2 Chapter 1 ■ Introduction
value. These elements are referred to as picture elements, image elements, pels,
and pixels. Pixel is the term most widely used to denote the elements of a digi-
tal image. We consider these definitions in more formal terms in Chapter 2.
Vision is the most advanced of our senses, so it is not surprising that images
play the single most important role in human perception. However, unlike
humans, who are limited to the visual band of the electromagnetic (EM) spec-
trum, imaging machines cover almost the entire EM spectrum, ranging from
gamma to radio waves. They can operate on images generated by sources that
humans are not accustomed to associating with images. These include ultra-
sound, electron microscopy, and computer-generated images.Thus, digital image
processing encompasses a wide and varied field of applications.
There is no general agreement among authors regarding where image pro-
cessing stops and other related areas, such as image analysis and computer vi-
sion, start. Sometimes a distinction is made by defining image processing as a
discipline in which both the input and output of a process are images.We believe
this to be a limiting and somewhat artificial boundary. For example, under this
definition, even the trivial task of computing the average intensity of an image
(which yields a single number) would not be considered an image processing op-
eration. On the other hand, there are fields such as computer vision whose ul-
timate goal is to use computers to emulate human vision, including learning
and being able to make inferences and take actions based on visual inputs.This
area itself is a branch of artificial intelligence (AI) whose objective is to emu-
late human intelligence.The field of AI is in its earliest stages of infancy in terms
of development, with progress having been much slower than originally antic-
ipated. The area of image analysis (also called image understanding) is in be-
tween image processing and computer vision.
There are no clear-cut boundaries in the continuum from image processing
at one end to computer vision at the other. However, one useful paradigm is
to consider three types of computerized processes in this continuum: low-,
mid-, and high-level processes. Low-level processes involve primitive opera-
tions such as image preprocessing to reduce noise, contrast enhancement, and
image sharpening. A low-level process is characterized by the fact that both
its inputs and outputs are images. Mid-level processing on images involves
tasks such as segmentation (partitioning an image into regions or objects),
description of those objects to reduce them to a form suitable for computer
processing, and classification (recognition) of individual objects. A mid-level
process is characterized by the fact that its inputs generally are images, but its
outputs are attributes extracted from those images (e.g., edges, contours, and
the identity of individual objects). Finally, higher-level processing involves
“making sense” of an ensemble of recognized objects, as in image analysis,
and, at the far end of the continuum, performing the cognitive functions nor-
mally associated with vision.
Based on the preceding comments, we see that a logical place of overlap be-
tween image processing and image analysis is the area of recognition of indi-
vidual regions or objects in an image. Thus, what we call in this book digital
image processing encompasses processes whose inputs and outputs are images