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590 22. Visual Perception
The human visual system is sufficiently able to determine the absolute size of
most viewed objects; our perception of size is dominated by the the actual physi-
cal size, and we have almost no conscious awareness of the corresponding retinal
size of objects. This is similar to lightness constancy, discussed earlier, in that
our perception is dominated by inferred properties of the world, not the low level
features actually sensed by photoreceptors in the retina. Gregory (1997) describes
a simple example of size constancy. Hold your two hands out in front of you, one
at arms length and the other at half that distance away from you (Figure 22.38(a)).
Your two hands will look almost the same size, even though the retinal sizes differ
by a factor of two. The effect is much less strong if the nearer hand partially oc-
Figure 22.39. Shape
constancy—the table looks
rectangular even though its
shape in the image is an ir-
regular four sided polygon.
cludes the more distant hand, particularly if you close one eye (Figure 22.38(b)).
The visual system also exhibits shape constancy, where the perception of geomet-
ric structure is close to actual object geometry than might be expected given the
distortions of the retinal image due to perspective (Figure 22.39).
22.4.3 Events
Most aspects of event perception are beyond the scope of this chapter, since they
involve complex non-visual cognitive processes. Three types of event perception
are primarily visual, however, and are also of clear relevance to computer graph-
ics. Vision is capable of providing information about how a person is moving in
the world, the existence of independently moving objects in the world, and the
potential for collisions either due to observer motion or due to objects moving
towards the observer.
Vision can be used to determine rotation and the direction of translation rel-
ative to the environment. The simplest case involves movement towards a flat
surface oriented perpendicularly to the line of sight. Presuming that there is suffi-
cient surface texture to enable the recovery of optic flow, the flow field will form
a symmetric pattern as shown in Figure 22.40(a). The location in the field of view
of the focus of expansion of the flow field will have an associated line of sight
corresponding to the direction of translation. While optic flow can be used to vi-
sually determine the direction of motion, it does not contain enough information
to determine speed. To see this, consider the situation in which the world is made
twice as large and the viewer moves twice as fast. The decrease in the magnitude
of flow values due to the doubling of distances is exactly compensated for by the
increase in the magnitude of flow values due to the doubling of velocity, resulting
in an identical flow field.
Figure 22.40(b) shows the optic flow field resulting from the viewer (or more
accurately, the viewer’s eyes) rotating around the vertical axis. Unlike the situa-