Digital Signal Processing Reference
In-Depth Information
Chapter 2
EXISTING TECHNIQUES OF
VISUAL PROCESSING
- Imagine men living in a cave with a long passageway stretching between
them and the cave's mouth, where it opens wide to the light. Imagine further
that since childhood the cave dwellers have had their legs and necks shackled
so as to be confined to the same spot. They are further constrained by blinders
that prevent them from turning their heads; they can see only directly in front
of them. Next, imagine a light from a fire some distance behind them and a
raised path along whose edge there is a low wall like the partition at the front
of a puppet stage. The wall conceals the puppeteers while they manipulate
their puppets above it.
- So far I can visualize it.
- Imagine, further, men behind the wall varying all sorts of objects along its
length and holding them above it. The objects include human and animal
images made of stone and wood and all other material. Presumably, those
who carry them sometimes speak and are sometimes silent.
- You describe a strange prison and strange prisoners.
- Like ourselves.
Plato's Republic, Book VII
Plato's metaphor of the cave is a literal fact to an engineer involved
with video processing. The video sequence is composed of the projec-
tions of physical objects. From only one perspective, we must make sense
of the world from a series of images as a static three dimensional array
of pixel intensities. For such a daunting problem, we are faced with a
volume of visual data that only compounds its difficulty. For instance,
a single image (a color CIF image, RGB channels of 320x200 resolution)
contains millions of points of data; a low resolution color video image
at 60 frames per second is on the order of megabytes per second. To
make sense of all this data, we must conceptually describe the behavior
of the video sequence in a condensed manner via preprocessing. Such
preprocessing is further removed from the ground truth than the raw
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