Some perspectives on visual depth perception

The process which people use to judge visually the relative and absolute distance of objects is quite complex and poorly understood, despite much investigation over a long period of time. However, one thing is clear--it's not simply a matter of triangulation using our two eyes; a variety of other factors seem to be brought into play with varying degrees of influence.Some psychologists believe that a wide variety of separate cues are taken into consideration when estimating distances. In this "constructionist" view, the various cues (which are used quite unconsciously) contribute independently to a mental "construction" process of perceived depth. Other researchers refute the entire concept of detecting depth specifically, preferring instead to incorporate it into a more general, dynamic mapping of the overall perceived scene. This latter viewpoint has become known as the "ecological approach" to visual perception.Many of the concepts are not new. Renaissance artists like Leonardo da Vinci were among the first to realize how depth could be rendered in two-dimensional paintings; they called it "artificial perspective." Since then, a great deal of work has gone into investigating this surprisingly complex topic, although much still remains to be done. However, it seems that some virtual reality developers and other computer graphics practitioners are unfamiliar with the terminology and the various existing concepts and knowledge in the field.The following review describes many of the principal depth cues, as understood by those adopting a constructionist viewpoint; it is followed by a short summary of the ecological approach. I hope this will assist those new to the field, and I offer my sincere apologies to others whose favorite concepts or definitions may have been poorly represented or, worse still, omitted entirely.