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Robot Vision


Abstract and Figures

1 Introduction 2 Image Formation & Image Sensing 3 Binary Images: Geometrical Properties 4 Binary Images: Topological Properties 5 Regions & Image Segmentation 6 Image Processing: Continuous Images 7 Image Processing: Discrete Images 8 Edges & Edge Finding 9 Lightness & Color 10 Reflectance Map: Photometric Stereo 11 Reflectance Map: Shape from Shading 12 Motion Field & Optical Flow 13 Photogrammetry & Stereo 14 Pattern Classification 15 Polyhedral Objects 16 Extended Gaussian Images 17 Passive Navigation & Structure from Motion 18 Picking Parts out of a Bin & Hand-Eye Systems
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In this chapter we discuss what a machine vision system is, and what
tasks it is suited for. We also explore the relationship of machine vision
to other fields that provide techniques for processing images or symbolic
descriptions of images. Finally, we introduce the particular view of machine
vision exploited in this text and outline the contents of subsequent chapters.
1.1 Machine Vision
Vision is our most powerful sense. It provides us with a remarkable amount
of information about our surroundings and enables us to interact intelli-
gently with the environment, all without direct physical contact. Through
it we learn the positions and identities of objects and the relationships be-
tween them, and we are at a considerable disadvantage if we are deprived of
this sense. It is no wonder that attempts have been made to give machines
a sense of vision almost since the time that digital computers first became
generally available.
Vision is also our most complicated sense. The knowledge we have ac-
cumulated about how biological vision systems operate is still fragmentary
and confined mostly to the processing stages directly concerned with sig-
nals from the sensors. What we do know is that biological vision systems
2 Introduction
are complex. It is not surprising, then, that many attempts to provide
machines with a sense of vision have ended in failure. Significant progress
has been made nevertheless, and today one can find vision systems that
successfully deal with a variable environment as parts of machines.
Figure 1-1. A machine vision system can make a robot manipulator much more
versatile by allowing it to deal with variations in part position and orientation. In
some cases simple binary image-processing systems are adequate for this purpose.
Most progress has been made in industrial applications, where the vi-
sual environment can be controlled and the task faced by the machine vision
system is clear-cut. A typical example would be a vision system used to
direct a robot arm to pick parts off a conveyor belt (figure 1-1).
Less progress has been made in those areas where computers have been
called upon to extract ill-defined information from images that even people
find hard to interpret. This applies particularly to images derived by other
than the usual optical means in the visual spectrum. A typical example of
such a task is the interpretation of X-rays of the human lung.
It is of the nature of research in a difficult area that some early ideas
have to be abandoned and new concepts introduced as time passes. While
1.2 Tasks for a Machine Vision System 3
frustrating at times, it is part of the excitement of the search for solutions.
Some believed, for example, that understanding the image-formation pro-
cess was not required. Others became too enamored of specific computing
methods of rather narrow utility. No doubt some of the ideas presented
here will also be revised or abandoned in due course. The field is evolving
too rapidly for it to be otherwise.
We cannot at this stage build a “universal” vision system. Instead,
we address ourselves either to systems that perform a particular task in a
controlled environment or to modules that could eventually become part of
a general-purpose system. Naturally, we must also be sensitive to practical
considerations of speed and cost. Because of the enormous volume of data
and the nature of the computations required, it is often difficult to reach a
satisfactory compromise between these factors.
Figure 1-2. The purpose of a machine vision system is to produce a symbolic
description of what is being imaged. This description may then be used to direct
the interaction of a robotic system with its environment. In some sense, the
vision system’s task can be viewed as an inversion of the imaging process.
1.2 Tasks for a Machine Vision System
A machine vision system analyzes images and produces descriptions of what
is imaged (figure 1.2). These descriptions must capture the aspects of the
objects being imaged that are useful in carrying out some task. Thus we
consider the machine vision system as part of a larger entity that interacts
4 Introduction
with the environment. The vision system can be considered an element of
a feedback loop that is concerned with sensing, while other elements are
dedicated to decision making and the implementation of these decisions.
The input to the machine vision system is an image, or several images,
while its output is a description that must satisfy two criteria:
It must bear some relationship to what is being imaged.
It must contain all the information needed for the some given task.
The first criterion ensures that the description depends in some way on the
visual input. The second ensures that the information provided is useful.
An object does not have a unique description; we can conceive of de-
scriptions at many levels of detail and from many points of view. It is
impossible to describe an object completely. Fortunately, we can avoid this
potential philosophical snare by considering the task for which the descrip-
tion is intended. That is, we do not want just any description of what is
imaged, but one that allows us to take appropriate action.
A simple example may help to clarify these ideas. Consider again the
task of picking parts from a conveyor belt. The parts may be randomly
oriented and positioned on the belt. There may be several different types of
parts, with each to be loaded into a different fixture. The vision system is
provided with images of the objects as they are transported past a camera
mounted above the belt. The descriptions that the system has to produce
in this case are simple. It need only give the position, orientation, and
type of each object. The description could be just a few numbers. In other
situations an elaborate symbolic description may be called for.
There are cases where the feedback loop is not closed through a ma-
chine, but the description is provided as output to be interpreted by a
human. The two criteria introduced above must still be satisfied, but it
is harder in this case to determine whether the system was successful in
solving the vision problem presented.
1.3 Relation to Other Fields
Machine vision is closely allied with three fields (figure 1-3):
Image processing.
Pattern classification.
Scene analysis.
1.3 Relation to Other Fields 5
Figure 1-3. Three ancestor paradigms of machine vision are image processing,
pattern classification, and scene analysis. Each contributes useful techniques, but
none is central to the problem of developing symbolic descriptions from images.
Image processing is largely concerned with the generation of new im-
ages from existing images. Most of the techniques used come from linear
systems theory. The new image may have noise suppressed, blurring re-
moved, or edges accentuated. The result is, however, still an image, usually
meant to be interpreted by a person. As we shall see, some of the tech-
niques of image processing are useful for understanding the limitations of
image-forming systems and for designing preprocessing modules for ma-
chine vision.
Pattern classification has as its main thrust the classification of a “pat-
6 Introduction
tern,” usually given as a set of numbers representing measurements of an
object, such as height and weight. Although the input to a classifier is not
an image, the techniques of pattern classification are at times useful for
analyzing the results produced by a machine vision system. To recognize
an object means to assign it to one of a number of known classes. Note,
however, that recognition is only one of many tasks faced by the machine
vision system. Researchers concerned with classification have created sim-
ple methods for obtaining measurements from images. These techniques,
however, usually treat the images as a two-dimensional pattern of bright-
ness and cannot deal with objects presented in an arbitrary attitude.
Figure 1-4. In scene analysis, a low-level symbolic description, such as a line
drawing, is used to develop a high-level symbolic description. The result may
contain information about the spatial relationships between objects, their shapes,
and their identities.
Scene analysis is concerned with the transformation of simple descrip-
tions, obtained directly from images, into more elaborate ones, in a form
more useful for a particular task. A classic illustration of this is the in-
terpretation of line drawings (figure 1-4). Here a description of the image
of a set of polyhedra is given in the form of a collection of line segments.
Before these can be used, we must figure out which regions bounded by
the lines belong together to form objects. We will also want to know how
objects support one another. In this way a complex symbolic description
of the image can be obtained from the simple one. Note that here we do
not start with an image, and thus once again do not address the central
issue of machine vision:
Generating a symbolic description from one or more images.
1.4 Outline of What Is to Come 7
1.4 Outline of What Is to Come
The generation of descriptions from images can often be conveniently bro-
ken down into two stages. The first stage produces a sketch, a detailed but
undigested description. Later stages produce more parsimonious, struc-
tured descriptions suitable for decision making. Processing in the first
stage will be referred to as image analysis, while subsequent processing of
the results will be called scene analysis. The division is somewhat arbi-
trary, except insofar as image analysis starts with an image, while scene
analysis begins with a sketch. The first thirteen chapters of the book are
concerned with image analysis, also referred to as early vision, while the
remaining five chapters are devoted to scene analysis.
The development of methods for machine vision requires some under-
standing of how the data to be processed are generated. For this reason we
start by discussing image formation and image sensing in chapter 2. There
we also treat measurement noise and introduce the concept of convolution.
Figure 1-5. Binary images have only two brightness levels: black and white.
While restricted in application, they are of interest because they are particularly
easy to process.
The easiest images to analyze are those that allow a simple separation
of an “object” from a “background.” These binary images will be treated
first (figure 1-5). Some industrial problems can be tackled by methods that
use such images, but this usually requires careful control of the lighting.
There exists a fairly complete theory of what can and cannot be accom-
plished with binary images. This is in contrast to the more general case of
gray-level images. It is known, for example, that binary image techniques
are useful only when possible changes in the attitude of the object are con-
fined to rotations in a plane parallel to the image plane. Binary image
8 Introduction
processing is covered in chapters 3 and 4.
Many image-analysis techniques are meant to be applied to regions of
an image corresponding to single objects, rather than to the whole image.
Because typically many surfaces in the environment are imaged together,
the image must be divided up into regions corresponding to separate entities
in the environment before such techniques can be applied. The required
segmentation of images is discussed in chapter 5.
In chapters 6 and 7 we consider the transformation of gray-level im-
ages into new gray-level images by means of linear operations. The usual
intent of such manipulations is to reduce noise, accentuate some aspect of
the image, or reduce its dynamic range. Subsequent stages of the machine
vision system may find the processed images easier to analyze. Such filter-
ing methods are often exploited in edge-detection systems as preprocessing
Figure 1-6. In order to use images to recover information about the world, we
need to understand image formation. In some cases the image formation process
can be inverted to extract estimates of the permanent properties of the surfaces
of the objects being imaged.
1.4 Outline of What Is to Come 9
Complementary to image segmentation is edge finding, discussed in
chapter 8. Often the interesting events in a scene, such as a boundary where
one object occludes another, lead to discontinuities in image brightness or
in brightness gradient. Edge-finding techniques locate such features. At
this point, we begin to emphasize the idea that an important aspect of
machine vision is the estimation of properties of the surfaces being imaged.
In chapter 9 the estimation of surface reflectance and color is addressed
and found to be a surprisingly difficult task.
Finally, we confront the central issue of machine vision: the generation
of a description of the world from one or more images. A point of view
that one might espouse is that the purpose of the machine vision system is
to invert the projection operation performed by image formation. This is
not quite correct, since we want not to recover the world being imaged, but
to obtain a symbolic description. Still, this notion leads us to study image
formation carefully (figure 1-6). The way light is reflected from a surface
becomes a central issue. The apparent brightness of a surface depends on
three factors:
Figure 1-7. The appearance of the image of an object is greatly influenced by
the reflectance properties of its surface. Perfectly matte and perfectly specular
surfaces present two extreme cases.
10 Introduction
Figure 1-8. The appearance of the image of a scene depends a lot on the lighting
conditions. To recover information about the world from images we need to un-
derstand how the brightness patterns in the image are determined by the shapes
of surfaces, their reflectance properties, and the distribution of light sources.
1.4 Outline of What Is to Come 11
Microstructure of the surface.
Distribution of the incident light.
Orientation of the surface with respect to the viewer and the light sources.
In figure 1-7 we see images of two spherical surfaces, one covered with a
paint that has a matte or diffuse reflectance, the other metallic, giving rise
to specular reflections. In the second case we see a virtual image of the
world around the spherical object. It is clear that the microstructure of
the surface is important in determining image brightness.
Figure 1-8 shows three views of Place Ville-Marie in Montreal. The
three pictures were taken from the same hotel window, but under different
lighting conditions. Again, we easily recognize that the same objects are
depicted, but there is a tremendous difference in brightness patterns be-
tween the images taken with direct solar illumination and those obtained
under a cloudy sky.
In chapters 10 and 11 we discuss these issues and apply the understand-
ing developed to the recovery of surface shape from one or more images.
Representations for the shape of a surface are also introduced there. In
developing methods for recovering surface shape, we often consider the
surface broken up into tiny patches, each of which can be treated as if it
were planar. Light reflection from such a planar patch is governed by three
angles if it is illuminated by a point source (figure 1-9).
The same systematic approach, based on an analysis of image bright-
ness, is used in chapters 12 and 13 to recover information from time-varying
images and images taken by cameras separated in space. Surface shape,
object motion, and other information can be recovered from images us-
ing the methods developed in these two chapters. The relations between
various coordinate systems, either viewer-centered or object-centered, are
uncovered in the discussion of photogrammetry in chapter 13, along with
an analysis of the binocular stereo problem. In using a machine vision
system to guide a mechanical manipulator, measurements in the camera’s
coordinate system must be transformed into the coordinate system of the
robot arm. This topic naturally fits into the discussion of this chapter also.
At this point, we turn from image analysis to scene analysis. Chapter
14 introduces methods for classifying objects based on feature measure-
ments. Line drawings obtained from images of polyhedral objects are an-
alyzed in chapter 15 in order to recover the spatial relationships between
the objects.
12 Introduction
Figure 1-9. The reflection of light from a point source by a patch of an object’s
surface is governed by three angles: the incident angle i, the emittance angle
e, and the phase angle g. Here N is the direction perpendicular, or normal, to
the surface, S the direction to the light source, and V the direction toward the
The issue of how to represent visually acquired information is of great
importance. In chapter 16 we develop in detail the extended Gaussian
image, a representation for surface shape that is useful in recognition and
allows us to determine the attitude of an object in space. Image sequences
can be exploited to recover the motion of the camera. As a by-product,
we obtain the shapes of the surfaces being imaged. This forms the topic
of chapter 17. (The reader may wonder why this chapter does not directly
follow the one on optical flow. The reason is that it does not deal with image
analysis and so logically belongs in the part of the book dedicated to scene
analysis.) Finally, in chapter 18 we bring together many of the concepts
developed in this book to built a complete hand–eye system. A robot
1.4 Outline of What Is to Come 13
arm is guided to pick up one object after another out of a pile of objects.
Visual input provides the system with information about the positions of
the objects and their attitudes in space. In this chapter we introduce some
new topics, such as methods for representing rotations in three-dimensional
space, and discuss some of the difficulties encountered in building a real-
world system.
Throughout the book we start by discussing elementary issues and well-
established techniques, progress to more advanced topics, and close with
less certain matters and subjects of current research. In the past, machine
vision may have appeared to be a collection of assorted heuristics and ad
hoc tricks. To give the material coherence we maintain a particular point
of view here:
Machine vision should be based on a thorough understanding of image
This emphasis allows us to derive mathematical models of the image-
analysis process. Algorithms for recovering a description of the imaged
world can then be based on these mathematical models.
An approach based on the analysis of image formation is, of course,
not the only one possible for machine vision. One might start instead from
existing biological vision systems. Artificial systems would then be based
on detailed knowledge of natural systems, provided these can be adequately
characterized. We shall occasionally discuss alternate approaches to given
problems in machine vision, but to avoid confusion we will not dwell on
Figure 1-10. In many cases, the development of a symbolic description of a
scene from one or more images can be broken down conveniently into two stages.
The first stage is largely governed by our understanding of the image-formation
process; the second depends more on the needs of the intended application.
14 Introduction
The transformation from image to sketch appears to be governed
mostly by what is in the image and what information we can extract di-
rectly from it (figure 1-10). The transformation from a crude sketch to
a full symbolic description, on the other hand, is mostly governed by the
need to generate information in a form that will be of use in the intended
1.5 References
Each chapter will have a section providing pointers to background reading,
further explanation of the concepts introduced in that chapters, and recent
results in the area. Books will be listed first, complete with authors and
titles. Papers in journals, conference proceedings, and internal reports of
universities and research laboratories are listed after the books, but without
title. Please note that the bibliography has two sections: the first for books,
the second for papers.
There are now numerous books on the subject of machine vision. Of
these, Computer Vision by Ballard & Brown [1982] is remarkable for its
broad coverage. Also notable are Digital Picture Processing by Rosenfeld
& Kak [1982], Computer Image Processing and Recognition by Hall [1979],
and Machine Perception [1982], a short book by Nevatia. A recent addition
is Vision in Man and Machine [1985] by Levine, a book that has a biological
vision point of view and emphasizes applications to biomedical problems.
Many books concentrate on the image-processing side of things, such
as Computer Techniques in Image Processing by Andrews [1970], Digi-
tal Image Processing by Gonzalez & Wintz [1977], and two books dealing
with the processing of images obtained by cameras in space: Digital Image
Processing by Castleman [1979] and Digital Image Processing: A Systems
Approach by Green [1983]. The first few chapters of Digital Picture Pro-
cessing by Rosenfeld & Kak [1982] also provide an excellent introduction
to the subject. The classic reference on image processing is still Pratt’s
encyclopedic Digital Image Processing [1978].
One of the earliest significant books in this field, Pattern Classification
and Scene Analysis by Duda & Hart [1973], contains more on the sub-
ject of pattern classification than one typically needs to know. Artificial
Intelligence by Winston [1984] has an easy-to-read, broad-brush chapter
on machine vision that makes the connection between that subject and
artificial intelligence.
A number of edited books, containing contributions from several re-
searchers in the field, have appeared in the last ten years. Early on there
1.5 References 15
was The Psychology of Computer Vision, edited by Winston [1975], now
out of print. Then came Digital Picture Analysis, edited by Rosenfeld
[1976], and Computer Vision Systems, edited by Hanson & Riseman [1978].
Several papers on machine vision can be found in volume 2 of Artificial In-
telligence: An MIT Perspective, edited by Winston & Brown [1979]. The
collection Structured Computer Vision: Machine Perception through Hier-
archical Computation Structures, edited by Tanimoto & Klinger, was pub-
lished in 1980. Finally there appeared the fine assemblage of papers Image
Understanding 1984, edited by Ullman & Richards [1984].
The papers presented at a number of conferences have also been col-
lected in book form. Gardner was the editor of a book published in 1979
called Machine-aided Image Analysis, 1978. Applications of machine vi-
sion to robotics are explored in Computer Vision and Sensor-Based Robots,
edited by Dodd & Rossol [1979], and in Robot Vision, edited by Pugh [1983].
Stucki edited Advances in Digital Image Processing: Theory, Application,
Implementation [1979], a book containing papers presented at a meeting
organized by IBM. The notes for a course organized by Faugeras appeared
in Fundamentals in Computer Vision [1983].
Because many of the key papers in the field were not easily accessible,
a number of collections have appeared, including three published by IEEE
Press, namely Computer Methods in Image Analysis, edited by Aggarwal,
Duda, & Rosenfeld [1977], Digital Image Processing, edited by Andrews
[1978], and Digital Image Processing for Remote Sensing, edited by Bern-
stein [1978].
The IEEE Computer Society’s publication Computer brought out a
special issue on image processing in August 1977, the Proceedings of the
IEEE devoted the May 1979 issue to pattern recognition and image pro-
cessing, and Computer produced a special issue on machine perception for
industrial applications in May 1980. A special issue (Volume 17) of the
journal Artificial Intelligence was published in book form under the title
Computer Vision, edited by Brady [1981]. The Institute of Electronics and
Communication Engineers of Japan produced a special issue (Volume J68-
D, Number 4) on machine vision work in Japan in April 1985 (in Japanese).
Not much is said in this book about biological vision systems. They
provide us, on the one hand, with reassuring existence proofs and, on the
other, with optical illusions. These startling effects may someday prove
to be keys with which we can unlock the secrets of biological vision sys-
tems. A computational theory of their function is beginning to emerge, to
a great extent due to the pioneering work of a single man, David Marr.
16 Introduction
His approach is documented in the classic book Vision: A Computational
Investigation into the Human Representation and Processing of Visual In-
formation [1982].
Human vision has, of course, always been a subject of intense curios-
ity, and there is a vast literature on the subject. Just a few books will
be mentioned here. Gregory has provided popular accounts of the subject
in Eye and Brain [1966] and The Intelligent Eye [1970]. Three books by
Gibson—The Perception of the Visual World [1950], The Senses Consid-
ered as Perceptual Systems [1966], and The Ecological Approach to Visual
Perception [1979]—are noteworthy for providing a fresh approach to the
problem. Cornsweet’s Visual Perception [1971] and The Psychology of Vi-
sual Perception by Haber & Hershenson [1973] are of interest also. The
work of Julesz has been very influential, particularly in the area of binoc-
ular stereo, as documented in Foundations of Cyclopean Perception [1971].
More recently, in the wonderfully illustrated book Seeing, Frisby [1982] has
been able to show the crosscurrents between work on machine vision and
work on biological vision systems. For another point of view see Perception
by Rock [1984].
Twenty years ago, papers on machine vision were few in number and
scattered widely. Since then a number of journals have become preferred
repositories for new research results. In fact, the journal Computer Graph-
ics and Image Processing, published by Academic Press, had to change
its name to Computer Vision, Graphics and Image Processing (CVGIP)
when it became the standard place to send papers in this field for review.
More recently, a new special-interest group of the Institute of Electrical and
Electronic Engineers (IEEE) started publishing the Transactions on Pat-
tern Analysis and Machine Intelligence (PAMI). Other journals, such as
Artificial Intelligence, published by North-Holland, and Robotics Research,
published by MIT Press, also contain articles on machine vision. There are
several journals devoted to related topics, such as pattern classification.
Some research results first see the light of day at an “Image Under-
standing Workshop” sponsored by the Defense Advanced Research Projects
Agency (DARPA). Proceedings of these workshops are published by Science
Applications Incorporated, McLean, Virginia, and are available through
the Defense Technical Information Center (DTIC) in Alexandria, Virginia.
Many of these papers are later submitted, possibly after revision and ex-
tension, to be reviewed for publication in one of the journals mentioned
The Computer Society of the IEEE organizes annual conferences on
1.5 References 17
Computer Vision and Pattern Recognition (CVPR) and publishes their
proceedings. Also of interest are the proceedings of the biannual Interna-
tional Joint Conference on Artificial Intelligence (IJCAI) and the national
conferences organized by the American Association for Artificial Intelli-
gence (AAAI), usually in the years in between.
The thorough annual surveys by Rosenfeld [1972, 1974, 1975, 1976,
1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984a, 1985] in Computer Vi-
sion, Graphics and Image Processing are extremely valuable and make it
possible to be less than complete in providing references here. The most
recent survey contained 1,252 entries! There have been many analyses of
the state of the field or of particular views of the field. An early survey
of image processing is that of Huang, Schreiber, & Tretiak [1971]. While
not really a survey, the influential paper of Barrow & Tenenbaum [1978]
presents the now prevailing view that machine vision is concerned with the
process of recovering information about the surfaces being imaged. More
recent surveys of machine vision by Marr [1980], Barrow & Tenenbaum
[1981a], Poggio [1984], and Rosenfeld [1984b] are recommended particu-
larly. Another paper that has been influential is that by Binford [1981].
Once past the hurdles of early vision, the representation of information
and the modeling of objects and the physical interaction between them
become important. We touch upon these issues in the later chapters of this
book. For more information see, for example, Brooks [1981] and Binford
There are many papers on the application of machine vision to in-
dustrial problems (although some of the work with the highest payoff is
likely not to have been published in the open literature). Several papers in
Robotics Research: The First International Symposium, edited by Brady
& Paul [1984], deal with this topic. Chin [1982] and Chin & Harlow [1982]
have surveyed the automation of visual inspection.
The inspection of printed circuit boards, both naked and stuffed, is a
topic of great interest, since there are many boards to be inspected and since
it is not a very pleasant job for people, nor one that they are particularly
good at. For examples of work in this area, see Ejiri et al. [1973], Daniels-
son & Kruse [1979], Danielsson [1980], and Hara, Akiyama, & Karasaki
[1983]. There is a similar demand for such techniques in the manufacture
of integrated circuits. Masks are simple black-and-white patterns, and
their inspection has not been too difficult to automate. The inspection of
integrated circuit wafers is another matter; see, for example, Hsieh & Fu
18 Introduction
Machine vision has been used in automated alignment. See Horn
[1975b], Kashioka, Ejiri, & Sakamoto [1976], and Baird [1978] for ex-
amples in semiconductor manufacturing. Industrial robots are regularly
guided using visually obtained information about the position and orienta-
tion of parts. Many such systems use binary image-processing techniques,
although some are more sophisticated. See, for example, Yachida & Tsuji
[1977], Gonzalez & Safabakhsh [1982], and Horn & Ikeuchi [1984]. These
techniques will not find widespread application if the user has to program
each application in a standard programming language. Some attempts have
been made to provide tools specifically suited to the vision applications; see,
for example, Lavin & Lieberman [1982].
Papers on the application of machine vision methods to the vectoriza-
tion of line drawings are mentioned at the end of chapter 4; references on
character recognition may be found at the end of chapter 14.
1.6 Exercises
1-1 Explain in what sense one can consider pattern classification, image pro-
cessing, and scene analysis as “ancestor paradigms” to machine vision. In what
way do the methods from each of these disciplines contribute to machine vision?
In what way are the problems addressed by machine vision different from those
to which these methods apply?
Image Formation & Image Sensing
In this chapter we explore how images are formed and how they are
sensed by a computer. Understanding image formation is a prerequisite for
full understanding of the methods for recovering information from images.
In analyzing the process by which a three-dimensional world is projected
onto a two-dimensional image plane, we uncover the two key questions of
image formation:
What determines where the image of some point will appear?
What determines how bright the image of some surface will be?
The answers to these two questions require knowledge of image projection
and image radiometry, topics that will be discussed in the context of simple
lens systems.
A crucial notion in the study of image formation is that we live in a
very special visual world. It has particular features that make it possi-
ble to recover information about the three-dimensional world from one or
more two-dimensional images. We discuss this issue and point out imag-
ing situations where these special constraint do not apply, and where it is
consequently much harder to extract information from images.
2.1 Two Aspects of Image Formation 19
We also study the basic mechanism of typical image sensors, and how
information in different spectral bands may be obtained and processed.
Following a brief discussion of color, the chapter closes with a discussion of
noise and reviews some concepts from the fields of probability and statis-
tics. This is a convenient point to introduce convolution in one dimension,
an idea that will be exploited later in its two-dimensional generalization.
Readers familiar with these concepts may omit these sections without loss
of continuity. The chapter concludes with a discussion of the need for
quantization of brightness measurements and for tessellations of the image
2.1 Two Aspects of Image Formation
Before we can analyze an image, we must know how it is formed. An image
is a two-dimensional pattern of brightness. How this pattern is produced
in an optical image-forming system is best studied in two parts: first, we
need to find the geometric correspondence between points in the scene and
points in the image; then we must figure out what determines the brightness
at a particular point in the image.
2.1.1 Perspective Projection
Consider an ideal pinhole at a fixed distance in front of an image plane
(figure 2-1). Assume that an enclosure is provided so that only light coming
through the pinhole can reach the image plane. Since light travels along
straight lines, each point in the image corresponds to a particular direction
defined by a ray from that point through the pinhole. Thus we have the
familiar perspective projection.
We define the optical axis, in this simple case, to be the perpendic-
ular from the pinhole to the image plane. Now we can introduce a con-
venient Cartesian coordinate system with the origin at the pinhole and
z-axis aligned with the optical axis and pointing toward the image. With
this choice of orientation, the zcomponents of the coordinates of points
in front of the camera are negative. We use this convention, despite the
drawback, because it gives us a convenient right-hand coordinate system
(with the x-axis to the right and the y-axis upward).
We would like to compute where the image Pof the point Pon some
object in front of the camera will appear (figure 2-1). We assume that no
other object lies on the ray from Pto the pinhole O. Let r=(x, y, z)Tbe
the vector connecting Oto P, and r=(x,y
,f)Tbe the vector connecting
Oto P. (As explained in the appendix, vectors will be denoted by boldface
20 Image Formation & Image Sensing
letters. We commonly deal with column vectors, and so must take the
transpose, indicated by the superscript T, when we want to write them in
terms of the equivalent row vectors.)
Figure 2-1. A pinhole camera produces an image that is a perspective projection
of the world. It is convenient to use a coordinate system in which the xy-plane is
parallel to the image plane, and the origin is at the pinhole O. The z-axis then
lies along the optical axis.
Here fis the distance of the image plane from the pinhole, while x
and yare the coordinates of the point Pin the image plane. The two
vectors rand rare collinear and differ only by a (negative) scale factor.
If the ray connecting Pto Pmakes an angle αwith the optical axis, then
the length of ris just
r=zsec α=(r·ˆz) sec α,
where ˆz is the unit vector along the optical axis. (Remember that zis
negative for a point in front of the camera.)
The length of ris
r=fsec α,
2.1 Two Aspects of Image Formation 21
and so 1
r·ˆz r.
In component form this can be written as
zand y
Sometimes image coordinates are normalized by dividing xand yby f
in order to simplify the projection equations.
Figure 2-2. When the scene depth is small relative to the average distance
from the camera, perspective projection can be approximated by orthographic
projection. In orthographic projection, rays from a point in the scene are traced
parallel to the projection direction until they intercept the image plane.
2.1.2 Orthographic Projection
Suppose we form the image of a plane that lies parallel to the image at
z=z0. Then we can define m, the (lateral) magnification, as the ratio
of the distance between two points measured in the image to the distance
between the corresponding points on the plane. Consider a small interval
(δx, δy, 0)Ton the plane and the corresponding small interval (δxy
22 Image Formation & Image Sensing
in the image. Then
where z0is the distance of the plane from the pinhole. The magnification
is the same for all points in the plane. (Note that m<1, except in the
case of microscopic imaging.)
A small object at an average distance z0will give rise to an image that
is magnified by m, provided that the variation in zover its visible surface
is not significant compared to z0. The area occupied by the image of
an object is proportional to m2. Objects at different distances from the
imaging system will, of course, be imaged with different magnifications.
Let the depth range of a scene be the range of distances of surfaces from
the camera. The magnification is approximately constant when the depth
range of the scene being imaged is small relative to the average distance of
the surfaces from the camera. In this case we can simplify the projection
equations to read
x=mx and y=my,
where m=f/(z0) and z0is the average value of z. Often the scaling
factor mis set to 1 or 1 for convenience. Then we can further simplify
the equations to become
x=xand y=y.
This orthographic projection (figure 2-2), can be modeled by rays parallel
to the optical axis (rather than ones passing through the origin). The
difference between perspective and orthographic projection is small when
the distance to the scene is much larger than the variation in distance
among objects in the scene.
The field of view of an imaging system is the angle of the cone of
directions encompassed by the scene that is being imaged. This cone of
directions clearly has the same shape and size as the cone obtained by
connecting the edge of the image plane to the center of projection. A
“normal” lens has a field of view of perhaps 25by 40.Atelephoto lens
is one that has a long focal length relative to the image size and thus a
narrow field of view. Conversely, a wide-angle lens has a short focal length
relative to the image size and thus a wide field of view. A rough rule of
thumb is that perspective effects are significant when a wide-angle lens is
used, while images obtained using a telephoto lenses tend to approximate
2.2 Brightness 23
orthographic projection. We shall show in exercise 2-11 that this rule is
not exact.
Figure 2-3. (a) Irradiance is the power per unit area falling on a surface. (b)
Radiance is the power emitted per unit area into a cone of directions having unit
solid angle. The term brightness is used informally for both concepts.
2.2 Brightness
The more difficult, and more interesting, question of image formation is
what determines the brightness at a particular point in the image. Bright-
ness is an informal term used to refer to at least two different concepts:
image brightness and scene brightness. In the image, brightness is related
to energy flux incident on the image plane and can be measured in a num-
ber of ways. Here we introduce the term irradiance to replace the informal
term image brightness. Irradiance is the power per unit area (W·m2
watts per square meter) of radiant energy falling on a surface (figure 2-3a).
In the figure, Edenotes the irradiance, while δP is the power of the radiant
energy falling on the infinitesimal surface patch of area δA. The blackening
of a film in a camera, for example, is a function of the irradiance. (As we
24 Image Formation & Image Sensing
shall discuss a little later, the measurement of brightness in the image also
depends on the spectral sensitivity of the sensor.) The irradiance at a par-
ticular point in the image will depend on how much light arrives from the
corresponding object point (the point found by following the ray from the
image point through the pinhole until it meets the surface of an object).
In the scene, brightness is related to the energy flux emitted from a
surface. Different points on the objects in front of the imaging system will
have different brightnesses, depending on how they are illuminated and
how they reflect light. We now introduce the term radiance to substitute
for the informal term scene brightness. Radiance is the power per unit
foreshortened area emitted into a unit solid angle (W·m2·sr1—watts per
square meter per steradian) by a surface (figure 2-3b). In the figure, Lis the
radiance and δ2Pis the power emitted by the infinitesimal surface patch
of area δA into an infinitesimal solid angle δω. The apparent complexity
of the definition of radiance stems from the fact that a surface emits light
into a hemisphere of possible directions, and we obtain a finite amount
only by considering a finite solid angle of these directions. In general the
radiance will vary with the direction from which the object is viewed. We
shall discuss radiometry in detail later, when we introduce the reflectance
We are interested in the radiance of surface patches on objects because
what we measure, image irradiance, turns out to be proportional to scene
radiance, as we show later. The constant of proportionality depends on the
optical system. To gather a finite amount of light in the image plane we
must have an aperture of finite size. The pinhole, introduced in the last
section, must have a nonzero diameter. Our simple analysis of projection
no longer applies, though, since a point in the environment is now imaged
as a small circle. This can be seen by considering the cone of rays passing
through the circular pinhole with its apex at the object point.
We cannot make the pinhole very small for another reason. Because
of the wave nature of light, diffraction occurs at the edge of the pinhole
and the light is spread over the image. As the pinhole is made smaller and
smaller, a larger and larger fraction of the incoming light is deflected far
from the direction of the incoming ray.
2.3 Lenses
In order to avoid the problems associated with pinhole cameras, we now
consider the use of a lens in an image-forming system. An ideal lens pro-
duces the same projection as the pinhole, but also gathers a finite amount
2.3 Lenses 25
of light (figure 2-4). The larger the lens, the larger the solid angle it sub-
tends when seen from the object. Correspondingly it intercepts more of
the light reflected from (or emitted by) the object. The ray through the
center of the lens is undeflected. In a well-focused system the other rays
are deflected to reach the same image point as the central ray.
Figure 2-4. To obtain finite irradiance in the image plane, a lens is used instead
of an ideal pinhole. A perfect lens generates an image that obeys the same
projection equations as that generated by a pinhole, but gathers light from a
finite area as well. A lens produces well-focused images of objects at a particular
distance only.
An ideal lens has the disadvantage that it only brings to focus light
from points at a distance zgiven by the familiar lens equation
where zis the distance of the image plane from the lens and fis the focal
length (figure 2-4). Points at other distances are imaged as little circles.
This can be seen by considering the cone of light rays passing through the
lens with apex at the point where they are correctly focused. The size of
the blur circle can be determined as follows: A point at distance zis
imaged at a point zfrom the lens, where
26 Image Formation & Image Sensing
and so
(zz)= f
If the image plane is situated to receive correctly focused images of objects
at distance z, then points at distance zwill give rise to blur circles of
where dis the diameter of the lens. The depth of field is the range of
distances over which objects are focused “sufficiently well,” in the sense
that the diameter of the blur circle is less than the resolution of the imaging
device. The depth of field depends, of course, on what sensor is used, but
in any case it is clear that the larger the lens aperture, the less the depth
of field. Clearly also, errors in focusing become more serious when a large
aperture is employed.
Figure 2-5. An ideal thick lens provides a reasonable model for most real lenses.
It produces the same perspective projection that an ideal thin lens does, except
for an additional offset, the lens thickness t, along the optical axis. It can be un-
derstood in terms of the principal planes and the nodal points at the intersections
of the principal planes and the optical axis.
Simple ray-tracing rules can help in understanding simple lens combi-
nations. As already mentioned, the ray through the center of the lens is
undeflected. Rays entering the lens parallel to the optical axis converge to
2.3 Lenses 27
a point on the optical axis at a distance equal to the focal length. This fol-
lows from the definition of focal length as the distance from the lens where
the image of an object that is infinitely far away is focused. Conversely,
rays emitted from a point on the optical axis at a distance equal to the focal
length from the lens are deflected to emerge parallel to the optical axis on
the other side of the lens. This follows from the reversibility of rays. At an
interface between media of different refractive indices, the same reflection
and refraction angles apply to light rays traveling in opposite directions.
A simple lens is made by grinding and polishing a glass blank so that
its two surfaces have shapes that are spherical. The optical axis is the line
through the centers of the two spheres. Any such simple lens will have
a number of defects or aberrations. For this reason one usually combines
several simple lenses, carefully lining up their individual optical axes, so as
to make a compound lens with better properties.
A useful model of such a system of lenses is the thick lens (figure 2-5).
One can define two principal planes perpendicular to the optical axis, and
two nodal points where these planes intersect the optical axis. A ray arriv-
ing at the front nodal point leaves the rear nodal point without changing
direction. This defines the projection performed by the lens. The distance
between the two nodal points is the thickness of the lens. A thin lens is
one in which the two nodal points can be considered coincident.
It is theoretically impossible to make a perfect lens. The projection
will never be exactly like that of an ideal pinhole. More important, exact
focusing of all rays cannot be achieved. A variety of aberrations occur. In
a well-designed lens these defects are kept to a minimum, but this becomes
more difficult as the aperture of the lens is increased. Thus there is a
trade-off between light-gathering power and image quality.
A defect of particular interest to us here is called vignetting. Imagine
several circular diaphragms of different diameter, stacked one behind the
other, with their centers on a common line (figure 2-6). When you look
along this common line, the smallest diaphragm will limit your view. As
you move away from the line, some of the other diaphragms will begin to
occlude more, until finally nothing can be seen. Similarly, in a simple lens,
all the rays that enter the front surface of the lens end up being focused
in the image. In a compound lens, some of the rays that pass through
the first lens may be occluded by portions of the second lens, and so on.
This will depend on the inclination of the entering ray with respect to the
optical axis and its distance from the front nodal point. Thus points in
the image away from the optical axis benefit less from the light-gathering
28 Image Formation & Image Sensing
power of the lens than does the point on the optical axis. There is a falloff
in sensitivity with distance from the center of the image.
Figure 2-6. Vignetting is a reduction in light-gathering power with increasing
inclination of light rays with respect to the optical axis. It is caused by apertures
in the lens system occluding part of the beam of light as it passes through the
lens system. Vignetting results in a smooth, but sometimes quite large, falloff in
sensitivity toward the edges of the image region.
Another important consideration is that the aberrations of a lens in-
crease in magnitude as a power of the angle between the incident ray and
the optical axis. Aberrations are classified by their order, that is, the power
of the angle that occurs in this relationship. Points on the optical axis may
be quite well focused, while those in a corner of the image are smeared out.
For this reason, only a limited portion of the image plane is usable. The
magnitude of an aberration defect also increases as a power of the distance
from the optical axis at which a ray passes through the lens. Thus the
image quality can be improved by using only the central portion of a lens.
One reason for introducing diaphragms into a lens system is to im-
prove image quality in a situation where it is not necessary to utilize fully
the light-gathering power of the system. As already mentioned, fixed di-
aphragms ensure that rays entering at a large angle to the optical axis do
not pass through the outer regions of any of the lenses. This improves
image quality in the outer regions of the image, but at the same time
greatly increases vignetting. In most common uses of lenses this is not
an important matter, since people are astonishingly insensitive to smooth
2.4 Our Visual World 29
spatial variations in image brightness. It does matter in machine vision,
however, since we use the measurements of image brightness (irradiance)
to determine the scene brightness (radiance).
2.4 Our Visual World
How can we hope to recover information about the three-dimensional world
using a mere two-dimensional image? It may seem that the available in-
formation is not adequate, even if we take several images. Yet biological
systems interact intelligently with the environment using visual informa-
tion. The puzzle is solved when we consider the special nature of our usual
visual world. We are immersed in a homogeneous transparent medium, and
the objects we look at are typically opaque. Light rays are not refracted
or absorbed in the environment, and we can follow a ray from an image
point through the lens until it reaches some surface. The brightness at
a point in the image depends only on the brightness of the corresponding
surface patch. Surfaces are two-dimensional manifolds, and their shape can
be represented by giving the distance z(x,y
) to the surface as a function
of the image coordinates xand y.
This is to be contrasted with a situation in which we are looking into
a volume occupied by a light-absorbing material of varying density. Here
we may specify the density ρ(x, y, z) of the material as a function of the
coordinates x,y, and z. One or more images provide enough constraint to
recover information about a surface, but not about a volume. In theory,
an infinite number of images is needed to solve the problem of tomography,
that is, to determine the density of the absorbing material.
Conditions of homogeneity and transparency may not always hold ex-
actly. Distant mountains appear changed in color and contrast, while in
deserts we may see mirages. Image analysis based on the assumption that
conditions are as stated may go awry when the assumptions are violated,
and so we can expect that both biological and machine vision systems will
be misled in such situations. Indeed, some optical illusions can be ex-
plained in this way. This does not mean that we should abandon these
additional constraints, for without them the solution of the problem of re-
covering information about the three-dimensional world from images would
be ambiguous.
Our usual visual world is special indeed. Imagine being immersed
instead in a world with varying concentrations of pigments dispersed within
a gelatinous substance. It would not be possible to recover the distributions
of these absorbing substances in three dimensions from one view. There
30 Image Formation & Image Sensing
just would not be enough information. Analogously, single X-ray images
are not useful unless there happens to be sharp contrast between different
materials, like bone and tissue. Otherwise a very large number of views
must be taken and a tomographic reconstruction attempted. It is perhaps
a good thing that we do not possess Superman’s X-ray vision capabilities!
By and large, we shall confine our attention to images formed by con-
ventional optical means. We shall avoid high-magnification microscopic
images, for instance, where many substances are effectively transparent,
or at least translucent. Similarly, images on a very large scale often show
the effects of absorption and refraction in the atmosphere. Interestingly,
other modalities do sometimes provide us with images much like the ones
we are used to. Examples include scanning electron microscopes (SEM)
and synthetic-aperture radar systems (SAR), both of which produce im-
ages that are easy to interpret. So there is some hope of analyzing them
using the methods discussed here.
In view of the importance of surfaces, we might hope that a machine
vision system could be designed to recover the shapes of surfaces given one
or more images. Indeed, there has been some success in this endeavor, as
we shall see in chapter 10, where we discuss the recovery of shape from
shading. Detailed understanding of the imaging process allows us to re-
cover quantitative information from images. The computed shape of a
surface may be used in recognition, inspection, or in planning the path of
a mechanical manipulator.
2.5 Image Sensing
Almost all image sensors depend on the generation of electron–hole pairs
when photons strike a suitable material. This is the basic process in bi-
ological vision as well as photography. Image sensors differ in how they
measure the flux of charged particles. Some devices use an electric field
in a vacuum to separate the electrons from the surface where they are lib-
erated (figure 2-7a). In other devices the electrons are swept through a
depleted zone in a semiconductor (figure 2-7b).
Not all incident photons generate an electron–hole pair. Some pass
right through the sensing layer, some are reflected, and others lose energy
in different ways. Further, not all electrons find their way into the detect-
ing circuit. The ratio of the electron flux to the incident photon flux is
called the quantum efficiency, denoted q(λ). The quantum efficiency de-
pends on the energy of the incident photon and hence on its wavelength λ.
It also depends on the material and the method used to collect the liber-
2.5 Image Sensing 31
ated electrons. Older vacuum devices tend to have coatings with relatively
low quantum efficiency, while solid-state devices are near ideal for some
wavelengths. Photographic film tends to have poor quantum efficiency.
Figure 2-7. Photons striking a suitable surface generate charge carriers that
are collected and measured to determine the irradiance. (a) In the case of a
vacuum device, electrons are liberated from the photocathode and attracted to
the positive anode. (b) In the case of a semiconductor device, electron–hole pairs
are separated by the built-in field to be collected in an external circuit.
2.5.1 Sensing Color
The sensitivity of a device varies with the wavelength of the incident light.
Photons with little energy tend to go right through the material, while
very energetic photons may be stopped before they reach the sensitive
layer. Each material has its characteristic variation of quantum efficiency
with wavelength.
For a small wavelength interval δλ, let the flux of photons with energy
equal to or greater than λ, but less than λ+δλ,beb(λ)δλ. Then the
number of electrons liberated is
32 Image Formation & Image Sensing
If we use sensors with different photosensitive materials, we obtain different
images because their spectral sensitivities are different. This can be helpful
in distinguishing surfaces that have similar gray-levels when imaged with
one sensor, yet give rise to different gray-levels when imaged with a differ-
ent sensor. Another way to achieve this effect is to use the same sensing
material but place filters in front of the camera that selectively absorb dif-
ferent parts of the spectrum. If the transmission of the ith filter is fi(λ),
the effective quantum efficiency of the combination of that filter and the
sensor is fi(λ)q(λ).
How many different filters should we use? The ability to distinguish
among materials grows as more images are taken through more filters.
The measurements are correlated, however, because most surfaces have a
smooth variation of reflectance with wavelength. Typically, little is gained
by using very many filters.
The human visual system uses three types of sensors, called cones,in
daylight conditions. Each of these cone types has a particular spectral
sensitivity, one of them peaking in the long wavelength range, one in the
middle, and one in the short wavelength range of the visible spectrum,
which extends from about 400 nm to about 700 nm. There is considerable
overlap between the sensitivity curves. Machine vision systems often also
use three images obtained through red, green, and blue filters. It should
be pointed out, however, that the results have little to do with human
color sensations unless the spectral response curves happen to be linear
combinations of the human spectral response curves, as discussed below.
One property of a sensing system with a small number of sensor types
having different spectral sensitivities is that many different spectral distri-
butions will produce the same output. The reason is that we do not measure
the spectral distributions themselves, but integrals of their product with
the spectral sensitivity of particular sensor types. The same applies to bio-
logical systems, of course. Colors that appear indistinguishable to a human
observer are said to be metameric. Useful information about the spectral
sensitivities of the human visual system can be gained by systematically
exploring metamers. The results of a large number of color-matching ex-
periments performed by many observers have been averaged and used to
calculate the so-called tristimulus or standard observer curves. These have
been published by the Commission Internationale de l’Eclairage (CIE) and
are shown in figure 2-8. A given spectral distribution is evaluated as fol-
lows: The spectral distribution is multiplied in turn by each of the three
functions x(λ), y(λ), and z(λ). The products are integrated over the visible
2.5 Image Sensing 33
wavelength range. The three results X,Y, and Zare called the tristimulus
values. Two spectral distributions that result in the same values for these
three quantities appear indistinguishable when placed side by side under
controlled conditions. (By the way, the spectral distributions used here are
expressed in terms of energy per unit wavelength interval, not photon flux.)
The actual spectral response curves of the three types of cones cannot
be determined in this way, however. There is some remaining ambiguity.
It is known that the tristimulus curves are fixed linear transforms of these
spectral response curves. The coefficients of the transformation are not
known accurately.
We show in exercise 2-14 that a machine vision system with the same
color-matching properties as the human color vision system must have sen-
sitivities that are linear transforms of the human cone response curves. This
in turn implies that the sensitivities must be linear transforms of the known
standard observer curves. Unfortunately, this rule has rarely been observed
when color-sensing systems were designed in the past. (Note that we are
not addressing the problem of color sensations; we are only interested in
having the machine confuse the same colors as the standard observer.)
2.5.2 Randomness and Noise
It is difficult to make accurate measurements of image brightness. In this
section we discuss the corrupting influence of noise on image sensing. In
order to do this, we need to discuss random variables and the probability
density distribution. We shall also take the opportunity to introduce the
concept of convolution in the one-dimensional case. Later, we shall en-
counter convolution again, applied to two-dimensional images. The reader
familiar with these concepts may want to skip this section.
Measurements are affected by fluctuations in the signal being mea-
sured. If the measurement is repeated, somewhat differing results may be
obtained. Typically, measurements will cluster around the “correct” value.
We can talk of the probability that a measurement will fall within a certain
interval. Roughly speaking, this is the limit of the ratio of the number of
measurements that fall in that interval to the total number of trials, as the
total number of trials tends to infinity. (This definition is not quite ac-
curate, since any particular sequence of experiments may produce results
that do not tend to the expected limit. It is unlikely that they are far off,
however. Indeed, the probability of the limit tending to an answer that is
not the desired one is zero.)
34 Image Formation & Image Sensing
Figure 2-8. The tristimulus curves allow us to predict which spectral distri-
butions will be indistinguishable. A given spectral distribution is multiplied by
each of the functions x(λ), y(λ), and z(λ), in turn, and the products integrated.
In this way we obtain the tristimulus values, X,Y, and Z, that can be used
to characterize the spectral distribution. Spectral distributions that lead to the
same tristimulus values appear the same when placed next to one another.
2.5 Image Sensing 35
Figure 2-9. (a) A histogram indicates how many samples fall into each of a
series of measurement intervals. If more and more samples are gathered, these
intervals can be made smaller and smaller while maintaining the accuracy of the
individual measurements. (b) In the limit the histogram becomes a continuous
function, called the probability distribution.
Now we can define the probability density distribution, denoted p(x).
The probability that a random variable will be equal to or greater than
x, but less than x+δx, tends to p(x)δx as δx tends to zero. (There
is a subtle problem here, since for a given number of trials the number
falling in the interval will tend to zero as the size of the interval tends
to zero. This problem can be sidestepped by considering the cumulative
36 Image Formation & Image Sensing
probability distribution, introduced below.) A probability distribution can
be estimated from a histogram obtained from a finite number of trials
(figure 2-9). From our definition follow two important properties of any
probability distribution p(x):
p(x)0 for all x, and
p(x)dx =1.
Often the probability distribution has a strong peak near the “correct,” or
“expected,” value. We may define the mean accordingly as the center of
area, μ, of this peak, defined by the equation
p(x)dx =
Since the integral of p(x) from minus infinity to plus infinity is one,
The integral on the right is called the first moment of p(x).
Next, to estimate the spread of the peak of p(x), we can take the second
moment about the mean, called the variance:
The square root of the variance, called the standard deviation, is a useful
measure of the width of the distribution.
Another useful concept is the cumulative probability distribution,
which tells us the probability that the random variable will be less than
or equal to x. The probability density distribution is just the derivative of
the cumulative probability distribution. Note that
x→∞ P(x)=1.
One way to improve accuracy is to average several measurements, assuming
that the “noise” in them will be independent and tend to cancel out. To
understand how this works, we need to be able to compute the probability
distribution of a sum of several random variables.
2.5 Image Sensing 37
Suppose that xis a sum of two independent random variables x1and
x2and that p1(x1) and p2(x2) are their probability distributions. How do
we find p(x), the probability distribution of x=x1+x2? Given x2,we
know that x1must lie between xx2and x+δx x2in order for xto lie
between xand x+δx (figure 2-10). The probability that this will happen
is p1(xx2)δx.Nowx2can take on a range of values, and the probability
that it lies in a particular interval x2to x2+δx2is just p2(x2)δx2.To
find the probability that xlies between xand x+δx we must integrate the
product over all x2.Thus
p(x)δx =
p1(xx2)δx p2(x2)dx2,
Figure 2-10. The probability distribution of the sum of two independent ran-
dom variables is the convolution of the probability distributions of the two vari-
ables. This can be shown by integrating the product of the individual probability
distributions over the narrow strip between x1+x2=xand x1+x2=x+δx.
38 Image Formation & Image Sensing
By a similar argument one can show that
in which the roles of x1and x2are reversed. These correspond to two ways
of integrating the product of the probabilities over the narrow diagonal strip
(figure 2-10). In either case, we talk of a convolution of the distributions
p1and p2, written as
We have just shown that convolution is commutative.
We show in exercise 2-16 that the mean of the sum of several random
variables is equal to the sum of the means, and that the variance of the
sum equals the sum of the variances. Thus if we compute the average of
Nindependent measurements,
each of which has mean μand standard deviation σ, the mean of the result
is also μ, while the standard deviation is σ/Nsince the variance of the
sum is 2. Thus we obtain a more accurate result, that is, one less
affected by “noise.” The relative accuracy only improves with the square
root of the number of measurements, however.
A probability distribution that is of great practical interest is the nor-
mal or Gaussian distribution
p(x)= 1
with mean μand standard deviation σ. The noise in many measurement
processes can be modeled well using this distribution.
So far we have been dealing with random variables that can take on
values in a continuous range. Analogous methods apply when the possible
values are in a discrete set. Consider the electrons liberated during a fixed
interval by photons falling on a suitable material. Each such event is inde-
pendent of the others. It can be shown that the probability that exactly n
are liberated in a time interval Tis
2.5 Image Sensing 39
for some m. This is the Poisson distribution. We can calculate the average
number liberated in time Tas follows:
(n1)! =
so the average is just m. We show in exercise 2-18 that the variance is also
m. The standard deviation is thus m, so that the ratio of the standard
deviation to the mean is 1/m. The measurement becomes more accurate
the longer we wait, since more electrons are gathered. Again, the ratio of
the “signal” to the “noise” only improves as the square root of the average
number of electrons collected, however.
To obtain reasonable results, many electrons must be measured. It can
be shown that a Poisson distribution with mean mis almost the same as
a Gaussian distribution with mean mand variance m, provided that mis
large. The Gaussian distribution is often easier to work with. In any case,
to obtain a standard deviation that is one-thousandth of the mean, one
must wait long enough to collect a million electrons. This is a small charge
still, since one electron carries only
e=1.602192 ...×1019 Coulomb.
Even a million electrons have a charge of only about 160 fC (femto-
Coulomb). (The prefix femto- denotes a multiplier of 1015.) It is not
easy to measure such a small charge, since noise is introduced in the mea-
surement process.
The number of electrons liberated from an area δA in time δt is
N=δA δt
where q(λ) is the quantum efficiency and b(λ) is the image irradiance in
photons per unit area. To obtain a usable result, then, electrons must be
collected from a finite image area over a finite amount of time. There is
thus a trade-off between (spatial and temporal) resolution and accuracy.
A measurement of the number of electrons liberated in a small area
during a fixed time interval produces a result that is proportional to the
irradiance (for fixed spectral distribution of incident photons). These mea-
surements are quantized in order to read them into a digital computer.
40 Image Formation & Image Sensing
This is done by analog-to-digital (A/D) conversion. The result is called a
gray-level. Since it is difficult to measure irradiance with great accuracy,
it is reasonable to use a small set of numbers to represent the irradiance
levels. The range 0 to 255 is often employed—requiring just 8 bits per
2.5.3 Quantization of the Image
Because we can only transmit a finite number of measurements to a com-
puter, spatial quantization is also required. It is common to make mea-
surements at the nodes of a square raster or grid of points. The image is
then represented as a rectangular array of integers. To obtain a reason-
able amount of detail we need many measurements. Television frames, for
example, might be quantized into 450 lines of 560 picture cells, sometimes
referred to as pixels.
Each number represents the average irradiance over a small area. We
cannot obtain a measurement at a point, as discussed above, because the
flux of light is proportional to the sensing area. At first this might appear
as a shortcoming, but it turns out to be an advantage. The reason is that
we are trying to use a discrete set of numbers to represent a continuous
distribution of brightness, and the sampling theorem tells us that this can
be done successfully only if the continuous distribution is smooth, that
is, if it does not contain high-frequency components. One way to make a
smooth distribution of brightness is to look at the image through a filter
that averages over small areas.
What is the optimal size of the sampling areas? It turns out that
reasonable results are obtained if the dimensions of the sampling areas are
approximately equal to their spacing. This is fortunate because it allows us
to pack the image plane efficiently with sensing elements. Thus no photons
need be wasted, nor must adjacent sampling areas overlap.
We have some latitude in dividing up the image plane into sensing
areas. So far we have been discussing square areas on a square grid. The
picture cells could equally well be rectangular, resulting in a different res-
olution in the horizontal and vertical directions. Other arrangements are
also possible. Suppose we want to tile the plane with regular polygons. The
tiles should not overlap, yet together they should cover the whole plane.
We shall show in exercise 2-21 that there are exactly three tessellations,
based on triangles, squares, and hexagons (figure 2-11).
It is easy to see how a square sampling pattern is obtained simply by
taking measurements at equal intervals along equally spaced lines in the
2.6 References 41
image. Hexagonal sampling is almost as easy, if odd-numbered lines are
offset by half a sampling interval from even-numbered lines. In television
scanning, the odd-numbered lines are read out after all the even-numbered
lines because of field interlace, and so this scheme is particularly easy to
implement. Hexagons on a triangular grid have certain advantages, which
we shall come to later.
Figure 2-11. The plane can be tiled with three regular polygons: the triangle,
the square, and the hexagon. Image tessellations can be based on these tilings.
The gray-level of a picture cell is the quantized value of the measured power
falling on the corresponding area in the image.
2.6 References
There are many standard references on basic optics, including Principles of
Optics: Electromagnetic Theory of Propagation, Interference and Diffrac-
tion of Light by Born & Wolf [1975], Handbook of Optics, edited by Driscoll
& Vaughan [1978], Applied Optics: A Guide to Optical System Design by
Levi [volume 1, 1968; volume 2, 1980], and the classic Optics by Sears
[1949]. Lens design and aberrations are covered by Kingslake in Lens De-
sign Fundamentals [1978]. Norton discusses the basic workings of a large
variety of sensors in Sensor and Analyzer Handbook [1982]. Barbe edited
Charge-Coupled Devices [1980], a book that includes some information on
the use of CCDs in image sensors.
There is no shortage of books on probability and statistics. One such
is Drake’s Fundamentals of Applied Probability Theory [1967].
Color vision is not treated in detail here, but is mentioned again in
chapter 9 where we discuss the recovery of lightness. For a general discus-
sion of color matching and tristimulus values see the first few chapters of
Color in Business, Science, and Industry by Judd & Wyszeck [1975].
42 Image Formation & Image Sensing
Some issues of color reproduction, including what constitutes an ap-
propriate sensor system, are discussed by Horn [1984a]. Further references
on color vision may be found at the end of chapter 9.
Straight lines in the three-dimensional world are projected as straight
lines into the two-dimensional image. The projections of parallel lines inter-
sect in a vanishing point. This is the point where a line parallel to the given
lines passing through the center of projection intersects the image plane.
In the case of rectangular objects, a great deal of information can be re-
covered from lines in the images and their intersections. See, for example,
Barnard [1983].
When the medium between us and the scene being imaged is not per-
fectly transparent, the interpretation of images becomes more complicated.
See, for example, Sjoberg & Horn [1983]. The reconstruction of absorbing
density in a volume from measured ray attenuation is the subject of to-
mography; a book on this subject has been edited by Herman [1979].
2.7 Exercises
2-1 What is the shape of the image of a sphere? What is the shape of the
image of a circular disk? Assume perspective projection and allow the disk to lie
in a plane that can be tilted with respect to the image plane.
2-2 Show that the image of an ellipse in a plane, not necessarily one parallel to
the image plane, is also an ellipse. Show that the image of a line in space is a line
in the image. Assume perspective projection. Describe the brightness patterns
in the image of a polyhedral object with uniform surface properties.
2-3 Suppose that an image is created by a camera in a certain world. Now
imagine the same camera placed in a similar world in which everything is twice
as large and all distances between objects have also doubled. Compare the new
image with the one formed in the original world. Assume perspective projection.
2-4 Suppose that an image is created by a camera in a certain world. Now
imagine the same camera placed in a similar world in which everything has half
the reflectance and the incident light has been doubled. Compare the new image
with the one formed in the original world. Hint: Ignore interflections, that is,
illumination of one part of the scene by light reflected from another.
2-5 Show that in a properly focused imaging system the distance ffrom the
lens to the image plane equals (1 + m)f, where fis the focal length and mis the
2.7 Exercises 43
magnification. This distance is called the effective focal length. Show that the
distance between the image plane and an object must be
m+2+ 1
How far must the object be from the lens for unit magnification?
2-6 What is the focal length of a compound lens obtained by placing two thin
lenses of focal length f1and f2against one another? Hint: Explain why an object
at a distance f1on one side of the compound lens will be focused at a distance
f2on the other side.
2-7 The f-number of a lens is the ratio of the focal length to the diameter of
the lens. The f-number of a given lens (of fixed focal length) can be increased
by introducing an aperture that intercepts some of the light and thus in effect
reduces the diameter of the lens. Show that image brightness will be inversely
proportional to the square of the f-number. Hint: Consider how much light is
intercepted by the aperture.
Figure 2-12. To determine the focal length and the positions of the principal
planes, a number of measurements are made. Here, an object lying in a plane a
distance xfrom an arbitrary reference point on one side of the lens is properly in
focus in a plane on the other side at a distance yfrom the reference point. The
two principal planes lie at distances aand bon either side of the reference point.
2-8 When a camera is used to obtain metric information about the world, it
is important to have accurate knowledge of the parameters of the lens, including
the focal length and the positions of the principal planes. Suppose that a pattern
in a plane at distance xon one side of the lens is found to be focused best on a
44 Image Formation & Image Sensing
plane at a distance yon the other side of the lens (figure 2-13). The distances
xand yare measured from an arbitrary but fixed point in the lens. How many
paired measurements like this are required to determine the focal length and
the position of the two principal planes? (In practice, of course, more than
the minimum required number of measurements would be taken, and a least-
squares procedure would be adopted. Least-squares methods are discussed in the
Suppose that the arbitrary reference point happens to lie between the two
principal planes and that aand bare the distances of the principal planes from
the reference point (figure 2-7). Note that a+bis the thickness of the lens, as
defined earlier. Show that
(ab +bf +fa)xi(f+b)+yi(f+a)+xiyi=0,
where xiand yiare the measurements obtained in the ith experiment. Suggest
a way to find the unknowns from a set of nonlinear equations like this. Can a
closed-form solution be obtained for f,a,b?
2-9 Here we explore a restricted case of the problem tackled in the previous
exercise. Describe a method for determining the focal length and positions of the
principal planes of a lens from the following three measurements: (a) the position
of a plane on which a scene at infinity on one side of the lens appears in sharp
focus; (b) the position of a plane on which a scene at infinity on the other side of
the lens appears in sharp focus; (c) the positions of two planes, one on each side
of the lens, such that one plane is imaged at unit magnification on the other.
2-10 Here we explore what happens when the image plane is tilted slightly.
Show that in a pinhole camera, tilting the image plane amounts to nothing
more than changing the place where the optical axis pierces the image plane
and changing the perpendicular distance of the projection center from the image
plane. What happens in a camera that uses a lens? Hint: Is a camera with an
(ideal) lens different from a camera with a pinhole as far as image projection is
How would you determine experimentally where the optical axis pierces the
image plane? Hint: It is difficult to find this point accurately.
2-11 It has been stated that perspective effects are significant when a wide-
angle lens is used, while images obtained using a telephoto lenses tend to ap-
proximate orthographic projection. Explain why these are only rough rules of
2-12 Straight lines in the three-dimensional world are projected as straight
lines into the two-dimensional image. The projections of parallel lines intersect
2.7 Exercises 45
in a vanishing point. Where in the image will the vanishing point of a particular
family of parallel lines lie? When does the vanishing point of a family of parallel
lines lie at infinity?
In the case of a rectangular object, a great deal of information can be recov-
ered from lines in the images and their intersections. The edges of a rectangular
solid fall into three sets of parallel lines, and so give rise to three vanishing points.
In technical drawing one speaks of one-point, two-point, and three-point perspec-
tive. These terms apply to the cases in which two, one, or none of three vanishing
points lie at infinity. What alignment between the edges of the rectangular object
and the image plane applies in each case?
2-13 Typically, imaging systems are almost exactly rotationally symmetric
about the optical axis. Thus distortions in the image plane are primarily ra-
dial. When very high precision is required, a lens can be calibrated to determine
its radial distortion. Commonly, a polynomial of the form
is fitted to the experimental data. Here r=x2+y2is the distance of a
point in the image from the place where the optical axis pierces the image plane.
Explain why no even powers of rappear in the polynomial.
2-14 Suppose that a color-sensing system has three types of sensors and that
the spectral sensitivity of each type is a sum of scaled versions of the human cone
sensitivities. Show that two metameric colors will produce identical signals in
the sensors.
Now show that a color-sensing system will have this property for all metamers
only if the spectral sensitivity of each of its three sensor types is a sum of scaled
versions of the human cone sensitivities. Warning: The second part of this prob-
lem is much harder than the first.
2-15 Show that the variance can be calculated as
x2p(x)dx μ2.
2-16 Here we consider the mean and standard deviation of the sum of two
random variables.
(a) Show that the mean of x=x1+x2is the sum μ1+μ2of the means of the
independent random variables x1and x2.
(b) Show that the variance of x=x1+x2is the sum σ2
2of the variances
of the independent random variables x1and x2.
46 Image Formation & Image Sensing
2-17 Suppose that the probability distribution of a random variable is
p(x)=(1/2w),if |x|≤w;
0,if |x|>w.
What is the probability distribution of the average of two independent values
from this distribution?
2-18 Here we consider some properties of the Gaussian and the Poisson distri-
(a) Show that the mean and variance of the Gaussian distribution
p(x)= 1
2πσ e1
are μand σ2respectively.
(b) Show that the mean and the variance of the Poisson distribution
are both equal to m.
2-19 Consider the weighted sum of independent random variables
where xihas mean mand standard deviation σ. Assume that the weights wi
add up to one. What are the mean and standard deviation of the weighted sum?
For fixed N, what choice of weights minimizes the variance?
2-20 A television frame is scanned in 1/30 second. All the even-numbered lines
in one field are followed by all the odd-numbered lines in the other field. Assume
that there are about 450 lines of interest, each to be divided into 560 picture cells.
At what rate must the conversion from analog to digital form occur? (Ignore time
intervals between lines and between successive frames.)
2-21 Show that there are only three regular polygons with which the plane can
be tiled, namely (a) the equilateral triangle, (b) the square, and (c) the hexagon.
(By tiling we mean covering without gaps or overlap.)
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