Perception & Cognition
Cues for Visual Space Perception
(Summary 1998, J.S.Longstaff)
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Monocular Focus
(Accommodation)
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- The visual image of an object is focused within a single eye by the bulging or flattening of the lens created by circumferential tension around the lens by the ciliary muscle.
- The more tension in the ciliary muscle to focus on an object, the closer it is perceived.
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Binocular Focus
(Vergence)
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- A visual image is focused between two eyes by convergent or divergent eye movements.
- More muscular tension is required for converging eye movements.
- The more tension to bring an object into binocular focus, the closer it is perceived to be.
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Binocular Disparity
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- Two eyes view the environment from slightly different points, receive different images.
- Two images are integrated into a unitary image in the brain.
- The more disparity between the two images, the closer an object is perceived to be.
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Motion Parallax
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- The closer an object is to the viewer, the faster its visual image will move past and the more its image will change (eg. a new viewpoint) when the observer moves (eg. turning the head or travelling).
- When a person moves, objects whose visual images move farther are perceived as being closer than objects whose visual images move less far.
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Occlusion
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- Objects farther from the observer will disappear from view (occlusion), or appear into view (disocclusion) from behind objects closer to the observer as a result of the objects or the observers motion.
- Proximity of several objects, closest, close, less close etc. is given from occlusions.
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Size / Distance
Relationship
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- An object which is close to the observer will fill a larger visual angle than the same object far from the observer (a fundamental principal to many others)
- When an object gradually fills a larger and larger visual angle, it is perceived as moving directly towards the observer. The bigger it gets, the closer it is.
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Linear Perspective
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- Parallel lines appear to converge with increasing distance from the viewer, thus:
- Lines or surfaces which appear to be parallel are perceived to have a constant distance from the eye.
- lines or surfaces which appear to converge towards the same point or line are perceived to be parallel and moving away from the eye.
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Slope of Regard
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- The visual projective line towards an object (the line of regard) in the lower visual field will be directed more and more steeply downward as the object becomes closer.
- The reverse is true for objects in the upper visual field.
- The analogous effect occurs for objects in the right or left visual field.
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Texture Gradients
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- As a uniform texture becomes further away from the eye, the textures visual image will gradually become more dense.
- Texture can occur on the object or in the environment (untextured object on a textured floor).
- If the visual image of a textured surface is uniform, the surface will be perceived as being at a constant distance from the eye.
- If the visual image of a textured surface gradually becomes more dense, the surface will be perceived as gradually moving away from the eye.
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Velocity Gradients
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- An object travelling at a constant speed far from an observer will move across the visual field slower than an object travelling at that same speed closer to the observer.
- An object travelling on an angle toward the observer at a constant speed will gradually move across the visual field faster and faster.
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Knowledge of
Object Sizes
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- Previous knowledge about the sizes of objects will influence the perception of how close or how far they are perceived to be from the viewer.
- A mouse filling a large visual angle will be perceived as being close, whereas a truck filling that same size visual angle is perceived to be farther away.
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Knowledge of
Object Distances
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- Previous knowledge (or perceptions) of the distance of an object from an observer will influence judgements of how big the object is perceived to actually be.
- When objects are known or perceived to be the same distance from the observer, then objects filling a large visual angle will be perceived to be larger than objects filling a smaller visual angle.
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