programs analyzing the optical stimuli.
They often provide several layers of cells. The angle subtended by an object changes as it changes distance. The intensity of light it projects varies with illumination conditions. This is the problem of perceptual constancy. The visual angle A of an object is the angle it subtends at the eye.