From: John Murray Subject: SCI: Visual Cues to Depth Perception Organization: University of Michigan Engineering, Ann Arbor At the recent VRAIS'93 conference in Seattle, the issue of depth perception arose in several conversations. The means by which humans determine the relative distances of objects in their field of view is not simply a matter of triangulation using two eyes. Psychologists believe that a variety of cues are taken into consideration; many such cues are used quite unconsciously. Renaissance artists were among the first to realize how depth could be rendered in two-dimensional paintings, so the concepts are not new. However, it appears that several virtual reality practitioners have not yet taken account of some of the existing knowledge in the field. I decided to prepare the following list of cues to assist those who are not familiar with the topic. I'd appreciate any other entries which you feel may be relevant, as well as additional comments or criticisms. I'll re-post the full document upon completion. Regards, John Murray, Center For Ergonomics University of Michigan, Ann Arbor. jxm@engin.umich.edu ++++++ CUES TO DEPTH PERCEPTION ------------------------ LINEAR PERSPECTIVE The convergence of supposedly parallel lines to a distant 'vanishing point' in a scene. More powerful impact can be obtained using several individual vanishing points. DETAIL PERSPECTIVE Patterns and textures of materials and surfaces grow finer and finer with distance from the observer. AERIAL PERSPECTIVE Distant objects such as mountains seem bluish in color, and atmospheric haze makes their contours appear more blurred. RELATIVE SIZE Objects which are assumed to be of similar size (people, vehicles, etc.) look smaller when they are further away. RELATIVE BRIGHTNESS The amount of light received by a viewer, which has been reflected or emitted by an object, is inversely proportional to the square of the viewer's distance from the object. So closer objects appear brighter. INTERPOSITION Things closer to us obscure those which are located further away. GESTALT PRINCIPLES Our tendency to seek organization and closure, recognize patterns, and so on. A drawing of two partially overlapping opaque squares, where one obscures a corner of the other, will be thought of as exactly that - not as an L-shape abutted to the edge of a square. SHADOWS When objects are lit from one direction, the shadows they cast provide information about their orientation and relative position. PLANE HEIGHT We generally view things from above, so objects which are closer to the horizon appear in our field of view to be above those which are nearer to us. MOTION PARALLAX When a viewer moves their head, the image of a distant stationary object sweeps across the retina (the light-sensitive surface at the back of the eye) at a different rate to the image of a closer object. STRUCTURE THROUGH MOTION The perceived contours of a rotating object can appear to change in both length and direction. Also, the rate at which points seem to move depends on whether they're crossing our viewfield, or approaching/receding. These kinetic depth effects give us clues about where the object's edges and surfaces are in relation to each other. BINOCULAR DISPARITY Each eye perceives the scene from a different viewpoint. Thus, the images projected onto the retinas are slightly different. The brain uses assumptions of depth to reconcile the disparity. ACCOMODATION The lens at the front of the eye changes shape to bring something at a specific distance into sharp focus on the retina. Objects situated at other distances become blurred. CONVERGENCE The degree of cross-eyed motion (or 'toe-in') of two eyes, which is required to maintain sight of something travelling towards us. EYE DOMINANCE One eye tends to prevail when we make fine alignments, even though both eyes are open. In cases of confusion, the view from the dominant eye takes precedence in the brain. (It's generally the eye which is habitually used with a camera or telescope.) REFERENCES "Sight and Mind" by Lloyd Kaufman (Oxford Press, 1974). "Engineering Psychology & Human Performance" by Chris Wickens (Harper Collins, 1992).