Graphics Reference
In-Depth Information
T
HE VISUAL DEBUGGING PRINCIPLE
:
Use visual displays to help you
debug and understand your graphics programs.
In graphics, output from the computer to the user is typically in the form of
light emitted by a display toward the user's eyes. The display might be a con-
ventional flat-panel display, a projector, a head-mounted display, or a heads-up
display for an aircraft pilot or automobile driver. In all cases, the light reaches
us through the eye. The eye's responses to that light are processed by the visual
system.
There are
other
modes of interaction as well, of course: Haptics (touch) and
sound are often used as part of the computer-to-human communication channel.
But the great bulk of the communication is through the visual system, which is
why we concentrate on it. The visual system is powerful in part because light,
which carries information to the visual system, has some special properties that are
not shared by sound, touch, smell, or taste. For instance, light isn't
directionally
diffuse:
A beam of light that starts in some direction travels in that direction only;
it can travel without a supporting medium, and when traveling through air (the
most common medium) it's largely uninfluenced by the air (although variations
in the air's index of refraction as a function of density can distort light—think of
seeing the desert “ripple” on a hot day). By contrast, the chemicals responsible
for smell and taste not only diffuse, but also are advected by moving air, and
sound's direction of propagation can be substantially altered by wind shear. Light
is remarkably good at carrying information from a source to our eyes. Touch, by
contrast, only works when the sensor (e.g., your finger) is collocated with the thing
being observed.
It's tempting to try to reduce the visual system's response to stimuli in various
ways that will make it easier to formulate a model of it. For example, because
the first step in our processing of light is detection by the sensory elements of
the eye, it's tempting to say, “The response of the visual system depends
only
on the incoming light; if you apply the same pattern of light, you get the same
response.” That's wrong, however, at both the physical and mental levels. At the
physical level, seeing a sunny beach after walking out of a dark restaurant, for
instance, causes you to squint your eyes reflexively, while seeing that same beach
after having been outdoors for a few minutes causes no such physical, physiolog-
ical, or psychological reaction. At the mental level, it's been shown that if you've
recently been shown an object, you'll notice another object like it in a jumble of
others more quickly. So, any model of visual processing must depend not only on
the
current
stimuli, but on the recent past as well. More significantly, our pattern
recognition ability is also influenced by training and learning. Once you've learned
to identify a shape, you will recognize it much faster the next time you encounter
it; a good example is the reading of the characters or glyphs that make up text.
Almost every aspect of the visual system is similarly complicated; there seem to
be no easy explanations. On the other hand, there is a wealth of experimental evi-
dence that helps us understand some of what the visual system is doing [Roc95].
In this chapter, we focus on the visual system and how it perceives the world, but
the discussion is necessarily abbreviated; we limit the discussion to the aspects of
the system that are likely to have an impact within graphics systems. The chapter
