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Buchroithner, Manfred 
4 PERCEPTION CUES 
The complex process of human spatial perception and cognition is based on physiological, psychological and 
other parameters. The human brain interprets information about spatial relations of the enviroment encoded in 
the attractions from visual, acoustic and tactile "sensors". From these the visual attractions are most important in 
this process. 
For the visual spatial perception the parameters termed depth cues are basic. One has to distinguish between 
physiological and psychological cues. 
The physiological cues: retinal disparity, convergence, accommodation, motion parallax and chromostereopsis 
are mainly affected by the structure of the human vision system and the relations between observer and objekt. 
Psychological cues are more affected by viewing experience and image contents. They include: retinal image 
size, linear perspective, aerial perspective, overlapping, shading and texture gradient. 
Although their exactly coaction is quite unknown, skilful combinations of selected cues facilitate the creation of 
artifical spatial perception. Only two of them - the retinal disparity and the convergence - are pure binocular 
cues. The other above mentioned cues can be summarised as monocular cues. Using only monocular cues, the 
spatial perception is possible from two-dimensional visualisations and monocular viewing, but in general this 
will cause a weaker depth sensation. 
Retinal disparity is caused by slightly different perspectives between the retinal images of the two eyes. Similar 
to the 3D measurement in stereoscopic photogrammetric models the spatial relations are obtained from the 
disparities of retinal images. 
Convergence is the angle between the viewing axes of the eyes when focusing a point or object. The muscular 
tension for the rotation of eyeballs gives for distances less than 10 meters (Okoshi 1976) informations about the 
distance of the focused object. 
Accomodation is the change of focal length of the crystalline lens in order to focus a point or object. The spent 
muscular tension for focused distances less than 2 meters is a cue for depth perception. Accomodation is closely 
connected to the convergence and effective only in combination with binocular cues. 
Motion parallax is a depth cue caused by the movement of the observer who is viewing a static scene. Objects, 
wich are closer to the observer seem to move faster than those objects, which have a larger distance to the 
viewpoint. This cue have a high significance for the spatial perception of the environment (Albertz 1997). 
Retinal image size of a object is benificial if the real object size is known or similar objects in different distances 
are viewed. The image size has a reciprocal relation to the object distance. 
Linear perspective in retinal images is determined by the optical parameter of the eye. Parallel and convergent 
lines are straightened to one far point and effect a stringently spatial perception (Kraak 1988). Only if the object 
is known or contextural knowledge about the scene is available, the foreshortening caused by the perspective is 
non-ambiguous. 
Aerial perspective is a cue resulting from the diffuse scattering of light on atmospheric particels. The viewing 
experience shows sharp objects with high contrast seem to be closer than unsharp objects with lower contrast. 
This cue is also partial responsible for the psychological effect, that red seems to be closer to the viewer as blue 
(Imhof 1965). The cue mainly intensify other depth cues (Sieber 1996). 
Overlapping has a high importance for the spatial structuring of the retinal image. A object with a closer outline 
is percepted more closer to the viewer than an other. Absolute distances are not derivable (Vlahos 1965). 
Because of the "Gesetz der guten Gestalt" (Law of the Good Form) (Albertz 1997) the cue is clear without 
ambiguity if the object is known or additional cues are available. 
Shading (illumination) summarises three illumination effects: reduced brightness by increasing distance for point 
sources (Vlahos 1965), shadows, cast shadows and chiarosuco (Sieber 1996). Shading is a ambiguous cue. If the 
position of the light source is unknown, for proper depth perception the form of the object have to be known (or 
vice versa). 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 
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