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22.3. Spatial Vision 573
partially because of the different sorts of information available from different vi-
sual cues and partly because of the different purposes to which the information
is put (Klatzky, 1998). Egocentric representations are defined with respect to the
viewer’s body. They can be subdivided into coordinate systems fixed to the eyes,
head, or body. Allocentric representations, also called exocentric representations,
are defined with respect to something external to the viewer. Allocentric frames
of reference can be local to some configuration of objects in the environment or
can be globally defined in terms of distinctive locations, gravity, or geographic
properties.
The distance from the viewer to a particular visible location in the environ-
ment, expressed in an egocentric representation, is often referred to as depth in
the perception literature. Surface orientation can be represented in either egocen-
tric or allocentric coordinates. In egocentric representations of orientation, the
term slant is used to refer to the angle between the line of sight to the point and
the surface normal at the point, while the term tilt refers to the orientation of the
projection of the surface normal onto a plane perpendicular to the line of sight.
Distance and orientation can be expressed in a variety of measurement scales.
Absolute descriptions are specified using a standard that is not part of the sensed
information itself. These can be culturally defined standards (e.g, meters), or
standards relative to the viewer’s body (e.g., eye height, the width of one’s shoul-
ders). Relative descriptions relate one perceived geometric property to another
(e.g., point a is twice as far away as point b). Ordinal descriptions are a special
Cue
a r o
Requirements for absolute depth
Accommodation x x x very limited range
Binocular convergence x x x limited range
Binocular disparity - x x limited range
Linear perspective, height x x x requires viewpoint height
in picture, horizon ratio
Familiar size x x x
Relative size - x x
Aerial perspective ? x x adaptation to local conditions
Absolute motion parallax ? x x requires viewpoint velocity
Relative motion parallax - - x
Texture gradients - x -
Shading - x -
Occlusion - - x
Figure 22.17. Common visual cues for absolute (a), relative (r), and ordinal (o) depth.