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594 22. Visual Perception
istic appearance of color in photographs depends on film color balanced for the
nature of the light source present when the photograph was taken and why real-
istic color in video requires a white-balancing step. While much is known about
how limitations in resolution, brightness, and dynamic range affect the detectabil-
ity of simple patterns, almost nothing is known about how these display properties
affect spatial vision or object identification.
We have a better understanding of other aspects of this problem, which psy-
chologists refer to as the perception of pictorial space (S. Rogers, 1995). One
difference between viewing images and viewing the real world is that accommo-
dation, binocular stereo, motion parallax, and perhaps other depth cues may indi-
cate that the surface under view is much different that the distances in the world
that it is intended to represent. The depths that are seen in such a situation tend
to be somewhere between the depths indicated by the pictorial cues in the image
and the distance to the image itself. When looking at a photograph or computer
display, this often results in a sense of scale smaller than intended. On the other
hand, seeing a movie in a big-screen theater produces a more compelling sense of
spaciousness than does seeing the same movie on television, even if the distance
to the TV is such that the visual angles are the same, since the movie screen is
farther away.
Computer graphics rendered using perspective projection has a viewpoint,
specified as a position and direction in model space, and a view frustum, which
specifies the horizontal and vertical field of view and several other aspects of the
viewing transform. If the rendered image is not viewed from the correct location,
the visual angles to the borders of the image will not match the frustum used in
creating the image. All visual angles within the image will be distorted as well,
causing a distortion in all of the pictorial depth and orientation cues based on
linear perspective. This effect occurs frequently in practice, when a viewer is po-
sitioned either too close or too far away from a photograph or display surface. If
the viewer is too close, the perspective cues for depth will be compressed, and the
cues for surface slant will indicate that the surface is closer to perpendicular to the
line of sight than is actually the case. The situation is reversed if the viewer is too
far from the photograph or screen. The situation is even more complicated if the
line of sight does not go through the center of the viewing area, as is commonly
the case in a wide variety of viewing situations.
The human visual system is able to partially compensate for perspective dis-
tortions arising from viewing an image at the wrong location, which is why we
are able to sit in different seats at a movie theater and experience a similar sense
of the depicted space. When controlling viewing position is particularly impor-
tant, viewing tubes can be used. These are appropriately sized tubes, mounted