18 Chapter 1 Introduction
1.1 WHAT IS DIGITAL IMAGE PROCESSING?
An image may be defined as a two-dimensional function, fxy(,), where x and y are
spatial (plane) coordinates, and the amplitude of f at any pair of coordinates (,)xy
is called the intensity or gray level of the image at that point. When x, y, and the
intensity values of f are all finite, discrete quantities, we call the image a digital image.
The field of digital image processing refers to processing digital images by means of
a digital computer. Note that a digital image is composed of a finite number of ele-
ments, each of which has a particular location and value. These elements are called
picture elements, image elements, pels, and pixels. Pixel is the term used most widely
to denote the elements of a digital image. We will 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) spectrum, 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 ultrasound, electron microscopy, and com-
puter-generated images. Thus, digital image processing encompasses a wide and var-
ied field of applications.
There is no general agreement among authors regarding where image process-
ing stops and other related areas, such as image analysis and computer vision, 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 sin-
gle number) would not be considered an image processing operation. On the other
hand, there are fields such as computer vision whose ultimate goal is to use comput-
ers 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 intel-
ligence (AI) whose objective is to emulate 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 anticipated. The area of image analysis (also called image
understanding) is in between 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 con-
sider three types of computerized processes in this continuum: low-, mid-, and high-
level processes. Low-level processes involve primitive operations 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 of 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
1.1
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