Graphics Reference
In-Depth Information
does however, conclude with a few brief remarks on the relationship between the
visual system and other perceptual modes, like hearing and touch.
Each section concludes with a paragraph or two labeled “Applications,” in
which ideas from the section are related to applications in graphics.
The human visual system (see Figure 5.1) consists of the eye (which focuses light
and contains sensors that respond to incoming light), the optic nerve, and parts of
the brain collectively called the
visual cortex.
The exact functioning of the parts
of the visual cortex is not completely known, but it is known that some “early
vision” parts (i.e., those that handle the first few steps in the processing of the
visual signal) detect sharp contrasts in brightness, small changes in orientation
and color, and
spatial frequencies,
that is, the number of alternations between
light and dark per centimeter. We could summarize this by saying that we are
adept at detecting and noting changes in what we might loosely call “patterns.”
The detection of orientation or color or frequency changes is
local,
that is, we
are sensitive to adjacent things having different colors, but small color differences
between things that are far apart in our visual field are not detected by the early-
vision system. Also in the early-vision system are parts that assemble the local
information into slightly larger-scale information (“This little bit of edge here and
this little bit next to it constitute a larger piece of boundary between two regions”).
Later regions of the visual cortex seem to be responsible for detecting motion,
objects (“This is the thing in the foreground; all that other stuff is in the back-
ground”), and shapes, handling “attention,” and providing control of the eye (i.e.,
muscle control to help the eye track an object of interest).
The simplicity suggested by Figure 5.1 is misleading: While there is certainly
a “pipeline” structure to the visual system at a large scale, a wealth of parallel
processing goes on as well, along with substantial feedback from later levels to
earlier ones.
The visual system performs many tasks extremely well, such as determining
size and orientation regardless of your viewpoint or distance, recognizing color
invariantly under a variety of lighting conditions, and recognizing shapes, even
in the presence of noise and distortions. It performs other tasks poorly, such as
determining absolute brightness, recognizing parallel lines, and detecting identi-
cal but nonadjacent colors. And some of these strengths and weaknesses seem
almost contradictory: We're great at noticing a tiny thing that's different from its
surroundings (e.g., a black pebble on white sand), but we're also great at
ignoring
many things that are different from their surroundings, which lets us watch old
films with lots of film grain and scratches and other noise and not be distracted.
It's natural to explain the visual system's particular “talents” on an evolutionary
basis, often based on the ideas that the visual system helps us (a) find food, and (b)
avoid predators.
1
Thus, for example, humans are very sensitive to motion (which
would help one detect predators that are trying to camouflage themselves), but
we're not particularly good at remembering colors from one day to the next. The
visual system is also very good at detecting color similarity under different light-
ing conditions (you want to be able to recognize food both at noon and at dusk, and
Eye
Optic nerve
Low-level
Mid-level
Visual cortex
High-level
Figure 5.1: The components of
the visual system.
1. Mating and obstacle avoidance may also be influences.
