Graphics Reference
In-Depth Information
object meets a surface actually
do
convey something about shape, as shown in
Figure 5.14. Such shadows are also very strong cues in helping us determine object
contact; a drawing without contact shadows can lead us to see objects as “floating
above” a surface rather than resting on it.
Applications.
Although a shadow may be quite faint, and hence not terri-
bly important in the
L
2
difference between two images, the perceptual difference
between the images can be huge. Rendering shadows is essential; getting them
exactly right is not.
(a)
(b)
Figure 5.12: When we truncate
the vertical lines so that the
obscuring strip seems to be a
plane parallel to one containing
the line, the illusion from Fig-
ure 5.11 disappears; the same
effect happens when the strip is
given a texture that indicates this
tilted orientation.
Perception is a huge subject, of which this brief overview only touches on a few
items of particular interest in graphics. There are physiological, mental, and philo-
sophical aspects to the subject; there are also large unexplored areas. Static per-
ception has been given a great deal of attention and study, but the effects of motion
(not only how humans perceive motion, but also what effects motion has on our
perception) are far less understood. Hoffman [Hof00] and Rock [Roc95] both pro-
vide fine overviews, but brain science is advancing at such a rate that you're prob-
ably best advised to look at recent journal articles rather than surveys in topics to
find out the best current thinking on the subject (which will surely change rapidly).
We've described constancy effects, but there are higher-level effects in vision
as well. To some degree, what you see is highly dependent on what you're look-
ing for. Simons and Chabris [SC99] showed that many viewers told to count how
often a basketball is passed by some players fail to notice a person in a gorilla
suit walking through the midst of them. Thus, semantic expectations regulate
perception.
We haven't discussed stereo viewing in detail because it's rather specialized.
In stereo the two eyes are presented with different images which the visual system
must resolve. Typically, differences between the images result in the powerful per-
cept of depth variation across the field of view. Unfortunately, as we mentioned
earlier, the images presented to the eyes by a stereo-graphics system are typically
displayed on flat surfaces that are not very distant from the eye; the adaptation of
the eyes' lenses to focus on this display plane gives a depth signal that is at odds
with the depths that the brain is inferring for the various things in the scene. This
kind of contradictory evidence being presented to the visual system makes it quite
difficult to know what a user will actually perceive. Furthermore, while stereo is
a key cue in depth perception for most people, there are people who lack stereo
(a)
(b)
(c)
Figure 5.13: (a) A ball moves
above a three-sided tray with-
out shadows; its motion is not
strongly determined. (b) and (c)
Shadows force an interpretation
of the motion as being in a ver-
tical or horizontal plane.
Figure 5.14: The appearance of a contact shadow tells us quite a lot about shapes and their
relationship. You perceive the two identically drawn forms quite differently when shown
their nonidentical shadows.
