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Color Imaging
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Color Imaging
Fundamentals and Applications
Erik Reinhard
Erum Arif Khan
Ahmet O
˘
guz Aky
¨
uz
Garrett Johnson
A K Peters, Ltd.
Wellesley, Massachusetts
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Contents
Preface xiii
I Principles 1
1 Introduction 3
1.1 Color in Nature . . . 4
1.2 Color in Society . . . 10
1.3 In this Book 12
1.4 Further Reading . . . 14
2 Physics of Light 17
2.1 Electromagnetic Theory 18
2.2 Waves 28
2.3 Polarization 38
2.4 Spectral Irradiance . 45
2.5 Reflection and Refraction . . . 47
2.6 Birefringence 63
2.7 Interference and Diffraction . . 66
2.8 Scattering . 78
2.9 Geometrical Optics . 84
2.10 Application: Image Synthesis . 96
2.11 Application: Modeling the Atmosphere 104
2.12 Summary . 119

4.9 Further Reading . . 250
5 Perception 251
5.1 Lightness, Brightness, and Related Definitions 252
5.2 Reflectance and Illumination 254
5.3 Models of Color Processing . 256
5.4 Visual Illusions . . 259
5.5 Adaptation and Sensitivity . 270
5.6 Visual Acuity . . . 279
5.7 Simultaneous Contrast . . . 282
5.8 Lightness Constancy 286
5.9 Color Constancy . . 295
5.10 Category-Based Processing . 298
5.11 Color Anomalies . 302
5.12 Application: Shadow Removal from Images . 309
5.13 Application: Graphical Design 312
5.14 Application: Telling Humans and Computers Apart . . . . . 314
5.15 Further Reading . . 314
II Color Models 317
6 Radiometry and Photometry 319
6.1 The Sensitivity of the Human Eye . . 320
6.2 Radiometric and Photometric Quantities . . . 322
6.3 The Efficacy of Optical Radiation . . 337
6.4 Luminance, Brightness, and Contrast . 340
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8.12 Application: Rendering 478
8.13 Application: Rendering and Color-Matching Paints 480
8.14 Application: Classification of Edges . . 484
8.15 Further Reading . . . 490
9 Illuminants 491
9.1 CIE Standard Illuminants and Sources . 491
9.2 Color Temperature . 503
9.3 Color-Rendering Index 508
9.4 CIE Metamerism Index 512
9.5 Dominant Wavelength 514
9.6 Excitation Purity . . 517
9.7 Colorimetric Purity . 517
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viii Contents
9.8 Application: Modeling Light-Emitting Diodes 518
9.9 Application: Estimating the Illuminant in an Image . . . . . 520
9.10 Further Reading . . 524
10 Chromatic Adaptation 525
10.1 Changes in Illumination . . . 526
10.2 Measuring Chromatic Adaptation . . . 530
10.3 Mechanisms of Chromatic Adaptation 532
10.4 Models of Chromatic Adaptation . . . 538
10.5 Application: Transforming sRGB Colors to D50 for an ICC

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Contents ix
13 High Dynamic Range Image Capture 709
13.1 Multi-Exposure Techniques . . 710
13.2 Response Curve Recovery . . 715
13.3 Noise Removal . . . 722
13.4 Ghost Removal . . . 726
13.5 Image Alignment . . 733
13.6 Single Capture High Dynamic Range Images . . 734
13.7 Direct High Dynamic Range Capture . . 737
13.8 Application: Drawing Programs 739
13.9 Application: Image-Based Material Editing . . . 740
13.10 Further Reading . . . 741
14 Display Technologies 743
14.1 Cathode-Ray Tubes (CRTs) . . 743
14.2 Liquid Crystal Displays (LCDs) 746
14.3 Transflective Liquid Crystal Displays . . 767
14.4 Plasma Display Panels (PDPs) 768
14.5 Light-Emitting Diode (LED) Displays . 770
14.6 Organic Light-Emitting Diode Displays 772
14.7 Field Emission Displays . . . 775
14.8 Surface-Conduction Electron-Emitter Displays . 776
14.9 Microcavity Plasma Devices . 777
14.10 Interferometric Modulator (IMOD) Displays . . . 777
14.11 Projection Displays . 779

16.3 Practical Applications 874
17 Dynamic Range Reduction 881
17.1 Spatial Operators . 885
17.2 Sigmoidal Compression . . . 888
17.3 Local Neighborhoods 892
17.4 Sub-Band Systems 895
17.5 Edge-Preserving Smoothing Operators 897
17.6 Gradient-Domain Operators . 899
17.7 Histogram Adjustment . . . 900
17.8 Lightness Perception 901
17.9 Counter Shading . 905
17.10 Post-Processing . . 906
17.11 Validation and Comparison . 910
17.12 Further Reading . . 926
IV Appendices 929
A Vectors and Matrices 931
A.1 Cross and Dot Product . . . 931
A.2 Vector Differentiation 933
A.3 Gradient of a Scalar Function 933
A.4 Divergence 934
A.5 Gauss’ Theorem . . 934
A.6 Curl . . . 935
A.7 Stokes’ Theorem . 936
A.8 Laplacian 937
A.9 Vector Identities . . 937
A.10 Homogeneous Coordinates . 937
B Trigonometry 939
B.1 Sum and Difference Formulae 939
B.2 Product Identities . 940
B.3 Double-Angle Formulae . . 941

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Preface
Color is one of the most fascinating areas to study. Color forms an integral part
of nature, and we humans are exposed to it every day. We all have an intuitive
understanding of what color is, but by studying the underlying physics, chemistry,
optics, and human visual perception, the true beauty and complexity of color can
be appreciated—at least to some extent. Such understanding is not just important
in these areas of research, but also for fields such as color reproduction, vision
science, atmospheric modeling, image archiving, art, photography, and the like.
Many of these application areas are served very well by several specifically
targeted books. These books do an excellent job of explaining in detail some as-
pect of color that happens to be most important for the target audience. This is
understandable as our knowledge of color spans many disciplines and can there-
fore be difficult to fathom.
It is our opinion that in application areas of computer science and computer
engineering, including such exciting fields as computer graphics, computer vi-
sion, high dynamic range imaging, image processing and game development, the
role of color is not yet fully appreciated. We have come across several applications
as well as research papers where color is added as an afterthought, and frequently
wrongly too. The dreaded RGB color space, which is really a collection of loosely
similar color spaces, is one of the culprits.
With this book, we hope to give a deep understanding of what color is, and
where color comes from. We also aim to show how color can be used correctly
in many different applications. Where appropriate, we include at the end of each
chapter sections on applications that exploit the material covered. While the book
is primarily aimed at computer-science and computer-engineering related areas,
as mentioned above, it is suitable for any technically minded reader with an in-
terest in color. In addition, the book can also be used as a text book serving a
graduate-level course on color theory. In any case, we believe that to be useful in

can and should record the full dynamic range of the scene, rather than just the
restricted range that can be reproduced on older display devices. In this book,
the vast majority of the photography was done in high dynamic range (HDR),
with each photograph tone-mapped for reproduction on paper. In addition, high
dynamic range imaging (HDRI) is integral to the writing of the text, with excep-
tions only made in specific places to highlight the differences between conven-
tional imaging and HDRI. Thus, the book is as future-proof as we could possibly
make it.
Acknowledgments
Numerous people have contributed to this book with their expertise and help.
In particular, we would like to thank Eric van Stryland, Dean and Director of
CREOL, who has given access to many optics labs, introduced us to his col-
leagues, and allowed us to photograph some of the exciting research undertaken
at the School of Optics, University of Central Florida. Karen Louden, Curator
and Director of Education of the Albin Polasek Museum, Winter Park, Florida,
has given us free access to photograph in the Albin Polasek collection.
We have sourced many images from various researchers. In particular, we
are grateful for the spectacular renderings given to us by Diego Gutierrez and
his colleagues from the University of Zaragoza. The professional photographs
donated by Kirt Witte (Savannah College of Art and Design) grace several pages,
and we gratefully acknowledge his help. Several interesting weather phenomena
were photographed by Timo Kunkel, and he has kindly allowed us to reproduce
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sweeney, Bryant Johnson, and Paul and Linda Johnson.
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Part I
Principles

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Chapter 1
Introduction
Color is a phenomenon that relates to the physics of light, chemistry of matter,
geometric properties of objects, as well as to human visual perception and cog-
nition. We may call sunlight yellow, and in this case we refer to a property of
light. A car may be painted red, in which case the color red is an attribute of the
car. When light enters the eye, a complex chain of events leads to the sensation of
color, a perceived quantity. Finally, color may be remembered, associated with
events, and reasoned about. These are cognitive aspects of color. Color means
different things under different circumstances [814].

netic waves helps to understand why diamonds shimmer, rubies are red, and the
feathers of the blue jay are blue. Even before light enters the eye, a lot has already
happened.
The complexities of color multiply when perception is taken into account. The
human eye is not a simple light detector by any stretch of the imagination. Human
vision is able to solve an inherently under-constrained problem: it tries to make
sense out of a 3D world using optical projections that are two-dimensional. To
reconstruct a three-dimensional world, the human visual system needs to make
a great many assumptions about the structure of the world. It is quite remark-
able how well this system works, given how difficult it is to find a computational
solution that only partially replicates these achievements.
When these assumptions are violated, the human visual system can be fooled
into perceiving the wrong thing. For instance, if a human face is lit from above, it
is instantly recognizable. If the same face is lit from below, it is almost impossi-
ble to determine whose face it is. It can be argued that whenever an assumption is
broken, a visual illusion emerges. Visual illusions are therefore important to learn
about how the human visual system operates. At the same time, they are impor-
tant, for instance in computer graphics, to understand which image features need
to be rendered correctly and which ones can be approximated while maintaining
realism.
Color theory is at the heart of this book. All other topics serve to underpin
the importance of using color correctly in engineering applications. We find that
too often color is taken for granted, and engineering solutions, particularly in
computer graphics and computer vision, therefore appear suboptimal. To redress
the balance, we provide chapters detailing all important issues governing color
and its perception, along with many examples of applications.
We begin this book with a brief assessment of the roles color plays in different
contexts, including nature and society.
1.1 Color in Nature
Living organisms are embedded in an environment with which they interact. To

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6 1. Introduction
Figure 1.2. Many animals are colored similar to their environment to evade predators.
Figure 1.3. This peacock uses bright colors to attract a mate; Paignton Zoo, Devon, UK.
(Photo by Brett Burridge (www.brettb.com).)
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1.1. Color in Nature 7
Figure 1.4. Many plant species grow brightly colored flowers to attract pollinators such
as insects and bees; Rennes, France, June 2005.
Figure 1.5. These beetles have a metallic color, presumably to discourage predators.
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1.1. Color in Nature 9
Figure 1.7. Color plays an important role in art. An example is this photograph of the
Tybee Light House, taken by Kirt Witte, which won the 2006 International Color Award’s
Masters of Color Photography award in the abstract category for professional photogra-
phers (see also www.theothersavannah.com).