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S.K. Ghosh
Digital Image
Processing
Digital Image
Processing

Alpha Science International Ltd.
Oxford, U.K.
α
S. K. Ghosh

S. K. Ghosh
Department of Civil Engineering
Indian Institute of Technology Roorkee
Roorkee
Copyright © 2013
ALPHA SCIENCE INTERNATIONAL LTD.
7200 The Quorum, Oxford Business Park North
Garsington Road, Oxford OX4 2JZ, U.K.
www.alphasci.com
All rights reserved. No part of this publication may be reproduced, stored
in a retrieval system, or transmitted in any form or by any means, electronic,
mechanical, photocopying, recording or otherwise, without prior written
permission of the publisher.
ISBN 978-1-84265-731-7
Printed in India
Digital Image Processing
314 pgs. | 122 figs. (19 figs. in colour) | 52 tbls.
E-ISBN 978-1-84265-996-0

This book is dedicated
to
my wife Chitrita
and
my children
Netri and Abhishek

Preface
Advent of photographic camera led to the birth of another passion for human
beings, to record all important events and beautiful locations and objects on film
and chemically process to yield photographs on paper. In the initial stages, to
analyze B&W photographs for information required normal human interpretation
skills. Categorization of grey level was limited making the interpretation processing
simple. With the availability of colour films and digital cameras, the interpretation
procedure became more complicated as the number of colours shades or tones
increased many fold. Further, change in media type from film based to digital
form, brought about a new phase in recording and storing information. These
products were known as digital images or simply images. Now it was possible to
analyze for information using digital computer and relevant mathematical based
algorithms; thus, the introduction of Digital Image Processing.
In the present day world, images have started to play an important role.
People like to share images across networks, analyze to extract critical
informations, store the same as reference for future comparisons. One area where
images have started to play a significant role is the use of satellite images for
extraction of information related to natural resources. Satellite data is available
both as single or multi-spectral data. In view of the same, the need for analysis of
digital images has gained prominence. The aim of this book is to provide the
fundamental concepts related to formation of digital image and its analysis for
extraction of information. This book can be divided into two parts.
The first four chapters deals with the fundamentals of image formation, its
characteristics, file format and some of the commercially available software.
Chapter 1 deals the principles of electromagnetic spectrum and its role in image
formation. Chapter 2 deals with the concept of image and its characteristics. In
order to store information in digital form, an image has to be designed in such a
manner that it can be archived and retrieved. Chapter 3 deals with image file
format and provides details of the various file formats that have been used to
store images. Further, it is equally important that the analyst develops customized
analysis procedures or uses commercially developed software. Chapter 4 provides
an overview of some of the commercially available image processing software.
The next part of the book deals with the analysis procedures for analyzing
satellite data. Chapter 5 outlines the need to examine the image quality by computing
both univariate and multivariate statistical parameters. As the electromagnetic
radiation has to pass through the earth's atmosphere, it interacts with the various

viii
viii
viii
viii
viii Preface
constituents of the earth's atmosphere causing overall degradation in the image
quality. Further, the motions of the Earth and the satellite cause a distortion in the
shape of the objects, so Chapter 6 deals with the various pre-processing steps
applied to satellite data. Chapter 7 is devoted to image enhancement technique in
order to improve the quality of the input image caused by background or sensor
quality. In general, it is observed that in case of multi-spectral data, it is beset with
problems of data redundancy. Use of arithmetical operators and transformations
tend to yield a new set of data having better information and the concepts of the
same have been discussed in Chapter 8.
Chapter 9 is devoted to image analysis or classification and is the heart of
the book. It discusses the various statistical based classification techniques and
algorithms and in the end provides details for assessing the accuracy of the
classified images. Finally, Chapter 10 deals various techniques of identifying the
various techniques of spatial filtering. The task of spatial filtering is to emphasize
or de-emphasize information on the basis of its tonal variation or frequencies.
An attempt has also been made to provide an understanding to each of the
processes or techniques by considering a sample satellite data in order to provide
a simple interpretation to the results. The same data has been used for all the tasks
to provide consistency. It may be noted that these interpretations are as per author's
understanding and many vary with person.
At the end, it may be noted that it is the passion of the author and many
years of teaching and research in this area has finally culminated into this book.
The basic idea has been to translate his knowledge and efforts for the benefits of
students, researchers and any person interested in fiddling with digital image.
S. K. Ghosh

Writing a book is a journey through a never ending pool of knowledge. It takes
lots of time to put your thought in a well structured way so that the reader can
understand them. Many times you have a setback when your computer hard disk
crashes or the pen drive containing all the files lands inside the washing machine.
Right throughout this venture of ups and downs, there have been many good
people who have helped me in their own way. A proper acknowledgement to all
these people is appropriate.
The first person to be acknowledged is my wife Chitrita. During difficult
times, her consoling words would motivate me to go back to my laptop and try
and accomplish my task of completing this book. My daughter Netri has been
one source of inspiration while My son Abhishek has been my best critic and
hope that in my future endeavours they play their role to perfection. No amount
of words can express my gratitude for my parents and my late in-laws for being
my constant guide. I would like to also thank my Ph.D supervisor Professor
George Fleming, Department of Civil and Environmental Engineering, University
of Strathclyde, Glasgow, UK, who indirectly inducted me in to this area of digital
image processing.
A teacher’s evaluator is his students. During this period a few hundreds of
students have played their role as the target group. I would like to especially
acknowledge the help extended by Ms. Vandita Srivastava, Scientist, Indian
Institute of Remote Sensing by developing those model makers in ERDAS
IMAGINE for different types of Vegetation Index.
I would like also acknowledge University of Roorkee and now Indian Institute
of Technology, Roorkee for providing the proper environment conducive for
good teaching and research.
Finally, I would like to thank my office assistant Mr. Amit Kumar who
spent hours typing, edit the manuscripts and drawing all the diagrams.
S. K. Ghosh
Acknowledgement

Contents
Preface vii
Acknowledgement ix
List of Figures xix
List of Tables xxiii
1. Concept of Images 1.1
1.1 Introduction 1.1
1.2 Electromagnetic Energy 1.1
1.3 Electromagnetic Spectrum and its Characteristics 1.2
1.4 Utility of EM Radiation in Image Acquisition 1.4
1.5 Image Processing 1.5
1.6 Basic Image Processing Technique 1.5
1.6.1 Image representation and modeling 1.6
1.6.2 Image enhancement 1.7
1.6.3 Image restoration 1.7
1.6.4 Image analysis 1.7
1.6.5 Image reconstruction 1.7
1.6.6 Image data compression 1.7
2. The Process of Imaging 2.1
2.2 Passive Sensors 2.1
2.2.1 Gamma-ray spectrometer 2.1
2.2.2 Aerial camera 2.2
2.2.3 Video camera 2.2
2.2.4 Multi-spectral scanner 2.3
2.2.5 Imaging spectrometer 2.3
2.2.6 Thermal scanner 2.3
2.2.7 Radiometer 2.3
2.3 Active Sensors 2.3
2.3.1 Laser scanner 2.4
2.3.2 Radar altimeter 2.4
2.3.3 Imaging radar 2.4
2.4 Platforms 2.4
2.5 Characteristics of Image 2.5

xii Contents
2.5.1 Sampling 2.5
2.5.2 Spatial resolution 2.6
2.5.3 Sampling pattern 2.7
2.6.3 Quantization 2.8
2.6 Colour Fundamentals 2.9
2.7 Colour Models 2.11
2.7.1 The RGB model 2.12
2.7.2 CMY model 2.14
2.7.3 HSI model 2.14
2.7.4 Conversion of colour from RGB to HSI 2.15
2.7.5 Converting colours from HSI to RGB 2.15
3. Image File Format 3.1
3.1 Storage Media 3.1
3.2 File Formats 3.1
3.3 Common Interchangeable Formats 3.2
3.4 Bitmap (BMP) 3.3
3.5 Tagged Image File Format (TIFF) 3.4
3.5.1 The TIFF file structure 3.4
3.5.2 TIFF data compression 3.9
3.5.3 TIFF classes 3.9
3.6 Graphic Interchange Format (GIF) 3.11
3.7 Joint Photographic Expert Graphic (JPEG) 3.11
3.8 Portable Network Graphics (PNG) 3.12
3.8.1 File structure of PNG 3.15
3.8.1.1 PNG file signature 3.15
3.8.1.2 Chunk layout 3.15
3.8.1.3 Chunks specifications 3.16
3.8.2 Primary Chunks 3.16
3.8.2.1 IHDR chunk 3.16
3.8.2.2 PLTE chunk 3.17
3.8.2.3 IDAT chunk 3.18
3.8.2.4 IEND chunk 3.18
3.8.3 Ancillary chunks 3.18
3.8.3.1 bKGD chunk 3.18
3.8.3.2 cHRM chunk 3.19
3.8.3.3 gAMA chunk 3.19
3.8.3.4 hIST chunk 3.19
3.8.3.5 pHYS chunk 3.19
3.8.3.6 sBIT chunk 3.20
3.8.3.7 tEXT chunk 3.20
3.8.3.8 tIME chunk 3.21
3.8.3.9 tRNS chunk 3.21
3.8.3.10 zTXT chunk 3.21
3.8.4 Summary of standard chunks 3.22
3.9 Shape File 3.22
3.9.1 The main file header 3.24

Contents xiii
3.9.1.1 Record headers 3.25
3.9.2 Index file 3.25
3.9.3 dBASE file 3.26
3.9.4 Description of main file record contents 3.26
3.9.4.1 Null shapes 3.26
3.9.4.2 Point 3.27
3.9.4.3 Multi point 3.27
3.9.4.4 PolyLine 3.27
3.9.4.5 Polygon 3.27
3.9.4.6 PointM 3.28
3.9.4.7 MultiPointM 3.28
3.9.4.8 PolyLineM 3.28
3.9.4.9 PolygonM 3.28
3.9.4.10 PointZ 3.28
3.9.4.11 MultiPointZ 3.28
3.9.4.12 PolyLineZ 3.29
3.9.4.12 PolyLineZ 3.29
3.9.4.14 MultiPatch 3.29
3.10 Satellite Tape Formats 3.30
4. Image Processing Software 4.1
4.2 ERDAS Imagine 4.2
4.2.1 Imagine essential 4.3
4.2.1.1 Data types and integration 4.3
4.2.1.2 Data visualization 4.3
4.2.1.3 Geometric correction 4.4
4.2.1.4 Simple classification 4.4
4.2.1.5 Map composer 4.4
4.2.1.6 General tools and utilities 4.4
4.2.2 IMAGINE Advantage 4.4
4.2.2.1 Ortho correction 4.5
4.2.2.2 Metric accuracy assessment (MAA) tools 4.5
4.2.2.3 Mosaicking 4.5
4.2.2.4 Image processing 4.5
4.2.2.5 Modeling language 4.5
4.2.2.6 Knowledge classifier 4.6
4.2.3 IMAGINE Professional 4.6
4.2.3.1 Spectral analysis 4.6
4.2.3.2 Expert classifier 4.6
4.2.3.3 Multispectral classifier 4.7
4.2.3.4 Radar interpreter 4.7
4.2.4 System specifications of ERDAS IMAGINE 9.0 4.8
4.3 ENVI 4.9
4.3.1 Generally review of ENVI functionality 4.9
4.3.2 Advantages of ENVI 4.11
4.4 IDRISI 4.12

xiv Contents
4.4.1 IDRISI system overview 4.13
4.4.2 Image processing menu 4.14
4.4.2.1 Restoration submenu 4.14
4.4.2.2 Enhancement submenu 4.15
4.4.3.2 Transformation submenu 4.15
4.4.3.4 Fourier analysis submenu 4.16
4.4.3.5 Signature development submenu 4.16
4.4.3.6 Hard classifiers submenu 4.17
4.4.3.7 Soft classifiers / mixture analysis submenu 4.19
4.4.3.8 Hyperspectral image analysis submenu 4.20
4.4.3.9 Accuracy assessment submenu 4.21
4.5 ER Mapper 4.21
4.5.1 Algorithms 4.21
4.5.1.1 View and enhance raster data 4.22
4.5.1.2 Filters 4.22
4.5.1.3 Contrast stretches (Transforms) 4.22
4.5.1.4 Formulae and statistics 4.22
4.5.1.5 View and edit vector data 4.22
4.5.1.6 Integrate data 4.23
4.5.1.7 Raster translators 4.23
4.5.1.8 Automatic data fusion and mosaicing 4.23
4.5.1.9 Hardcopy including stereo pair generation 4.23
4.5.1.10 Map composition 4.23
4.5.1.11 Classify raster images 4.24
4.5.1.12 Visualize in 3-D 4.24
4.5.1.13 Traverse 4.24
4.5.1.14 Application based toolbars and batch scripts 4.24
4.5.1.15 Raster to vector polygon conversion 4.24
4.5.1.16 Geocoding 4.24
4.5.1.17 Gridding 4.24
4.5.1.18 Fourier transformation 4.24
4.5.1.19 Customizable functions 4.25
4.5.2 Virtual datasets 4.25
4.5.3 Compression wizard 4.25
4.6 Concluding Remarks 4.25
5. Initial Statistics 5.1
5.2 Univariate Statistics 5.1
5.2.1 Histogram 5.2
5.2.2 Cumulative histogram 5.3
5.2.3 Minimum and maximum value 5.4
5.2.4 Mean and standard deviation 5.4
5.2.5 Median 5.5
5.2.6 Mode 5.5
5.2.7 Skewness 5.6
5.2.8 Kurtosis 5.6

Contents xv
5.3 Multivariate Image Statistics 5.7
5.3.1 Scatterplot 5.8
5.3.2 Covariance matrix 5.8
5.3.3 Correlation 5.9
5.4 Illustrative Example 5.10
5.4.1 Discussion on univariate statistics 5.10
5.4.2 Multi - variate statistics 5.11
5.4.3 Concluding remarks 5.14
6. Pre-Processing of Data 6.1
6.2 Radiometric Corrections 6.2
6.2.1 Missing scan lines 6.2
6.2.2 De-striping methods 6.3
6.3 Atmospheric Correction Methods 6.4
6.4 Geometric Correction and Registration 6.6
6.4.1 Orbital geometry model 6.8
6.4.1.1 Aspect ratio 6.8
6.4.1.2 Skew correction 6.8
6.4.1.3 Earth rotation correction 6.9
6.4.2 Transformation based on ground control points 6.11
6.5 Resampling 6.12
7. Enhancement Techniques 7.1
7.2 Contrast Stretch or Enhancement 7.1
7.3 Linear Enhancement 7.2
7.3.1 Min-Max stretch 7.2
7.3.2 Percentile stretching 7.4
7.3.3 Piece wise linear stretch 7.4
7.4 Non Linear Enhancement 7.5
7.4.1 Histogram equalization 7.5
7.4.2 Gaussian equalization 7.6
7.4.3 Logarithmic contrast enhancement 7.7
7.4.4 Exponential contrast enhancement 7.7
7.5 Comparison of Enhancement Method 7.7
8. Image Transformations 8.1
8.1 Basic Arithmetic Operators 8.1
8.1.1 Image addition 8.1
8.1.2 Image subtraction 8.2
8.1.3 Image multiplication 8.3
8.1.4 Image division 8.3
8.2 Vegetation Indices 8.4
8.3 Classification of Vegetation Indices 8.5

xvi Contents
8.4 The Slope-based Vegetation Index 8.6
8.4.1 Ratio vegetation index (RATIO) 8.6
8.4.2 The normalized difference vegetation index (NDVI) 8.7
8.4.3 The transformed vegetation index (TVI) 8.7
8.4.4 The corrected transformed vegetation index (CTVI) 8.8
8.4.5 Thiams transformed vegetation index (TTVI) 8.8
8.4.6 Ratio vegetation index (RVI) 8.8
8.4.7 The normalized ratio vegetation index (NRVI) 8.8
8.4.8 Other indices 8.9
8.5 The Distance-based Vegetation Index 8.9
8.5.1 The perpendicular vegetation index (PVI) 8.10
8.5.2 Difference vegetation index (DVI) 8.12
8.5.3 The Ashburn vegetation index (AVI) 8.12
8.5.4 The weighted difference vegetation index (WDVI) 8.13
8.5.5 The soil-adjusted vegetation index (SAVI) 8.13
8.5.6 The modified soil-adjusted vegetation indices
(MSAVI1 and MSAVI2) 8.14
8.5.7 Atmospherically resistant vegetation index (ARVI) 8.17
8.5.8 Soil and atmospherically resistant
vegetation index (SARVI) 8.18
8.5.9 Enhanced vegetation Index 8.19
8.6 Special Indices 8.19
8.6.1 Normalized difference water index 8.19
8.6.2 Normalized difference snow index 8.24
8.6.3 Normalized burn ratio 8.28
8.7 The Orthogonal Transformations 8.30
8.7.1 Principal component analysis 8.30
8.7.2 Tasseled-cap components 8.34
8.7.5 The concept of n-space indices 8.36
8.7.5.1 Calculation of n-space coefficients 8.37
8.8.1 Ratio and NDVI images 8.40
8.8.2 Vegetation indices 8.41
8.8.2.1 Transformed vegetation index 8.41
8.8.2.2 Corrected transformed vegetation index 8.42
8.8.2.3 Thiams transformed vegetation index 8.42
8.8.2.4 Ratio vegetation index (RVI) 8.43
8.8.2.5 Normalized ratio vegetation index 8.43
8.8.2.6 Infrared index (II) 8.44
8.8.2.7 Moisture stress Index 8.44
8.8.2.8 Perpendicular vegetation Index 1 8.45
8.8.3 Principal component analysis images 8.45
8.8.4 Tassel cap transformation images 8.48
9. Image Classification 9.1
9.2 Supervised Classification 9.2

Contents xvii
9.2.1 Classification scheme 9.3
9.2.2 Training site selection and statistics extraction 9.5
9.2.2.1 Guidelines for training data 9.5
9.2.2.2 Idealized sequence for selecting training data 9.7
9.2.3 Training data statistics 9.8
9.2.4 Feature selection 9.8
9.2.5 Selection of appropriate classification algorithm 9.12
9.2.5.1 The parallelopiped classifier 9.12
9.2.5.2 The minimum-distance to means classifier 9.14
9.2.5.3 The maximum likelihood classifier 9.14
9.3 Unsupervised Classification 9.15
9.3.1 Distance based clustering methods 9.18
9.3.2 Model-based clustering methods 9.18
9.3.3 Density-based clustering method 9.19
9.3.4 Condensation-based method for clustering 9.20
9.3.5 Subspace clustering methods 9.21
9.3.6 Feature selection for clustering 9.22
9.3.7 Clustering pattern classification by distance functions 9.23
9.3.7.1 Minimum distance pattern classification 9.23
9.3.7.2 Maximin distance algorithm 9.26
9.3.7.4 K-means algorithm 9.28
9.3.7.5 ISODATA algorithm 9.29
9.4 Classification Accuracy Assessment 9.32
9.4.1 Error matrix 9.33
9.5 Illustration Example 9.38
9.5.1 Selection of training dataset 9.38
9.5.2 Feature selection 9.38
9.5.3 Image classification 9.43
9.5.4 Assessment of accuracy 9.44
10. Spatial Filtering 10.1
10.2 Process of Filtering 10.2
10.3 Noise Removal Filtering 10.4
10.3.1 Mean filter 10.6
10.3.2 Weighted mean filter 10.6
10.3.3 Median filter 10.7
10.3.4 Mode filter 10.7
10.3.5 Olympic filter 10.7
10.3.6 Multi level median (MLM) filter 10.7
10.3.7 P-median (PM) filter 10.7
10.3.8 Adaptive mean P-median (AMPM) filter 10.8
10.4 Edge Detection 10.8
10.4.1 Classification of Edge Detection Techniques 10.10
10.4.2 Non-directional Filters 10.10
10.4.2.1 Laplacian filter 10.10
10.4.2.2 High boost filter 10.12

xviii Contents
10.4.3 Simple Directional Filtering 10.13
10.4.4 Gradient filtering 10.14
10.4.4.1 Roberts operator 10.15
10.4.4.2 Prewitt operator 10.15
10.4.4.3 Sobel operator 10.16
10.4.4.4 Kirsch operator 10.18
10.4.5 Zero Crossing Filtering 10.18
10.4.5.1 LoG filter 10.19
10.4.5.2 DDoG filter 10.21
References R.1
Index I.1
About the Author A.1

List of Figures
Fig. 1.1 Electromagnetic Spectrum 1.2
Fig. 1.2 The wave model of electromagnetic energy 1.2
Fig. 1.3 Radiations from a blackbody 1.4
Fig. 1.4 Schematic description of image representation and modeling 1.6
Fig. 2.1 Overview of the sensors 2.2
Fig. 2.2 Connectivity of different sampling pattern 2.7
Fig. 2.3 A log-polar array of pixels 2.8
Fig. 2.4 Chromaticity diagram 2.11
Fig. 2.5 Typical color gamut of color monitors (triangle)
and color printed devices (irregular region) 2.12
Fig. 2.6 The RGB Colour Model 2.13
Fig. 2.7 HSI Model 2.14
Fig. 2.8 Separation of primaries 2.16
Fig. 3.1 Simple file format model 3.2
Fig. 3.2 TIFF Data Structures 3.5
Fig. 3.3 Image File Header 3.6
Fig. 3.4 Organization of the Main File 3.24
Fig. 3.5 Organization of the Index File 3.26
Fig. 3.6 An Example Polygon Instance 3.28
Fig. 3.7 Multi-Patch Part Examples 3.30
Fig. 3.8 Layout of sample dataset 3.31
Fig. 3.9 Band Interleaved by Pixel (BIP) data format 3.31
Fig. 3.10 Band Interleaved by Line (BIL) data format 3.31
Fig. 3.11 Band Sequence (BSQ) data format 3.32
Fig. 4.1 Basic Utilities in a image processing software 4.2
Fig. 4.2 Application Window of IDRISI 4.13
Fig. 5.1 Different types of histograms 5.2
Fig. 5.2 Representation of a histogram 5.3
Fig. 5.3 Different types of Skewness 5.6
Fig. 5.4 Different types of Kurtosis 5.7
Fig. 5.5 Different types of scatter plot 5.9
Fig. 5.6 TM Band 1 5.14
Fig. 5.7 TM Band 2 5.14
Fig. 5.8 TM Band 3 5.14

xx List of Figures
Fig. 5.9 TM Band 4 5.14
Fig. 5.10 TM Band 5 5.15
Fig. 5.11 TM Band 7 5.15
Fig. 5.12 FCC of TM Band 4, 3 and 2 5.15
Fig. 5.13 Histogram of TM Band 1 5.15
Fig. 5.19 Scatter plot – TM 1 & 2 5.18
Fig. 5.21 Scatter plot - TM 1 & 4 5.18
Fig. 6.1 Components of the signal received by a
satellite-mounted sensor 6.4
Fig. 6.2 Regression method for computation of
atmospheric path radiance 6.6
Fig. 6.3 Effects of Earth rotation on the geometry of a
line-scanned image 6.7
Fig. 6.4 Earth Rotation Correction 6.10
Fig. 7.1 Concept of Min-Max stretch 7.3
Fig. 7.2 Concept of Percentile stretching 7.4
Fig. 7.3 Piecewise linear contrast stretch 7.5
Fig. 7.4 Various contrast enhancement techniques applied to TM Band 1 7.9
Fig. 7.5 Various Contrast enhancement Technique applied to TM Band 4 7.9
Fig. 8.1 Concept of Vegetation Index 8.6
Fig. 8.2 The Perpendicular Vegetation Index 8.10
Fig. 8.3 Distance from the Soil Line 8.12
Fig. 8.4 Light and dark soil influences on SAVI values as a function
of the shift origin correction factor 8.14
Fig. 8.5 Effect on soil moisture on NDVI, SAVI and WDVI 8.15
Fig. 8.6 Completion of Red-NIR Space with vegetation density 8.16
Fig. 8.7 Laboratory-measured green and dry vegetation
reflectance spectra 8.21
Fig. 8.8 Liquid water transmittances for different water thicknesses 8.21
Fig. 8.9 Sensitivity of NDWI to liquid water thickness 8.22

List of Figures xxi
Fig. 8.10 Scatter diagram between reflectance of wet and dry
soil at 1.2 µm and 0.86 µm 8.22
Fig. 8.11 Relationship of NDWI and reflectance at 0.86 µm 8.23
Fig. 8.12 The mean NDWI for mixtures of wet soils with
green vegetation and drier soil with green vegetation as
a function of vegetation area fraction 8.24
Fig. 8.13 Effects different crystal radii on snow reflectance 8.25
Fig. 8.14 TM Tasseled Cap transformation axes system 8.35
Fig. 8.15 Approximate locations of some classes in TM
Tasseled Cap feature space 8.35
Fig. 8.16 Ratio image 8.41
Fig. 8.17 NDVI Image 8.41
Fig. 8.18 Transformed Vegetation Index 8.42
Fig. 8.19 Corrected Transformed Vegetation Index 8.42
Fig. 8.20 Thiams Transformed Vegetation Index 8.43
Fig. 8.21 Ratio Vegetation Index 8.43
Fig. 8.22 Normalized Ratio Vegetation Index 8.44
Fig. 8.23 Infrared Index 8.44
Fig. 8.24 Moisture Stress Index 8.45
Fig. 8.25 Perpendicular Vegetation Index 1 8.45
Fig. 8.26 Principal Component Analysis Images 8.47
Fig. 8.27 Tassel Cap Transformation Images 8.48
Fig. 8.28 FCC of the TM band 4, 3 and 2 8.49
Fig. 8.29 FCC generated from the first three Principal
Component Analysis 8.50
Fig. 8.30 FCC generated from Brightness, Greenness
and Wetness images 8.51
Fig. 9.1 Schematic representation of overlapping information
from training data 9.9
Fig. 9.2 Clustering procedure 9.15
Fig. 9.3 A general classification of clustering methods 9.18
Fig. 9.4 Two “patterns” in a two dimensional measurement space 9.23
Fig. 9.5 Decision boundaries defined by a single-prototype,
minimum distance classifier 9.25
Fig. 9.6 Partitioning of feature space by maximum algorithm 9.28
Fig. 9.7 Schematic representation of an error matrix 9.34
Fig. 9.8 Histogram plot of different features in different
Training data in different TM Bands 9.39
Fig. 9.9 Minimum Distance to Means classifier 9.44
Fig. 9.10 Maximum Likelihood classifier 9.44
Fig. 10.1 Low pass filtering and high pass filtering 10.2
Fig. 10.2 An example of small image (left) and kernel (right)
to illustrate convolution 10.3
Fig. 10.3 1-D Gaussian distribution with mean value of 0
and standard deviation of 1 10.5
Fig. 10.4 Layout of Mean filter 10.6
Fig. 10.5 1st
and 2nd
derivative of an edge illustrated
in one dimensional 10.9

xxii List of Figures
Fig. 10.6 Representation of a Laplacian Filter procedure 10.11
Fig. 10.7 (a) Simple Laplacian mask 10.12
Fig. 10.7 (b) Laplacian mask with orientation in variant 10.12
Fig. 10.8 Simple Directional Filter 10.13
Fig. 10.9 Roberts operator 10.15
Fig. 10.10 Prewitt Operator 10.16
Fig. 10.11 Sobel operator 10.17
Fig. 10.12 Kirsch Operator (8 masks) 10.18
Fig. 10.13 Intensity profit of an ideal Step Edge 10.20
Fig. 10.14 Response of Log Filter to a Step Edge 10.21

List of Tables
Table 1.1 Data Volumes of Image Sources (in Millions of Bytes) 1.8
Table 1.2 Storage Capacities (in Millions of Bytes) 1.8
Table 3.1 Summary of Tags in TIFF 3.8
Table 3.2 Tag Value 3.9
Table 3.3 Additional tags required for TIFF conformant images 3.10
Table 3.4 Comparison between JPEG and GIF 3.13
Table 3.5 Details of IHDR Chunk 3.16
Table 3.6 Allowed combination of colour type and allowed bit depth 3.17
Table 3.7 List of predefined keywords 3.21
Table 3.8 Details of tIME Chunk 3.21
Table 3.9 Structure of zTXT chunk 3.22
Table 3.10 Properties and ordering constraints of standards chunk 3.22
Table 3.11 Description of the Main File Header 3.24
Table 3.12 Values for Shape Type 3.25
Table 3.13 Description of Main File Record Headers 3.25
Table 3.14 Description of Index Records 3.26
Table 3.15 Multi Patch Part values 3.30
Table 4.1 System Specifications of ERDAS IMAGINE 9.0
for Windows OS 4.8
Table 4.2 Recommended System specifications ERDAS IMAGINE 9.0
for UNIX OS 4.8
Table 5.1 Initial Statistics of the 6-band TM Dataset 5.11
Table 5.2 Variance Co-variance Matrix 5.12
Table 5.3 Correlation Matrix 5.12
Table 6.1 Comparative statement of Resampling Methods 6.13
Table 7.1 Look up Table 7.2
Table 8.1 Tasseled-cap coefficients for Landsat-1 MSS 8.36
Table 8.2 Formulae of Different Indices 8.39
Table 8.3 Eigen values of the Principal Component Analysis 8.48
Table 8.4 Eigen vectors of Principal Component Analysis 8.48
Table 8.5 Degree of correlation between each band and component 8.48
Table 9.1 U.S. Geological survey land use /land cover
classification system 9.3
Table 9.2 The Four levels of Remotely Sensed Data to be used 9.4
Table 9.3 Different feature selection methods 9.13
Table 9.4 Alternate definitions of discriminate function 9.25
Table 9.5 A sample Error Matrix 9.34

xxiv List of Tables
Table 9.6 Percentage errors of Ommission and Commission 9.35
Table 9.7 Confidence limits of Ommission and Commission 9.35
Table 9.8 Sample computation of κ 9.36
Table 9.9 List of accuracy measures 9.37
Table 9.10 Statistics for Dry Sand 9.40
Table 9.11 Statistics for Agriculture 9.40
Table 9.12 Statistics for High density forest 9.40
Table 9.13 Statistics for Medium density forest 9.41
Table 9.14 Statistics for Light dense forest 9.41
Table 9.15 Statistics for New forest 9.41
Table 9.16 Statistics for Urban 9.42
Table 9.17 Statistics for Water 9.42
Table 9.18 Statistics for Shallow water 9.42
Table 9.19 Statistics for Wet sand 9.43
Table 9.20 Classification results using different classifiers using
different band combinations 9.43
Table 9.21 ERROR MATRIX of 4-band combination using Maximum
Likelihood Classifier 9.46
Table 9.22 Accuracy of Classification for 4-band combination using
Maximum Likelihood Classifier 9.47
Table 9.23 ERROR MATRIX of 4-band combination using Minimum
Distance to Mean Classifier 9.48
Table 9.24 Accuracy of Classification for 4-band combination
using Minimum Distance to Mean Classifier 9.49
Table 9.25 ERROR MATRIX of 3-band combination using Maximum
Likelihood Classifier 9.50
Table 9.26 Accuracy of Classification for 3-band combination using
Maximum Likelihood Classifier 9.51
Table 9.27 ERROR MATRIX of 3-band combination using Minimum
Distance to Mean Classifier 9.52
Table 9.29 Error Matrix of 3-band combination using MLC 9.54
Table 9.31 Error Matrix of 4-band combination using MLC 9.56
Table 9.33 Error Matrix of 4-band combination using
Minimum Distance to Means 9.58
Table 9.35 Error Matrix of 3-band combination using Minimum
Distance to Mans 9.60

1
Concept of Images
1.1 INTRODUCTION
Simply, images are pictures where information is recorded by a camera or sensor
and presented visually either in hard or soft copy form. Pictures are important as
these can be an extraordinarily effective medium for storage and communication
of information. Photographs help human beings to create a permanent record of
their visual experiences, and also to share these experiences with others. A
photograph, in general, provides the necessary information which otherwise may
require a lengthy, tedious and ambiguous verbal description of the area
photographed. Normally, human beings rely on their eyes to receive information
related to a surrounding, and the brain is adept at visual data processing. Thus, it
is very aptly said that a photograph or a picture is worth a thousand words.
The process of acquiring a photograph is similar to the process of normal
human vision. In both human vision and photography, a light source is required
to illuminate a scene. The light interacts with the objects in the scene and the
reflected light reaches the observer, whereupon it is detected by the eyes in case
of human vision or by a camera in case of photography. Information about the
objects in the scene is recorded as variations in the intensity and colour of the
detected light. It is important to note is that, although a scene is three-
dimensional in nature, the image of that scene is always a two-dimensional one.
1.2 ELECTROMAGNETIC ENERGY
Apart from light, there are other forms of energy that can be used to create
images. Light is a small portion between 0.4µm to 0.7µm portion of the
electromagnetic (EM) spectrum known as the visible portion. It is important to
note that this is the only portion of the spectrum that can be associated with the

1.2 Digital Image Processing
concept of colour. Blue, green, and red are known as the primary colours or
wavelengths of the visible spectrum. However electromagnetic (EM) spectrum
ranges from gamma rays to radiowaves (Fig.1.1).
Fig. 1.1: Electromagnetic spectrum
1.3 ELECTROMAGNETIC SPECTRUM AND ITS
CHARACTERISTICS
Electromagnetic energy can be modeled by waves or by energy bearing particles
called photons. In the wave model, electromagnetic energy is considered to
propagate through space in the form of sinusoidal waves. These waves are
characterized by electrical field (E) and magnetic field (M) both of which is
perpendicular to each other, and hence for this reason the term electromagnetic
energy is used. The vibration of both fields is perpendicular to the direction of
travel of the wave (Fig. 1.2). Both fields propagate through space at the speed of
light c which is 299,790,000 ms-1
, and can be rounded off to 3×108
ms-1
.
Fig. 1.2 The wave model of electromagnetic energy
The wavelength λ of electromagnetic waves, are particularly important for
understanding remote sensing, is defined as the distance between successive
Electric field
Distance
Wavelength
E
M
Magnetic field
Velocity of light c
0.4 0.5 0.6 0.7 (µm)
Blue
Green
Red
Visible
UV Near-infrared
10-6
10-5
10-4
10-3
10-1
1 10 102
103
104
105
106
107
108
109
10-2
Wavelength (µm)
Cosmic
rays
γ-rays
X-rays
Television
and
radio
Microwave
Thermal
IR
Mid-IR
Near-IR
Ultraviolet
(UV)
Visible

Concept of Images 1.3
wave crests. Wavelength is measured in metres (m) or some fraction of metres,
such as nanometers (nm, 10-9
metres) or micrometers (µm, 10-6
metres).
The frequency ν of the electromagnetic energy is the number of cycles of a
wave passing a fixed point over a specific period of time. Frequency is normally
measured in hertz (Hz), which is equivalent to number of cycles per second.
Since the speed of light is constant, wavelength and frequency are inversely
related to each other.
ν =
λ
c
... (1.1)
Most characteristics of EM energy can be described using the wave model.
For some purposes, however, EM energy modeled by particle (photons) theory is
more convenient to use. This approach is considered when quantifying the
amount of energy measured by multi-spectral sensor. The amount of energy held
by a photon of a specific wavelength is given by
Q = hν
or = h
λ
c
…(1.2)
where Q is the energy of a photon J and h is Planck’s constant (6.6262×10-34
Js).
From Eq. (1.1) it follows that the longer the wavelength, the lower is its energy
content. Gamma rays (around 10-9
m) are the most energetic, and radio waves
(around >1 m) are the least energetic. It may be noted that it is easier to measure
shorter wavelengths than the larger wavelengths.
All matter with a temperature above Absolute zero (0° K) radiate EM energy
due to molecular agitation in which movement of the molecules is taking place.
This means that the Sun, and also the Earth, radiates energy in the form of
waves. The matter capable of absorbing and re-emitting all EM energy is known
as a blackbody. For blackbodies, both the emissivity (ε) and the absorptance (α)
are equal to 1.
The amount of energy radiated by an object depends on its absolute
temperature and emissivity, and it is a function of the wavelength. The radiation
emitted by a blackbody at different temperatures is shown in Fig. 1.3. The area
below the curve represents the total amount of energy emitted at a specific
temperature. It can be concluded that a higher temperature corresponds to a
greater contribution of shorter wavelengths. The peak radiation at 400° C is
around 4 µm while at 1000° C it is 2.5 µm. The emitting ability of a real material
compared to that of the blackbody, is referred to as the emissivity of a material.
In reality, blackbodies are hardly found in nature, and most natural objects have
emissivity less than one. This means that only a part, usually between 80-98% of
the received energy, is re-emitted, and the remaining part of the energy is
absorbed.
One of the useful properties of EM radiation, for imaging purposes, is its
ability to travel in straight lines. Thus geometric characteristics of objects in a

scene can be preserved in images. Further, EM radiation can interact with matter
in different ways, depending on its wavelength. Images acquired at different
wavelengths may have very different properties, and we may need to be aware of
these differences when seeking appropriate image processing techniques.
1.4 UTILITY OF EM RADIATION IN IMAGE ACQUISITION
The visible portion of the spectrum occurs between wavelengths of
approximately 400 and 700 nanometres (nm). Within this region, wavelength is
perceived as colour; light at 550 nm appears green, whereas light at 700 nm is
seen as red. At shorter wavelengths, EM radiation carries larger energies. In the
X-ray region of the spectrum (at a wavelength λ, of around 10-10
m), it carries
sufficient energy to penetrate a significant volume of material. X-ray images
therefore reveal the internal structure of objects that are opaque to light and
commonly used to image internal parts of a human body.
At wavelengths around 10-12
m, EM radiation has radioactive properties and
known as gamma rays. Gamma rays are highly penetrating and, have medical
applications. The information is represented as a function of absorbivity of
radioactive tracer. This tracer is absorbed in varying amounts by different tissues
in the body, according to their level of activity. Thereafter, a gamma camera is
used to collect gamma ray photons emitted by body tissues to form an image.
The diseased areas such as a tumour will appear as a bright region in image.
EM radiation having wavelengths longer than light can be used for
acquiring information. Warm objects emit large quantities of infrared (IR)
radiation and can be used to locate people or moving vehicles even in conditions
●
●
●
Wavelength, µm
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
If T increases, peak moves towards
shorter wavelengths and the total
area under the curve increases
T=1273°K=1000°C
T=1073°K=800°C
T=873°K=600°C
T=673°K=400°C
wλ
(watts/cm
2
/µm)
1
2
3
4
●
Fig. 1.3 Radiations from a blackbody

of total darkness. Microwave energy is able to penetrate through cloud and fog
and can be used to study earth surface during cloudy or foggy conditions.
Apart from EM energy, images can be generated whenever an information
has a spatial nature. Digital Elevation Model (DEM) is a common representation
of elevation in a pictorial/ image form. In this case, instead of depicting
information as a function of some property of the object such as reflectivity,
emissivity or absorbivity, variation of ground elevation is represented.
1.5 IMAGE PROCESSING
Image processing is a general term for the wide range of techniques that are used
to manipulate, modify or classify images in various ways. In general, a digital
image acquired through satellite or a digital camera is used for analysis through
computers, and hence the term digital image processing. In this book, the focus
would be to use digital satellite images for highlighting the various procedures of
digital image analysis.
In general, Digital Image Processing refers to processing of a two -
dimensional picture in digital form by a digital computer. A digital image can be
defined as an array of real or complex numbers represented by a finite number of
bits. An image can be acquired through the process of scanning an existing
photograph by a scanner, digital camera or by a digital sensor on board a satellite
or aircraft. This digital image can then be processed and/or displayed on a high-
resolution computer monitor.
Digital image processing has a broad spectrum of applications, such as
remote sensing, image transmission and storage for business applications,
medical processing, radar, sonar, and acoustic image processing, robotics, and
automated inspection of industrial parts. Images acquired by satellites are useful
in identifying and mapping earth resources; prediction of agricultural crops,
urban growth, and weather, flood and fire control; and many other environmental
applications. Image transmission and storage application are frequently used in
broadcast of television programmes, teleconferencing between users, sending of
facsimile messages (FAX), internet and intranet. In the field of medical
applications, use of X-rays, angiograms, tomography, nuclear magnetic
resonance (NMR), and ultrasonic scanning are some of the use of digital image
processing.
Radar and sonar images can be used for detection and recognition of
various types of targets or in guidance and maneuvering of aircraft or missile
systems. Robot vision for industrial automation, image synthesis, cartoon
making or fashion design are some of the application areas of digital image
processing.
1.6 BASIC IMAGE PROCESSING TECHNIQUE
The basic image processing techniques can be enumerated as follows:
i. Image representation and modeling
ii. Image enhancement
iii. Image restoration

iv. Image analysis
v. Image reconstruction
vi. Image data compression
1.6.1 Image Representation and Modeling
A digital image can represent a typical informational characteristic such as
reflected, emitted or absorbed energy of an object, elevation, rainfall attribution,
electrical conductivity etc. of an area. Thus, any information having spatial
characteristics can be considered as a image. The spatial dimension of a digital
image is spatially represented as a pixel (picture element) and it is defined as
spatial resolution of the image. Fig 1.4 shows a schematic description of image
representation and modeling.
Fig. 1.4: Schematic description of image representation and modeling
In image representation, fidelity is an important consideration as it defines
the quality of the image, and specifies the contrast, spatial resolution, colour etc.
It is helpful in designing the sensor since it defines which parameters are to be
measured with high accuracy. Further, the sampling rate of image is an
importation criteria, as it defines the manner by which useful information in an
image is to be preserved, and is dependent upon the band width of an image. In
general, a television signal is about 4 MHz and from sampling theorem, it
requires a minimum sampling rate of 8MHz. At 30 frames per sec, it implies that
each frame should contain 266,000 pixels approximately. So, for a raster image
of 512 lines, each image should be of the size of 512 × 512 pixels.
Statistical model describes an image, which is often characterized by its
mean and covariance functions. This allows for the development of algorithms
that can be used for useful identification for an entire class of images. Often the
images are assumed to be stationary, so that the mean and covariance functions
can easily be estimated. Stationary models are useful in data compression
problems.
In global modeling, an image is considered as a composition of several
objects. Various objects in the scene are detected (for example, by segmentation
techniques), and the model gives the rules for defining the relationship among
Perception model Local model Global
Image representation and modeling
* Visual Perception of
contrast spatial
frequencies and colour
* Image fidelity models
* Temporal perception
* Scene perception
* Sampling and reconstruction
* Image quantization
* Deterministic models
* Series expansions/unitary
* Statistical models
* Scene analysis/artificial
intelligence models
* Sequential and
clustering model
* Image understanding
models

various objects. Such representations fall under the category of image
understanding models, which is not a subject of study in this text.
1.6.2 Image Enhancement
The role of image enhancement is to highlight certain image features so that it
can be used for subsequent analysis or for image display. Some of the image
enhancement techniques are contrast and edge enhancement, pseudo-colouring,
noise filtering, sharpening, and magnifying. Image enhancement is useful in
feature extraction, image analysis, and visual information display. It may be
noted that enhancement process does not increase the inherent information
content of data; it simply emphasizes certain specified portion of the image.
Enhancement algorithms are generally interactive and application-dependent.
1.6.3 Image Restoration
The purpose of image restoration is to remove or minimize known degradations
in an image, such as deblurring of images caused by the limitations of a sensor or
its environment, noise filtering, and correction of geometric distortion or non--
linearities due to sensors.
1.6.4 Image Analysis
Image analysis is performed to make quantitative measurements from an image
and produce a description of it. Image analysis techniques require extraction of
certain features that aid in the identification of the object. Quantitative
measurements of object features allow classification and description of the
image.
1.6.5 Image Reconstruction
In image reconstruction, 2D or 3D object is reconstructed from several one-
dimensional images. Each image is obtained by passing rays of energy through
the object at different angles and viewed simultaneously to obtain a 2D or 3D
view. Such techniques are important in medical imaging (CT scanners),
astronomy, radar imaging, geological exploration and nondestructive testing of
assemblies.
1.6.6 Image Data Compression
To acquire an image of object or scene, voluminous data is generated (Table 1.1)
and to store the same would require devices having enormous storage capacity
(Table 1.2). Generally, the access speeds of storage devices are, inversely
proportional to their capacity. Image data compression techniques are concerned
with reduction of the number of bits required to store or transmit images without
any appreciable loss of information. Image storage is required commonly for
educational and business documents, medical images are used in patient

monitoring systems. Due to their wide applications, data compression is of great
importance in digital image processing
Table 1.1 Data Volumes of Image Sources (in Millions of Bytes)
National archives 12 5 x 109
1 h of color television 28 x103
Encyclopedia Britannica 125 x 103
Book (200 pages of text characters) 13
One page viewed as an image 13
Table 1.2 Storage Capacities (in Millions of Bytes)
Human brain 125,000,000
Magnetic cartridge 250,000
Optical disc memory 12,500
Magnetic disc 760
2400-ft magnetic tape 200
Floppy disc 125
Solid-state memory modules 025
With the help of this background knowledge regarding images and image
processing in brief, the next chapter looks at the process of imaging i.e. the data
acquisition.

2
The Process of Imaging
2.1 INTRODUCTION
Imaging is short form for image acquisition. It is the process of sensing any
surroundings and its subsequent representation of measurements made, leading
to the formation of an image. One of the important aspects in imaging is the
source of energy, depending on this; it can be categorized as passive or active
imaging. To record information, a device is required and is known as a sensor. A
sensor can be grouped into two groups. Passive sensors depend on an external
source of energy usually the Sun. A passive sensor can record energy from 10-12
m (gamma rays) to over 1 m (micro and radio waves). Active sensors have their
own source of energy. Measurements by active sensors are controlled as they do
not depend upon the illumination condition. Fig. 2.1 shows a schematic
representation of different type of sensors.
2.2 PASSIVE SENSORS
Passive sensor is that sensor which does not have its independent energy sources
and relies on some external source such as Sun, to illuminate the objects. The
reflected energy from these objects is recorded by the sensor. Aerial camera is a
typical sensor as it can act both as passive or active sensor depending upon the
illumination condition.
2.2.1 Gamma-ray Spectrometer
The gamma – ray spectrometer measures the amount of gamma rays emitted by
the upper soil or rock layers due to radioactive decay. The energy measured in
specific wavelength band provides information on the abundance of radio
isotopes that relate to specific minerals. Therefore, the main application of
gamma-ray spectrometer is found in mineral exploration. Further, gamma rays

have very short wavelength (pico-m), thus due to large atmospheric absorption
characteristics of these waves, this type of energy can one be measured up to a
few hundred meters above the Earth’s surface.
2.2.2 Aerial Camera
The aerial camera uses a optical lens and film system and is mostly mounted in
aircraft for aerial photography. Low orbiting satellites and NASA Space Shuttle
missions also used conventional camera. The film types used in the camera
enable electromagnetic energy in the range between 400 nm and 900 nm to be
recorded. Aerial photographs are used in a wide range of applications. The rigid
and regular geometry of aerial photographs allows for the possibility to acquire
stereo-photography. This has enabled the development of photogrammetric
procedures for obtaining precise 3D coordinates. Although aerial photos are used
in many applications, yet principal applications include medium and large scale
topographic mapping and cadastral mapping. Presently, analogue photos are
often scanned stored and processed in digital systems.
2.2.3 Video Camera
Video camera is sometimes used to record image data. Most video sensors are
only sensitive to be visible colours, although a few are able to record the near
infrared part of the spectrum. Until recently, only analogue video cameras were
available. Today, digital video cameras are increasingly available, some of which
are applied in remote sensing. Mostly, video images serve to provide low cost
image data for qualitative purpose, for example, to provide additional visual
information about an area captured by another sensor.
Fig. 2.1 Overview of the sensors
Visible
domain
Optical
domain
Microwave
domain
gamma ray
spectrometer
- multispectral
scanner thermal
scanner
passive
microwave
Passive
sensors - aerial
camera
radar
altimeter
laser
scanner
Active
sensors imaging
radar
wavelength

The Process of Imaging 2.3
2.2.4 Multi-spectral Scanner
The multi-spectral scanner is an instrument that mainly measures the reflected
sunlight in the optical domain. A scanner systematically scans the Earth’s
surface thereby measuring the energy reflected from the viewed area. This is
done simultaneously for several wavelength bands, hence the name multi-
spectral scanner. A wavelength band is an interval of the electromagnetic
spectrum for which the average reflected energy is measured. The reason for
measuring a number of distinct wavelength bands is that each band is related to
specific characteristics of the Earth surface. For example, reflection
characteristics of ‘blue’ light give information about the mineral composition,
reflection characteristics of ‘infrared light’ tells something about the type and
health of vegetation. The definition of the waveband of scanner therefore
depends on the applications for which the sensor has been designed.
2.2.5 Imaging Spectrometer
The principle of the imaging spectrometer is similar to that of the multi-spectral
scanner, except that a spectrometer can measure only in very narrow (5-10 nm)
spectral bands. This results in an almost continuous reflectance curve per pixel
rather than the values for relatively broad spectral bands. The spectral curves
measures depend on the chemical composition of the material. Imaging
spectrometer data, therefore, can be used to determine mineral composition of
the surface or the chlorophyll content of the surface water.
2.2.6 Thermal Scanner
Thermal scanners measure the thermal data in the range of 10-14 µm.
Wavelengths in this range are directly related to the temperature of an object.
Data on cloud, land and sea surface temperature are extremely useful for weather
forecasting. For this reason, most remote sensing systems designed for
meteorology include a thermal scanner. Thermal scanners can also be used to
study the effects of drought (“water stress”) on agricultural crops, or to monitor
the temperature of cooling water discharged from the thermal power plants.
Another application is in the detection of coal fires, forest fire, geyser etc.
2.2.7 Radiometer
EM energy with very long wavelength (1-100 cm) is emitted from the soil and
rocks, on or just below the Earths surface. The depth from which this energy is
emitted depends on the properties, such as water content, of the specific material.
Radiometers are used to detect this energy. The resulting data can be used in
mineral exploration, soil mapping and soil moisture estimation.
2.3 ACTIVE SENSORS
Active sensors are those sensors which have their own energy source to
illuminate the objects on ground and then record the reflected energy from these

objects. Some of the commonly used active sensors are discussed in the
following section.
2.3.1 Laser Scanner
Laser scanners are mounted on aircraft and use a laser beam (infrared light) to
measure the distance from the aircraft to points located on the ground. This
distance measurement is then combined with exact information on the aircraft’s
position to calculate the elevation of ground points. Laser scanning is mainly
used to produce detailed and high resolution Digital Terrain Models (DTM) for
topographic mapping. Laser scanning is increasingly used for other purposes,
such as the production of detailed 3D models of city buildings and for measuring
tree heights in forestry.
2.3.2 Radar Altimeter
Radar altimeters are used to measure the topographic profile parallel to the
satellite orbit. They provide profiles (single lines of measurements) rather than
‘image’ data. Radar altimeters operate in the 1-6 cm domain and are able to
determine height with a precision of 2-4 cm. Radar altimeters are useful for
measuring relatively smooth surface such as oceans and for small scale mapping
of continental terrain models.
2.3.3 Imaging Radar
Radar instruments operate in the 1-100 cm domain. As in multi-spectral
scanning, different wavelength bands are related to particular characteristics of
the Earth’s surface. The radar backscatter is influenced by the illuminating
signal. The radar backscatter is influenced by the illuminating signal (microwave
parameters) and the illuminated surface characteristics (orientation, roughness,
dielectric constant/moisture content). Since radar is an active sensor system and
the applied wavelength are able to penetrate clouds, it has all-weather day-and-
night acquisition capability. The combination of two radar images of the same
areas can provide information about terrain heights. Combining two radar
images acquired at different times can be used to precisely assess changes in
height or vertical deformations (SAR Interferometry).
2.4 PLATFORMS
In remote sensing, the sensor is mounted on a platform. In general, remote
sensing sensors are attached to moving platform such as aircraft and satellites.
Static platforms are occasionally used in an experimental context. For example,
by using a multi-spectral sensor mounted to a pole, the changing reflection
characteristics of a specific crop during the day or season can be assessed.
Airborne observations are carried out using aircraft with specific modifications
to carry sensors. An aircraft that carries an aerial camera or a scanner needs a
hole in the floor of the aircraft. Sometimes, Ultra Light Vehicles (ULVs),

balloons, zeppelins or kites are used for airborne remote sensing. Airborne
observations are possible from 100 m up to 30-40 km height. Until recently, the
navigation of an aircraft was one of the most difficult and crucial parts of
airborne remote sensing. In recent years, the availability of satellite navigation
technology has significantly improved the quality of flight execution.
For space-borne remote sensing satellites are used. Satellites are launched
into space with rockets. Satellites for Earth Observation are positioned in orbits
between 150-36,000 km altitudes. The specific orbit depends on the objectives of
the mission, e.g. continuous observation of large areas or detailed observation of
smaller areas. For detailed discussion on various satellites system, the reader is
advised to refer to any standard remote sensing book (Jensen, 2000, Chandra &
Ghosh, 2005)
2.5 CHARACTERISTICS OF IMAGE
The optics of an imaging system focuses a continuous, two-dimensional pattern
of varying light intensity and colour onto a sensor. Pattern is defined in a
coordinate system whose origin is conventionally defined by the upper-left
corner of the image and a function, f (x, y). For monochrome images, the value
of the function at any pair of coordinates, x and y, is the intensity of the light
detected at that point. In the case of colour images, f (x, y) is a vector-valued
function.
The function f (x, y) must be translated into a discrete array of numerical
data if it is to undergo computer processing. This digital representation is only an
approximation of the original image, as this will allow the analyst to manipulate
the image using a computer. Translation of f (x, y) into an appropriate numerical
form is accomplished by the processes of sampling and quantization. For
standard video signals, both processes are usually carried out by a single piece of
hardware, known as an Analogue to Digital Converter (ADC).
2.5.1 Sampling
Sampling is the process of measuring the value of the image function I(x, y) at
discrete intervals in space. Each sample corresponds to a small, square area of
the image, known as a pixel. A digital image is a two-dimensional array of these
pixels. Pixels are indexed by x and y coordinates, with x and y taking integer
values.
In a CCD sensor, which consists of an array of photo detectors, the pattern
of sampling is already defined by the layout of the photo detectors. However, in
conventional video cameras, the incoming radiation gets converted into an
analogue video signal for compatibility as per the specification of video
equipment in use today.
A single frame from a standard video signal is already discrete in the y
dimension, consisting of either 525 or 625 lines of data. Sampling the signal
therefore involves measuring its amplitude at regular time intervals during the
segments of the signal that correspond to each line. This makes the image
discrete spatially in the x dimension. Video standards enforce a particular

sampling rate for a video signal. For example, a RS-170 video signal, for
instance, has 485 active lines and that each frame must have an aspect ratio of
4:3, so there must be 485 × (4/3) = 646 samples per line. In practice, a few
lines and samples are trimmed from the signal to give an array of pixels with
dimensions 640 × 480. To produce such an image, a temporal sampling rate of
around 12 MHz is required.
With a digital still picture camera, things are simple, as there is no need to
convert samples from the CCD into an analogue form and then resample.
Further, there is no requirement to conform to broadcast video standards. Such
cameras can typically produce images with dimensions of 1024 × 768, 1280 ×
1024 pixels etc. These dimensions are chosen to suit display standards
originating from the computer industry (e.g., SVGA). Much higher resolutions
than those of broadcast video are possible, and a 4:3 aspect ratio is not enforced
although this is often preferred.
Other types of imaging equipment operate under different conditions. In
medicine, for example, radio-isotope imaging devices produce images that are,
sampled very coarsely. This is because images are formed from gamma ray
photons emitted by radioactive material inside the patient. For safety reasons, the
quantity of this material is small, hence there are relatively few photons emitted.
It is therefore necessary to integrate photon counts over a relatively large area in
order to obtain statistically meaningful results. For example, the size of the chest
be represented by a 64 × 64 pixel array.
2.5.2 Spatial Resolution
The spatial resolution of an image is the physical size of a pixel in that image;
i.e., the area in the scene that is represented by a single pixel in the image. For a
given field of view, dense sampling will produce a high resolution image in
which there are many pixels, each of which represents the contribution of a very
small part of the scene; coarse sampling, on the other hand, will produce a low
resolution image in which there are few pixels, each representing the
contribution of a relatively large part of the scene to the image. Spatial resolution
dictates the amount of useful information that can be extracted from an image.
Quality of a digital image also depends upon the spatial frequency of the image.
Spatial frequency can be defined as the rate of change with which information f
(x, y) changes.
A gradual change in f (x, y) characterizes low spatial frequencies and can be
represented adequately by coarsely-sampled image while rapid changes are
characterized by high spatial frequencies and can be represented accurately by
densely-sampled image. However, the appropriate sampling for an image is
defined by Nyquist criterion. Essentially, it states that the sampling frequency
should be at least twice the highest spatial frequency found in the image. If an
image is sampled coarsely, such that the Nyquist criterion is not met, then the
image may suffer from the effects of aliasing.
In general, the advance knowledge of the highest spatial frequency present
in an image is not known. Consequently, the sampling process is normally

preceded by anti-aliasing. This is a filtering operation designed to remove
frequencies that exceed half the sampling rate achieved by the ADC hardware,
thereby guaranteeing that the Nyquist criterion is met.
2.5.3 Sampling Pattern
When sampling an image, it is not only important to know the sampling rate, but
also the physical arrangement of the samples. A rectangular pattern, in which
pixels are aligned horizontally and vertically into rows and columns, is by far the
most common. Unfortunately, rectangular sampling pattern leads to ambiguities
in pixel connectivity. A second problem with rectangular patterns is an
inconsistency in distance measurement. Suppose that each pixel in Fig.2.2 (a)
represents a region of the scene that is 1 cm wide and 1 cm high. Then, the
distance between pixels C and D is 1 cm; however, the distance between pixels
B and C is not 1 cm but √2 cm, by simple trigonometry. Hence, the actual
distance traveled is defined by a fixed number of pixels in the image depending
upon on the direction moved.
Fig 2.2 Connectivity of different sampling pattern (Efford, 2000)
These problems may be solved by a hexagonal sampling pattern (Fig.2.2b).
Here, diagonal neighbours are properly connected and the distance traveled in an
image does not depend on direction. Despite these advantages, a hexagonal
pattern is seldom used. It cannot portray accurately the large number of
horizontal and vertical features found in many images, and, in any case, sensors
and display hardware generally do not support hexagonal sampling.
The rectangular and hexagonal patterns described above are uniform, with
the result that one part of an image is as important as any other part. This is
useful in images intended for eventual human interpretation, for which
prediction of where viewers will direct their attention is impossible. In other
situations, where attention can be predicted or controlled, a non-uniform
sampling scheme may be profitable. In particular, a log-polar sampling pattern
has some interesting and useful properties. Fig. 2.3 shows an array of pixels that
conforms to this pattern. The pixels of this array are sectors with a fixed angular
A
B E
F C D
A
B E
F C D
(a) Rectangular (b) hexagonal

size and a radial size that increases logarithmically with increasing distance from
the centre. This gives high resolution near the centre of the array and low
resolution in the periphery. Such an arrangement satisfies the conflicting
requirements of good resolution and wide field of view. However, a camera
using a sensor with this sampling pattern must always point towards the most
interesting or important part of the scene, to ensure that it lies in the centre to the
array and is therefore imaged at the highest possible resolution. This is known as
an attentive vision strategy. The human visual system supports attentive vision
by means of eye, head and even body movements, thereby ensuring that the
features of interest are always imaged using the fovea.
Fig 2.3 A log-polar array of pixels
2.6.3 Quantization
It is usual to digitize the values of the image function, f (x, y), in addition to its
spatial coordinates. This process of quantization involves replacing a
continuously varying f (x, y) with a discrete set of quantization levels. The
accuracy with which variations in f (x, y) are represented is determined by the
number of quantization levels that are used. By using more levels, a better
approximation of the image information can be achieved.
Conventionally, a set of n quantization levels comprises of the integers 0, 1,
2, . … . ,n – 1 with 0 and n – 1 being usually displayed or printed as black and
white, respectively, and intermediate levels rendered in various shades of grey.
Quantization levels are therefore commonly referred to as grey levels. The
collective term for all the grey levels, ranging from black to white, is a grey
scale.
For convenient and efficient processing by a computer, the number of grey
levels, n, is usually an integral power of two and maybe expressed as.
n = 2b
, ... (2.1)
where b is the number of bits used for quantization. A typical value of b equal
to 8, gives images with 256 possible grey levels ranging from 0 (black) to 255
(white). Some ADCs are not capable of quantizing to 8 bits, producing 6-bit or 7

bit images. However these may subsequently be represented in memory using 8
bits per pixel. The specialized equipment used in medicine and astronomy may
produce images quantized using 10 or even 12 bits.
2.6 COLOUR FUNDAMENTALS
The process by which the human brain perceives and interprets colour is a
primarily a physio-psychological phenomenon that is not yet fully understood.
The physical nature of colour can be expressed by a formal basis supported by
experimental and theoretical results.
Basically, the colours that humans and some other animals perceive in an
object are determined by the nature of the light reflected from the object. Visible
light is composed of a relatively narrow band of frequencies in the
electromagnetic spectrum. A body that reflects light is balanced in all visible
wavelengths appears white to the observer, while, a body that reflects in a
limited range of the visible spectrum tends to exhibits some shades of colour. An
object appears green as it reflect light with in the wavelength range of 500 to 570
nm range while it absorbs most of the energy at other wavelengths.
Characterization of light is central to the science of colour. If the light is
achromatic (void of colour), its only attribute is its intensity, or amount.
Achromatic light is what viewers see on a black and white television set, and
grey level which refers to a scalar measure of intensity that ranges from black, to
grey, and finally to white.
Chromatic light spans the electromagnetic spectrum from approximately 400
to 700 nm. Three basic quantities are used to describe the quality of a chromatic
light source: radiance, luminance, and brightness. Radiance is the total amount of
energy that flows from the light source, and it is usually measured in watts (W).
Luminance, measured in lumens (lm), a measure of the amount of energy that an
observer perceives from a light source. For example, light emitted from a source
operating in the far infrared region of the spectrum could have significant energy
(radiance), but an observer would hardly perceive it as its luminance would be
almost zero. Finally, brightness is a subjective descriptor that is practically
impossible to measure. It embodies the achromatic notion of intensity and is one
of the key factors in describing colour sensation.
The characteristics generally used to distinguish one colour from another are
brightness, hue, and saturation. Brightness represents the chromatic notion of
intensity. Hue is an attribute of colour of the dominant wavelength in a mixture
of light waves as perceived by an observer. So, when an object is referred to as
red, orange, or yellow, the observer is specifying to its hue. Saturation refers to
the relative purity or the amount of white light mixed with a hue. The pure
spectrum colours are fully saturated. Colours such as pink (red and white) and
lavender (violet and white) are less saturated, with the degree of saturation being
inversely proportional to the amount of white light added.
Hue and saturation when taken together is called chromaticity, and,
therefore, a colour may be characterized by its brightness and chromaticity. The
amounts of red, green, and blue needed to form any particular colour are called
the tristimulus values and are denoted by, X, Y and Z, respectively. A colour can

then be specified by its trichromatic coefficients, as defined by
Z
Y
X
X
x
+
+
=
… (2.2)
Z
Y
X
Y
y
+
+
=
... (2.3)
and Z
Y
X
Z
z
+
+
=
…(2.4)
It is noted from these equations that
.
1
z
y
x =
+
+ … (2.5)
Another approach for specifying colours is to use CIE chromaticity diagram
(Fig. 2.4), which shows colour composition as a function of x (red) and y
(green). For any value of x and y, the corresponding value of z (blue) is obtained
from Eq. (2.5) by noting that z = 1– (x + y).
The positions of the various spectrum colours from violet at 380 nm to red at
780 nm are indicated around the boundary of the tongue-shaped chromaticity
diagram. These are the pure colours shown in spectrum of Fig 2.4. Any point
within the diagram represents some mixture of spectrum colours. The point of
equal energy shown in Fig. 2.4 corresponds to equal fractions of the three
primary colours; it represents the CIE standard for white light. Any point located
on the boundary of the chromaticity chart is fully saturated. As a point moves
ways from the boundary and approaches the point of equal energy, more white
light is added to the colours and it becomes less saturated, thus, at the point of
equal energy, saturation is equal to zero.
The chromaticity diagram is useful for colour mixing since a straight line
segment joining any two points in the diagram defines all the different colour
variations that can be obtained by combining these two colours additively. If a
straight line is drawn line drawn from the red to the green points shown in
Fig. 2.4 then, if there is more red light than green light, the exact point
representing the new colour will be on the line segment, but it well be closer to
the red point than to the green point. Similarly, a line drawn from the point of
equal energy to any point on the boundary of the chart will define all the shades
of that particular spectrum colour.
Extension of this procedure to three colours is straight forward. To
determine the range of colours that can be obtained from any three give colours
in the chromaticity diagram, we simply draw connecting lines to each of the
three colour points. The result is a triangle, and any colour inside the triangle can
be produce by various combinations of the three initial colours. A triangle with
vertices at any three fixed colours cannot enclose the entire colour region in
Fig. 2.4 This observation supports graphically the remark made earlier that not
all colours can be obtained with three single, fixed primaries.

Fig. 2.4 Chromaticity diagram. (Courtesy of the General Electric Co. Lamp
Business Division)
The triangle in Fig. 2.5 shows a typical range of colours (called the colour
gamut) produced by RGB monitors. The irregular region inside the triangle is
representative of the colour gamut of today’s high-quality colour printing
devices. The boundary of the colour printing gamut is irregular because colour
printing is a combination of additive and subtractive colour mixing, a process
that is much more difficult to control than that of displaying colours on a
monitor, which is based on the addition of three highly controllable light
primaries.
2.7 COLOUR MODELS
The purpose of a colour model (also called colour space or colour system) is to
facilitate the specification of colours in some standard, generally accepted way.
In essence, a colour model is a specification of a coordinate system and a
subspace within that system where each colour is represented by a single point.
Most colour models in use today are oriented either toward hardware (such as for
colour monitors and printers) or toward application where colour manipulation is
a goal (such as in the creation of colour graphics for animation).
In terms of digital image processing, the hardware-oriented models most
commonly used in practice are the RGB (red, green, blue) model for colour
monitors and a broad class of colour video cameras; the CMY (cyan, magenta,
yellow) and CMYK (cyan, magenta, yellow, black,) models for colour printing;

and the HSI (hue, saturation, intensity) model, which corresponds closely within
the way humans describe and interpret colour. The HSI model also has the
advantage that it decouples the colour and grey-scale information in an image,
making it suitable for many of the grey-scale techniques. There are numerous
colour models in use today due to the fact that colour science is a broad field that
encompasses many areas of application. It is tempting to dwell on some of these
models here simply because they are interesting and informative. However,
keeping to the task at hand, the models discussed in this chapter are leading
models for image processing. Having mastered the material in this chapter, the
reader will have no difficulty in understanding additional colour models in use
today.
Fig. 2.5 Typical color gamut of color monitors (triangle) and color printed devices
(irregular region)
2.7.1 The RGB Model
Technology for creating and displaying colour is based on the empirical obser-
vation that a wide variety of colours can be obtained by mixing red, green and
blue light in different proportions. For this reason, red (R), green (G) and blue
(B) are described as the primary colours of the additive colour system. However
not all colours can obtained in this way. Thus, colour image can be formed by
making three measurements of scene brightness at each pixel using the red,
G
R
.0 .1 .2 .3 .4 .5 .6 .7 .8
x-axis
.1
.2
.3
.4
.5
.6
.7
.8
.9
520
530
540
550
560
570
580
590
600
610
620
640
780
380
450
460
470
480
490
500
510
B
.0
y-axis

green and blue components of the detected light. This can be done either using a
colour camera where the sensor is able to measure radiation at red, green and
blue wavelengths for all points in the image or by using a monochrome camera
in conjunction with three special filters that blocks all but a narrow band of
wavelengths centered on red, green and blue, respectively.
In a colour image conforming to the RGB model, the value of each f (x, y) is
a vector of three components, corresponding to R, G and B. In a normalized
model, these components can vary between 0.0 and 1.0. R, G and B can be
regarded as orthogonal axes defining a three-dimensional colour space. Every
possible value of f (x, y) is a point in this colour cube. The primary colours red,
green and blue are at the corners (1, 0, 0), (0, 1, 0) and (0, 0.1); the colours cyan,
magenta and yellow are at the opposite corners while black is at the origin and
white is at the corner furthest from the origin (Fig. 2.6). Points on a straight line
joining the origin to the most distant corner represent various shades of grey.
Since each of the three components red, green and blue is normally
quantized using 8 bits, an image made up of these components is commonly
described as a 24-bit colour image. As each primary colour is represented to a
precision of 1 part in 256, it is possible to specify any arbitrary colour to a
precision of about 1 part in 16 million. Hence, in a 24 bit image 16.7 million
colours are available.
Despite its importance in image acquisition and display, the RGB model is
of limited use when processing colour images, because it is not a perceptual
model. In perceptual terms, colour and intensity are distinct from one another,
but the R, G and B components each contain both colour and intensity
information. Models which decouple these two different types of information
tend to be more useful for image processing.
Magenta
(1,0,1)
Red (1,0,0)
Blue (0,0,1)
Cyan
(0,1,1)
Yellow (1,1,0)
Black (0,0,0)
White
(1,1,1)
G
B
R
1
1
1
0
Green (0,1,0)
Fig. 2.6 The RGB colour model

2.7.2 CMY Model
The CMY model has as its primaries Cyan (C), Magenta (M) and Yellow (Y).
These are the primary colours of the subtractive system that describes how
colour is produced from pigments. A CMY colour is derived from an RGB
colour as follows:
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
−
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
=
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎣
⎡
B
G
R
1
1
1
Y
M
C
… (2.6)
Theoretical almost any colour can be produced on paper by mixing cyan,
magenta and yellow pigments. However this approach cannot produce a
satisfactory black, so a fourth component labeled as K and representing black
pigment is added, resulting in the CMYK model. This is the model that is used
when generating hardcopy versions of digital images using colour printers.
2.7.3 HSI Model
HSI model is more suitable than the RGB model for many image processing
tasks. The three components are hue (H), saturation (S) and intensity (I). H and S
specify colour where H specifies the dominant pure colour as perceived by an
observer while S measures the degree to which that pure colour has been diluted
by white light. Since colour and intensity are independent, it is possible to
manipulate one without affecting the other.
Fig. 2.7 HSI model
I
Green S H
Cyan
Blue Magenta
Yellow
Red
Black
White

HSI colour space is described by a cylindrical coordinate system and is
commonly represented as a 'double cone' (Fig. 2.7). A colour is a single point
inside or on the surface of the double cone, while the height of the point
corresponds to intensity. If a point lies in a horizontal plane, a vector can defined
in this plane from the axis of the cones to that point, the saturation is the length
of this vector and hue is its orientation, expressed as an angle in degrees.
2.7.4 Conversion of Colour from RGB to HSI
Given an image in RGB colour format, the Hue (H) component of each RGB
pixel is obtained by using the equation
⎩
⎨
⎧
>
−
≤
=
G
B
if
360
G
B
if
H
θ
θ
… (2.7)
where
( ) ( )
[ ]
( ) ( )( )
[ ]
.
B
G
B
R
G
R
B
R
G
R
2
1
cos 2
/
1
2
1
⎪
⎪
⎭
⎪
⎪
⎬
⎫
⎪
⎪
⎩
⎪
⎪
⎨
⎧
−
−
+
−
−
+
−
= −
θ
… (2.8)
The saturation (S) component is given by
( )
( )
[ ]
B
,
G
,
R
min
B
G
R
3
1
S
+
+
−
=
… (2.9)
and, the intensity (I) component is given by
( ).
B
G
R
3
1
I +
+
=
… (2.10)
It is assumed that the RGB values have been normalized to the range
[0, 1] and that angle θ is measured with respect to the red axis of the HSI space,
as indicated in (Fig. 2.7) Hue can be normalized to the range [0,1] by dividing by
3600
all values resulting from Eq 2.7. The other two HSI components already are
in this range if the give RGB values are in the interval [0,1].
2.7.5 Converting Colours from HSI to RGB
Given values of HSI in the interval [0,1], we want to find the corresponding
RGB values in the same range. The transforming equations depend on the values
of H. There are three sectors of interest, corresponding to the 120° intervals in
the separation of primaries (Fig 2.8).
First multiply H by 360°, such that the hue returns to its original range of
[0°, 360°]. Depending on the value of H, three cases can be defined.

Fig. 2.8 Separation of primaries
Case I
If Hue (H) has value between 0° to 120° (Fig 2.8(a)) i.e., H is between Red and
Green colour, the RGB components can be expressed by the following
equations:
)
S
1
(
I
B −
= … (2.11)
( )⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
−
+
=
H
60
cos
H
cos
S
1
I
R 0
… (2.12)
and
).
B
R
(
1
G +
−
= … (2.13)
Case II
If the colour point is located such that Hue lies within the range of (120° < H
<240°) i.e., Hue is within GB sector (Fig. 2.8(b)) than first subtracting 120° from
it:
°
−
= 120
H
H … (2.14)
Then the RGB components are calculated as
)
S
1
(
I
R −
= … (2.15)
( )⎥
⎦
⎤
⎢
⎣
⎡
−
°
+
=
H
H
S
I
G
60
cos
cos
1 … (2.16)
and )
G
R
(
1
B +
−
= … (2.17)
Case III
If the colour point is located such that Hue lies (240°< H <360°), then BR sector
i.e., H is in this range, Fig. (2.8(c)) first subtract 240° from it
R
G
B
R
G
B
G
G
B
(a) (b) (c)

°
−
= 240
H
H … (2.18)
Then the RGB components are computed as follows:
)
S
1
(
I
G −
= ... (2.19)
⎥
⎦
⎤
⎢
⎣
⎡
−
°
+
=
)
60
cos(
cos
1
H
H
S
I
B … (2.20)
and
).
B
G
(
1
R +
−
= … (2.21)
The next chapter focuses at the theories related to storing of an image. To do
so, the image has to have a defined format so that, it can be acquired, stored,
retrieved and archived properly without loss of information.

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become plain that he would never be restored, or would be restored
at least under strict limitations. The clergy went back, it must be
owned, to their old theory, as soon as they found that it would do
them no harm. It is principally to the general baseness and
profligacy of the times that Clarendon is indebted for his high
reputation. He was, in every respect, a man unfit for his age, at once
too good for it and too bad for it. He seemed to be one of the
ministers of Elizabeth, transplanted at once to a state of society
widely different from that in which the abilities of such ministers had
been serviceable. In the sixteenth century, the Royal prerogative had
scarcely been called in question. A Minister who held it high was in
no danger, so long as he used it well. That attachment to the Crown,
that extreme jealousy of popular encroachments, that love, half
religious half political, for the Church, which, from the beginning of
the second session of the Long Parliament, showed itself in
Clarendon, and which his sufferings, his long residence in France,
and his high station in the government, served to strengthen, would,
a hundred years earlier, have secured to him the favour of his
sovereign without rendering him odious to the people. His probity,
his correctness in private life, his decency of deportment, and his
general ability, would not have misbecome a colleague of
Walsingham and Burleigh. But, in the times on which he was cast,
his errors and his virtues were alike out of place. He imprisoned men
without trial. He was accused of raising unlawful contributions on the
people for the support of the army. The abolition of the act which
ensured the frequent holding of Parliaments was one of his favourite
objects. He seems to have meditated the revival of the Star
Chamber and the High Commission Court. His zeal for the
prerogative made him unpopular; but it could not secure to him the
favour of a master far more desirous of ease and pleasure than of
power. Charles would rather have lived in exile and privacy, with
abundance of money, a crowd of mimics to amuse him, and a score
of mistresses, than have purchased the absolute dominion of the
world by the privations and exertions to which Clarendon was
constantly urging him. A councillor who was always bringing him
papers and giving him advice, and who stoutly refused to

compliment Lady Castlemaine and to carry messages to Mistress
Stewart, soon became more hateful to him than ever Cromwell had
been. Thus, considered by the people as an oppressor, by the Court
as a censor, the Minister fell from his high office with a ruin more
violent and destructive than could ever have been his fate, if he had
either respected the principles of the Constitution or flattered the
vices of the King.
Mr. Hallam has formed, we think, a most correct estimate of the
character and administration of Clarendon. But he scarcely makes a
sufficient allowance for the wear and tear which honesty almost
necessarily sustains in the friction of political life, and which, in times
so rough as those through which Clarendon passed, must be very
considerable. When these are fairly estimated, we think that his
integrity may be allowed to pass muster. A high-minded man he
certainly was not, either in public or in private affairs. His own
account of his conduct in the affair of his daughter is the most
extraordinary passage in autobiography. We except nothing even in
the Confessions of Rousseau. Several writers have taken a perverted
and absurd pride in representing themselves as detestable; but no
other ever laboured hard to make himself despicable and ridiculous.
In one important particular Clarendon showed as little regard to the
honour of his country as he had shown to that of his family. He
accepted a subsidy from France for the relief of Portugal. But this
method of obtaining money was afterwards practised to a much
greater extent, and for objects much less respectable, both by the
Court and by the Opposition.
These pecuniary transactions are commonly considered as the
most disgraceful part of the history of those times; and they were no
doubt highly reprehensible. Yet, in justice to the Whigs and to
Charles himself, we must admit that they were not so shameful or
atrocious as at the present day they appear. The effect of violent
animosities between parties has always been an indifference to the
general welfare and honour of the State. A politician, where factions
run high, is interested not for the whole people, but for his own

section of it. The rest are, in his view, strangers, enemies, or rather
pirates. The strongest aversion which he can feel to any foreign
power is the ardour of friendship, when compared with the loathing
which he entertains towards those domestic foes with whom he is
cooped up in a narrow space, with whom he lives in a constant
interchange of petty injuries and insults, and from whom, in the day
of their success, he has to expect severities far beyond any that a
conqueror from a distant country would inflict. Thus, in Greece, it
was a point of honour for a man to cleave to his party against his
country. No aristocratical citizen of Samos or Corcyra would have
hesitated to call in the aid of Lacedæmon. The multitude, on the
contrary, looked every where to Athens. In the Italian states of the
thirteenth and fourteenth centuries, from the same cause, no man
was so much a Pisan or a Florentine as a Ghibeline or a Guelf. It may
be doubted whether there was a single individual who would have
scrupled to raise his party from a state of depression, by opening the
gates of his native city to a French or an Arragonese force. The
Reformation, dividing; almost every European country into’ two
parts, produced similar effects. The Catholic was too strong for the
Englishman, the Huguenot for the Frenchman. The Protestant
statesmen of Scotland and France called in the aid of Elizabeth; and
the Papists of the League brought a Spanish army into the very
heart of France. ‘The commotions to which the French Revolution
gave rise were followed by the same consequences. The Republicans
in every part of Europe were eager to see the armies of the National
Convention and the Directory appear among them, and exulted in
defeats which distressed and humbled those whom they considered
as their worst enemies, their own rulers. The princes and nobles of
France, on the other hand, did their utmost to bring foreign invaders
to Paris. A very short time has elapsed since the Apostolical party in
Spain invoked, too successfully, the support of strangers.
The great contest which raged in England during the seventeenth
century extinguished, not indeed in the body of the people, but in
those classes which were most actively engaged in politics, almost
all national feelings. Charles the Second and many of his courtiers

had passed a large part of their lives in banishment, living on the
bounty of foreign treasuries, soliciting foreign aid to reestablish
monarchy in their native country. The King’s own brother had fought
in Flanders, under the banners of Spain, against the English armies.
The oppressed Cavaliers in England constantly looked to the Louvre
and the Escurial for deliverance and revenge. Clarendon censures
the continental governments with great bitterness for not interfering
in our internal dissensions. It is not strange, therefore, that, amidst
the furious contests which followed the Restoration, the violence of
party feeling should produce effects which would probably have
attended it even in an age less distinguished by laxity of principle
and indelicacy of sentiment. It was not till a natural death had
terminated the paralytic old age of the Jacobite party that the evil
was completely at an end. The Whigs long looked to Holland, the
High Tories to France. The former concluded the Barrier Treaty; the
latter entreated the Court of Versailles to send an expedition to
England. Many men who, however erroneous their political notions
might be, were unquestionably honourable in private life, accepted
money without scruple from the foreign powers favourable to the
Pretender.
Never was there less of national feeling among the higher orders
than during the reign of Charles the Second. That Prince, on the one
side, thought it better to be the deputy of an absolute king than the
King of a free people. Algernon Sydney, on the other hand, would
gladly have aided France in all her ambitious schemes, and have
seen England reduced to the condition of a province, in the wild
hope that a foreign despot would assist him to establish his darling
republic. The King took the money of France to assist him in the
enterprise which he meditated against the liberty of his subjects,
with as little scruple as Frederic of Prussia or Alexander of Russia
accepted our subsidies in time of war. The leaders of the Opposition
no more thought themselves disgraced by the presents of Lewis,
than a gentleman of our own time thinks himself disgraced by the
liberality of powerful and wealthy members of his party who pay his
election bill. The money which the King received from France had

been largely employed to corrupt members of Parliament. The
enemies of the court might think it fair, or even absolutely necessary,
to encounter bribery with bribery. Thus they took the French
gratuities, the needy among them for their own use, the rich
probably for the general purposes of the party, without any scruple.
If we compare their conduct not with that of English statesmen in
our own time, but with that of persons in those foreign countries
which are now situated as England then was, we shall probably see
reason to abate something of the severity of censure with which it
has been the fashion to visit those proceedings. Yet, when every
allowance is made, the transaction is sufficiently offensive. It is
satisfactory to find that Lord Russell stands free from any imputation
of personal participation in the spoil. An age so miserably poor in all
the moral qualities which render public characters respectable can ill
spare the credit which it derives from a man, not indeed conspicuous
for talents or knowledge, but honest even in his errors, respectable
in every relation of life, rationally pious, steadily and placidly brave.
The great improvement which took place in our breed of public
men is principally to be ascribed to the Revolution. Yet that
memorable event, in a great measure, took its character from the
very vices which it was the means of reforming. It was assuredly a
happy revolution, and a useful revolution; but it was not, what it has
often been called, a glorious revolution. William, and William alone,
derived glory from it. The transaction was, in almost every part,
discreditable to England. That a tyrant who had violated the
fundamental laws of the country, who had attacked the rights of its
greatest corporations, who had begun to persecute the established
religion of the state, who had never respected the law either in his
superstition or in his revenge, could not be pulled down without the
aid of a foreign army, is a circumstance not very grateful to our
national pride. Yet this is the least degrading part of the story. The
shameless insincerity of the great and noble, the warm assurances
of general support which James received, down to the moment of
general desertion, indicate a meanness of spirit and a looseness of
morality most disgraceful to the age. That the enterprise succeeded,

at least that it succeeded without bloodshed or commotion, was
principally owing to an act of ungrateful perfidy, such as no soldier
had ever before committed, and to those monstrous fictions
respecting the birth of the Prince of Wales which persons of the
highest rank were not ashamed to circulate. In all the proceedings of
the Convention, in the conference particularly, we see that littleness
of mind which is the chief characteristic of the times. The resolutions
on which the two Houses at last agreed were as bad as any
resolutions for so excellent a purpose could be. Their feeble and
contradictory language was evidently intended to save the credit of
the Tories, who were ashamed to name what they were not
ashamed to do. Through the whole transaction no commanding
talents were displayed by any Englishman; no extraordinary risks
were run; no sacrifices were made for the deliverance of the nation,
except the sacrifice which Churchill made of honour, and Anne of
natural affection. It was in some sense fortunate, as we have
already said, for the Church of England, that the Reformation in this
country was effected by men who cared little about religion. And, in
the same manner, it was fortunate for our civil government that the
Revolution was in a great measure effected by men who cared little
about their political principles. At such a crisis, splendid talents and
strong passions might have done more harm than good. There was
far greater reason to fear that too much would be attempted, and
that violent movements would produce an equally violent reaction,
than that too little would be done in the way of change. But
narrowness of intellect and flexibility of principle, though they may
be serviceable, can never be respectable.
If in the Revolution itself there was little that can properly be
called glorious, there was still less in the events which followed. In a
church which had as one man declared the doctrine of resistance
unchristian, only four hundred persons refused to take the oath of
allegiance to a government founded on resistance. In the preceding
generation, both the Episcopal and the Presbyterian clergy, rather
than concede points of conscience not more important, had resigned
their livings by thousands.

The churchmen, at the time of the Revolution, justified their
conduct by all those profligate sophisms which are called Jesuitical,
and which are commonly reckoned among the peculiar sins of
Popery, but which in fact are every where the anodynes employed by
minds rather subtle than strong, to quiet those internal twinges
which they cannot but feel and which they will not obey. As the oath
taken by the clergy was in the teeth of their principles, so was their
conduct in the teeth of their oath. Their constant machinations
against the Government to which they had sworn fidelity brought a
reproach on their order and on Christianity itself. A distinguished
prelate has not scrupled to say that the rapid increase of infidelity at
that time was principally produced by the disgust which the faithless
conduct of his brethren excited in men not sufficiently candid or
judicious to discern the beauties of the system amidst the vices of its
ministers.
But the reproach was not confined to the Church. In every political
party, in the Cabinet itself, duplicity and perfidy abounded. The very
men whom William loaded with benefits and in whom he reposed
most confidence, with his seals of office in their hands, kept up a
correspondence with the exiled family. Orford, Leeds, and
Shrewsbury were guilty of this odious treachery. Even Devonshire is
not altogether free from suspicion. It may well be conceived that, at
such a time, such a nature as that of Marlborough would riot in the
very luxury of baseness. His former treason, thoroughly furnished
with all that makes infamy exquisite, placed him under the
disadvantage which attends every artist from the time that he
produces a masterpiece. Yet his second great stroke may excite
wonder, even in those who appreciate all the merit of the first. Lest
his admirers should be able to say that at the time of the Revolution
he had betrayed his King from any other than selfish motives, he
proceeded to betray his country. He sent intelligence to the French
court of a secret expedition intended to attack Brest. The
consequence was that the expedition failed, and that eight hundred
British soldiers lost their lives from the abandoned villany of a British
general. Yet this man has been canonized by so many eminent

writers that to speak of him as he deserves may seem scarcely
decent.
The reign of William the Third, as Mr. Hallam happily says, was the
Nadir of the national prosperity. It was also the Nadir of the national
character. It was the time when the rank harvest of vices sown
during thirty years of licentiousness and confusion was gathered in;
but it was also the seed-time of great virtues.
The press was emancipated from the censorship soon after the
Revolution; and the Government immediately fell under the
censorship of the press. Statesmen had a scrutiny to endure which
was every day becoming more and more severe. The extreme
violence of opinions abated. The Whigs learned moderation in office;
the Tories learned the principles of liberty in opposition. The parties
almost constantly approximated, often met, sometimes crossed each
other. There were occasional bursts of violence; but, from the time
of the Revolution, those bursts were constantly becoming less and
less terrible. The severity with which the Tories, at the close of the
reign of Anne, treated some of those who had directed public affairs
during the war of the Grand Alliance, and the retaliatory measures of
the Whigs, after the accession of the House of Hanover, cannot be
justified; but they were by no means in the style of the infuriated
parties, whose alternate murders had disgraced our history towards
the close of the reign of Charles the Second. At the fall of Walpole
far greater moderation was displayed. And from that time it has
been the practice, a practice not strictly according to the theory of
our Constitution, but still most salutary, to consider the loss of office,
and the public disapprobation, as punishments sufficient for errors in
the administration not imputable to personal corruption. Nothing, we
believe, has contributed more than this lenity to raise the character
of public men. Ambition is of itself a game sufficiently hazardous and
sufficiently deep to inflame the passions without adding property,
life, and liberty to the stake. Where the play runs so desperately
high as in the seventeenth century, honour is at an end. Statesmen,
instead of being as they should be, at once mild and steady, are at

once ferocious and inconsistent. The axe is for ever before their
eyes. A popular outcry sometimes unnerves them, and sometimes
makes them desperate; it drives them to unworthy compliances, or
to measures of vengeance as cruel as those which they have reason
to expect. A Minister in our times need not fear either to be firm or
to be merciful. Our old policy in this respect was as absurd as that of
the king in the Eastern tale who proclaimed that any physician who
pleased might come to court and prescribe for his diseases, but that
if the remedies failed the adventurer should lose his head. It is easy
to conceive how many able men would refuse to undertake the cure
on such conditions; how much the sense of extreme danger would
confuse the perceptions, and cloud the intellect, of the practitioner,
at the very crisis which most called for self-possession, and how
strong his temptation would be, if he found that he had committed a
blunder, to escape the consequences of it by poisoning his patient.
But in fact it would have been impossible, since the Revolution, to
punish any Minister for the general course of his policy, with the
slightest semblance of justice; for since that time no Minister has
been able to pursue any general course of policy without the
approbation of the Parliament. The most important effects of that
great change were, as Mr. Hallam has most truly said and most ably
shown, those which it indirectly produced. Thenceforward it became
the interest of the executive government to protect those very
doctrines which an executive government is in general inclined to
persecute. The sovereign, the ministers, the courtiers, at last even
the universities and the clergy, were changed into advocates of the
right of resistance. In the theory of the Whigs, in the situation of the
Tories, in the common interest of all public men, the Parliamentary
constitution of the country found perfect security. The power of the
House of Commons, in particular, has been steadily on the increase.
Since supplies have been granted for short terms and appropriated
to particular services, the approbation of that House has been as
necessary in practice to the executive administration as it has al-
ways been in theory to taxes and to laws.

Mr. Hallam appears to have begun with the reign of Henry the
Seventh, as the period at which what is called modern history, in
contradistinction to the history of the middle ages, is generally
supposed to commence. He has stopped at the accession of George
the Third, “from unwillingness,” as he says, “to excite the prejudices
of modern politics, especially those connected with personal
character.” These two eras, we think, deserved the distinction on
other grounds. Our remote posterity, when looking back on our
history in that comprehensive manner in which remote posterity
alone can, without much danger of error, look back on it, will
probably observe those points with peculiar interest. They are, if we
mistake not, the beginning and the end of an entire and separate
chapter in our annals. The period which lies between them is a
perfect cycle, a great year of the public mind. In the reign of Henry
the Seventh, all the political differences which had agitated England
since the Norman conquest seemed to be set at rest. The long and
fierce struggle between the Crown and the Barons had terminated.
The grievances which had produced the rebellions of Tyler and Cade
had disappeared. Vilanage was scarcely known. The two royal
houses, whose conflicting claims had long convulsed the kingdom,
were at length united. The claimants whose pretensions, just or
unjust, had disturbed the new settlement, were overthrown. In
religion there was no open dissent, and probably very little secret
heresy. The old subjects of contention, in short, had vanished; those
which were to succeed had not yet appeared.
Soon, however, new principles were announced; principles which
were destined to keep England during two centuries and a half in a
state of commotion. The Reformation divided the people into two
great parties. The Protestants were victorious. They again
subdivided themselves. Political factions were engrafted on
theological sects. The mutual animosities of the two parties
gradually emerged into the light of public life. First came conflicts in
Parliament; then civil war; then revolutions upon revolutions, each
attended by its appurtenance of proscriptions, and persecutions, and
tests; each followed by severe measures on the part of the

conquerors; each exciting a deadly and festering hatred in the
conquered. During the reign of George the Second, things were
evidently tending to repose. At the close of that reign, the nation
had completed the great revolution which commenced in the early
part of the sixteenth century, and was again at rest. The fury of
sects had died away. The Catholics themselves practically enjoyed
toleration; and more than toleration they did not yet venture even to
desire. Jacobitism was a mere name. Nobody was left to fight for
that wretched cause, and very few to drink for it. The Constitution,
purchased so dearly, was on every side extolled and worshipped.
Even those distinctions of party which must almost always be found
in a free state could scarcely be traced. The two great bodies which,
from the time of the Revolution, had been gradually tending to
approximation, were now united in emulous support of that splendid
Administration which smote to the dust both the branches of the
House of Bourbon. The great battle for our ecclesiastical and civil
polity had been fought and won. The wounds had been healed. The
victors and the vanquished were rejoicing together. Every person
acquainted with the political writers of the last generation will
recollect the terms in which they generally speak of that time. It was
a glimpse of a golden age of union and glory, a short interval of rest,
which had been preceded by centuries of agitation, and which
centuries of agitation were destined to follow.
How soon faction again began to ferment is well known. In the
Letters of Junius, in Burke’s Thoughts on the Cause of the
Discontents, and in many other writings of less merit, the violent
dissensions which speedily convulsed the country are imputed to the
system of favouritism which George the Third introduced, to the
influence of Bute, or to the profligacy of those who called
themselves the King’s friends. With all deference to the eminent
writers to whom we have referred, we may venture to say that they
lived too near the events of which they treated to judge correctly.
The schism which was then appearing in the nation, and which has
been from that time almost constantly widening, had little in
common with those schisms which had divided it during the reigns

of the Tudors and the Stuarts. The symptoms of popular feeling,
indeed, will always be in a great measure the same; but the principle
which excited that feeling was here new. The support which was
given to Wilkes, the clamour for reform during the American war, the
disaffected conduct of large classes of people at the time of the
French Revolution, no more resembled the opposition which had
been offered to the government of Charles the Second, than that
opposition resembled the contest between the Roses.
In the political as in the natural body, a sensation is often referred
to a part widely different from that in which it really resides. A man
whose leg is cut off fancies that he feels a pain in his toe. And in the
same manner the people, in the earlier part of the late reign,
sincerely attributed their discontent to grievances which had been
effectually lopped off. They imagined that the prerogative was too
strong for the Constitution, that the principles of the Revolution were
abandoned, that the system of the Stuarts was restored. Every
impartial man must now acknowledge that these charges were
groundless. The conduct of the Government with respect to the
Middlesex election would have been contemplated with delight by
the first generation of Whigs. They would have thought it a splendid
triumph of the cause of liberty that the King and the Lords should
resign to the lower House a portion of the legislative power, and
allow it to incapacitate without their consent. This, indeed, Mr. Burke
clearly perceived. “When the House of Commons,” says he, “in an
endeavour to obtain new advantages at the expense of the other
orders of the state, for the benefit of the commons at large, have
pursued strong measures, if it were not just, it was at least natural,
that the constituents should connive at all their proceedings;
because we ourselves were ultimately to profit. But when this
submission is urged to us in a contest between the representatives
and ourselves, and where nothing can be put into their scale which
is not taken from ours, they fancy us to be children when they tell us
that they are our representatives, our own flesh and blood, and that
all the stripes they give us are for our good.” These sentences
contain, in fact, the whole explanation of the mystery. The conflict of

the seventeenth century was maintained by the Parliament against
the Crown. The conflict which commenced in the middle of the
eighteenth century, which still remains undecided, and in which our
children and grandchildren will probably be called to act or to suffer,
is between a large portion of the people on the one side, and the
Crown and the Parliament united on the other.
The privileges of the House of Commons, those privileges which,
in 1642, all London rose in arms to defend, which the people
considered as synonymous with their own liberties, and in
comparison of which they took no account of the most precious and
sacred principles of English jurisprudence, have now become nearly
as odious as the rigours of martial law. That power of committing
which the people anciently loved to see the House of Commons
exercise, is now, at least when employed against libellers, the most
unpopular power in the Constitution. If the Commons were to suffer
the Lords to amend money-bills, we do not believe that the people
would care one straw about the matter. If they were to suffer the
Lords even to originate money-bills, we doubt whether such a
surrender of their constitutional rights would excite half so much
dissatisfaction as the exclusion of strangers from a single important
discussion. The gallery in which the reporters sit has become a
fourth estate of the realm. The publication of the debates, a practice
which seemed to the most liberal statesman of the old school full of
danger to the great safeguards of public liberty, is now regarded by
many persons as a safeguard tantamount, and more than
tantamount, to all the rest together.
Burke, in a speech on parliamentary reform which is the more
remarkable because it was delivered long before the French
Revolution, has described, in striking language, the change in public
feeling of which we speak. “It suggests melancholy reflections,” says
he, “in consequence of the strange course we have long held, that
we are now no longer quarreling about the character, or about the
conduct of men, or the tenour of measures; but we are grown out of
humour with the English Constitution itself; this is become the object

of the animosity of Englishmen. This constitution in former days
used to be the envy of the world; it was the pattern for politicians;
the theme of the eloquent; the meditation of the philosopher in
every part of the world. As to Englishmen, it was their pride, their
consolation. By it they lived, and for it they were ready to die. Its
defects, if it had any, were partly covered by partiality, and partly
borne by prudence. Now all its excellencies are forgot, its faults are
forcibly dragged into day, exaggerated by every artifice of
misrepresentation. It is despised and rejected of men; and every
device and invention of ingenuity or idleness is set up in opposition,
or in preference to it.” We neither adopt nor condemn the language
of reprobation which the great orator here employs. We call him only
as a witness to the fact. That the revolution of public feeling which
he described was then in progress is indisputable; and it is equally
indisputable, we think, that it is in progress still.
To investigate and classify the causes of so great a change would
require far more thought, and far more space, than we at present
have to bestow. But some of them are obvious. During the contest
which the Parliament carried on against the Stuarts, it had only to
check and complain. It has since had to govern. As an attacking
body, it could select its points of attack, and it naturally chose those
on which it was likely to receive public support. As a ruling body, it
has neither the same liberty of choice, nor the same motives to
gratify the people. With the power of an executive government, it
has drawn to itself some of the vices, and all the unpopularity of an
executive government. On the House of Commons above all,
possessed as it is of the public purse, and consequently of the public
sword, the nation throws all the blame of an ill conducted war, of a
blundering negotiation, of a disgraceful treaty, of an embarrassing
commercial crisis. The delays of the Court of Chancery, the
misconduct of a judge at Van Diemen’s Land, any thing, in short,
which in any part of the administration any person feels as a
grievance, is attributed to the tyranny! or at least to the negligence,
of that all-powerful body. Private individuals pester it with their
wrongs and claims. A merchant appeals to it from the Courts of Rio

Janeiro or St. Petersburgh. A historical painter complains to it that
his department of art finds no encouragement. Anciently the
Parliament resembled a member of opposition, from whom no places
are expected, who is not expected to confer favours and propose
measures, but merely to watch and censure, and who may,
therefore, unless he is grossly injudicious, be popular with the great
body of the community. The Parliament now resembles the same
person put into office, surrounded by petitioners whom twenty times
his patronage would not satisfy, stunned with complaints, buried in
memorials, compelled by the duties of his station to bring forward
measures similar to those which he was formerly accustomed to
observe and to check, and perpetually encountered by objections
similar to those which it was formerly his business to raise.
Perhaps it may be laid down as a general rule that a legislative
assembly, not constituted on democratical principles, cannot be
popular long after it ceases to be weak. Its zeal for what the people,
rightly or wrongly, conceive to be their interests, its sympathy with
their mutable and violent passions, are merely the effects of the
particular circumstances in which it is placed. As long as it depends
for existence on the public favour, it will employ all the means in its
power to conciliate that favour. While this is the case, defects in its
constitution are of little consequence. But, as the close union of such
a body with the nation is the effect of an identity of interests not
essential but accidental, it is in some measure dissolved from the
time at which the danger which produced it ceases to exist.
Hence, before the Revolution, the question of Parliamentary
reform was of very little importance. The friends of liberty had no
very ardent wish for reform. The strongest Tories saw no objections
to it. It is remarkable that Clarendon loudly applauds the changes
which Cromwell introduced, changes far stronger than the Whigs of
the present day would in general approve. There is no reason to
think, however, that the reform effected by Cromwell made any
great difference in the conduct of the Parliament. Indeed, if the
House of Commons had, during the reign of Charles the Second,

been elected by universal suffrage, or if all the seats had been put
up to sale, as in the French Parliaments, it would, we suspect, have
acted very much as it did. We know how strongly the Parliament of
Paris exerted itself in favour of the people on many important
occasions; and the reason is evident. Though it did not emanate
from the people, its whole consequence depended on the support of
the people.
From the time of the Revolution the House of Commons has been
gradually becoming what it now is, a great council of state,
containing many members chosen freely by the people, and many
others anxious to acquire the favour of the people; but, on the
whole, aristocratical in its temper and interest. It is very far from
being an illiberal and stupid oligarchy; but it is equally far from being
an express image of the general feeling. It is influenced by the
opinion of the people, and influenced powerfully, but slowly and
circuitously. Instead of outrunning the public mind; as before the
Revolution it frequently did, it now follows with slow steps and at a
wide distance. It is therefore necessarily unpopular; and the more so
because the good which it produces is much less evident to common
perception than the evil which it inflicts. It bears the blame of all the
mischief which is done, or supposed to be done, by its authority or
by its connivance. It does not get the credit, on the other hand, of
having prevented those innumerable abuses which do not exist
solely because the House of Commons exists. A large part of the
nation is certainly desirous of a reform in the representative system.
How large that part may be, and how strong its desires on the
subject may be, it is difficult to say. It is only at intervals that the
clamour on the subject is loud and vehement. But it seems to us
that, during the remissions, the feeling gathers strength, and that
every successive burst is more violent than that which preceded it.
The public attention may be for a time diverted to the Catholic
claims or the Mercantile code; but it is probable that at no very
distant period, perhaps in the lifetime of the present generation, all
other questions will merge in that which is, in a certain degree,
connected with them all.

Already we seem to ourselves to perceive the signs of unquiet
times, the vague presentiment of something great and strange
which pervades the community, the restless and turbid hopes of
those who have every thing to gain, the dimly hinted forebodings of
those who have every thing to lose. Many indications might be
mentioned, in themselves indeed as insignificant as straws; but even
the direction of a straw, to borrow the illustration of Bacon, will show
from what quarter the storm is setting in.
A great statesman might, by judicious and timely reformations, by
reconciling the two great branches of the natural aristocracy, the
capitalists and the landowners, and by so widening the base of the
government as to interest in its defence the whole of the middle
class, that brave, honest, and sound-hearted class, which is as
anxious for the maintenance of order and the security of property, as
it is hostile to corruption and oppression, succeed in averting a
struggle to which no rational friend of liberty or of law can look
forward without great apprehensions. There are those who will be
contented with nothing but demolition; and there are those who
shrink from all repair. There are innovators who long for a President
and a National Convention; and there are bigots who, while cities
larger and richer than the capitals of many great kingdoms are
calling out for representatives to watch over their interests, select
some hackneyed jobber in boroughs, some peer of the narrowest
and smallest mind, as the fittest depositary of a forfeited franchise.
Between these extremes there lies a more excellent way. Time is
bringing round another crisis analogous to that which occurred in the
seventeenth century. We stand in a situation similar to that in which
our ancestors stood under the reign of James the First. It will soon
again be necessary to reform that we may preserve, to save the
fundamental principles of the Constitution by alterations in the
subordinate parts. It will then be possible, as it was possible two
hundred years ago, to protect vested rights, to secure every useful
institution, every institution endeared by antiquity and noble
associations, and, at the same time, to introduce into the system

improvements harmonizing with the original plan. It remains to be
seen whether two hundred years have made us wiser.
We know of no great revolution which might not have been
prevented by compromise early and graciously made. Firmness is a
great virtue in public affairs; but it has its proper sphere.
Conspiracies and insurrections in which small minorities are
engaged, the outbreakings of popular violence unconnected with any
extensive project or any durable principle, are best repressed by
vigour and decision. To shrink from them is to make them
formidable. But no wise ruler will confound the pervading taint with
the slight local irritation. No wise ruler will treat the deeply seated
discontents of a great party, as he treats the fury of a mob which
destroys mills and power-looms. The neglect of this distinction has
been fatal even to governments strong in the power of the sword.
The present time is indeed a time of peace and order. But it is at
such a time that fools are most thoughtless and wise men most
thoughtful. That the discontents which have agitated the country
during the late and the present reign, and which, though not always
noisy, are never wholly dormant, will again break forth with
aggravated symptoms, is almost as certain as that the tides and
seasons will follow their appointed course. But in all movements of
the human mind which tend to great revolutions there is a crisis at
which moderate concession may amend, conciliate, and preserve.
Happy will it be for England if, at that crisis, her interests be
confided to men for whom history has not recorded the long series
of human crimes and follies in vain.
END OF VOL. 1.

TRANSCRIBER'S NOTE: The 1860 six
volume print set had the index for all six
volumes at the end to volume six. This
PG edition has the complete index for all
volumes at the end of each volume.
A B C D E F G H I J K L M N O P Q R S T U V W XYZ

A.
A priori reasoning, 8 9 10 20 21 59
Abbt and abbot, difference between,
76
Academy, character of its doctrines,
411
Academy, French, (the), 2 3 ; has been
of no benefit to literature, 23 ; its
treatment of Corneille and Voltaire, 23 21
; the scene of the fiercest animosities, 23
Academy of the Floral Games, at
Toulouse, 136 137 ; Acting, Garrick's,
quotation from Fielding illustrative of, i.
332; the true test of excellence in,133
Adam, Robert, court architect to
George III., 11
Addington, Henry, speaker of the
House of Commons, 282 ; made First
Lord of the Treasury, 282 ; his
administration, 282 281 ; coolness
between him and Pitt, 285 286 ; their
quarrel, 287 ; his resignation, 290 112 ;
raised to the Peerage, 112 ; raised to the
Peerage, 293
Addison, Joseph, review of Miss Aikin's
life of, 321 122 ; his character, 323 321 ;
sketch of his father's life, 321 325 ; his

birth and early life, 325 327 ; appointed
to a scholarship in Magdalene College,
Oxford, 327 ; his classical attainments,
327 330 ; his Essay on the Evidences of
Christianity, 330 ; his Latin poems, 331
332 ; contributes a preface to Dryden's
Georgies, 335 ; his intention to take
orders frustrated. 335 ; sent by the
government to the Continent, 333 ; his
introduction to Boileau, 310 ; leaves
Paris and proceeds to Venice, 311 315 ;
his residence in Italy, 315 350 ;
composes his Epistle to Montague (then
Lord Halifax), 350 ; his prospects
clouded by the death of William III., 351
; becomes tutor to a young English
traveller, 351 ; writes his Treatise on
Medals, 351 ; repairs to Holland, 351 ;
returns to England, 351 ; his cordial
reception and introduction into the Kit
Cat Club, 351 ; his pecuniary difficulties,
352 ; engaged by Godolphin to write a
poem in honour of Marlborough's
exploits, 351 355 ; is appointed to a
Commissionership, 355 ; merits of his
"Campaign," 356 ; criticism of his Travels
in Italy, 329 359 ; his opera of
Rosamond, 361 ; is made Undersecretary
of State, and accompanies the Earl of
Halifax to Hanover, 361 302 ; his election
to the House of Commons, 362 ; his
failure as a speaker, 362 ; his popularity
and talents for conversation, 365 367 ;
his timidity and constraint among
strangers, 367 ; his favorite associates,
368 371 ; becomes Chief Secretary for

Ireland under Wharton, 371 ; origination
of the Tatler, 373 371 ; his characteristics
as a writer, 373 378 ; compared with
Swift and Voltaire as a master of the art
of ridicule, 377 379 ; his pecuniary
losses, 382 383 ; loss of his
Secretaryship, 382 ; resignation of his
Fellowship, 383 ; encouragement and
disappointment of his advances towards
a great lad 383 ; returned to Parliament
without a contest, 383 ; his Whig
Examiner, 384 ; intercedes with the
Tories on behalf of Ambrose Phillipps and
Steele, 384 ; his discontinuance of the
Tatler and commencement of the
Spectator, 384 ; his part in the Spectator,
385 ; his commencement and
discontinuance of the Guardian, 389 ; his
Cato, 345 390 394 365 366 ; his
intercourse with Pope, 394 395 ; his
concern for Steele, 396 ; begins a new
series of the Spectator, 397 ; appointed
secretary to the Lords Justices of the
Council on the death of Queen Anne. 397
; again appointed Chief Secretary for
Ireland, 399 ; his relations with Swift and
Tickell, 399 400 ; removed to the Board
of Trade, 401 ; production of his
Drummer, 401 ; his Freeholder, 402 ; his
estrangement from Pope, 403 404 ; his
long courtship of the Countess Dowager
of Warwick and union with her, 411 412 ;
takes up his abode at Holland House,
412 ; appointed Secretary of State bv
Sunderland, 413 ; failure of his health,
413 418 ; resigns his post, 413 ; receives

a pension, 414 ; his estrangement from
Steele and other friends, 414 415 ;
advocates the bill for limiting the number
of Peers, 415 ; refutation of a calumny
upon him, 417 ; intrusts his works to
Tickell, and dedicates them to Greggs,
418 ; sends for Gay on his death-bed to
ask his forgiveness, 418 419 ; his death
and funeral, 420 ; Tickell's eulogy on his
death, 421 ; superb edition of his works,
421 ; his monument in Poet's Corner,
Westminster Abbey, 422 ; praised by
Dryden, 369
Addison, Dr. Lancelot, sketch of his
life, 325 325
Adiaphorists, a sect of German
Protestants, 7 8
Adultery, how represented by the
Dramatists of the Restoration, 357
Advancement of Learning, by Bacon,
its publication, 383
Æschines, his character, 193 194
Æschylus and the Greek Drama, 210
229
Afghanistan, the monarchy of,
analogous to that of England in the 10th
century, 29 ; bravery of its inhabitants,
23 ; the English the only army in India
which could compete with them, 30 ;
their devastation in India, 207

Agricultural and manufacturing
laborers, comparison of their condition,
145 148
Agitjari, the singer, 256
Aiken, Miss, review of her Life of
Addison, 321 422
Aix, its capture, 244
Akenside, his epistle to Curio, 183
Albigenses, 310 311
Alcibiades, suspected of assisting at a
mock celebration of the Eleusinian
mysteries, 49
Aldrich, Dean, 113
Alexander the Great compared with
Clive, 297
Altieri, his greatness, 61 ; influence of
Dante upon his style, 61 62 ; comparison
between him and Cowper, 350 ; his
Rosmunda contrasted with Shakspere's
Lady Macbeth, 175 ; influence of
Plutarch and the writers of his school
upon, i. 401. 401
Allahabad, 27
Allegories of Johnson and Addison,
252
Allegory, difficulty of making it
interesting, 252

Allegro and Penseroso, 215
Alphabetical writing, the greatest of
human inventions, 453 ; comparative
views of its value by Plato and Bacon,
453 454
America, acquisitions of the Catholic
Church in, 300 ; its capabilities, 301
American Colonies, British, war with
them, 57 59 ; act for imposing stamp
duties upon them, 58 65 ; their
disaffection, 76 ; revival of the dispute
with them, 105 ; progress of their
resistance, 106
Anabaptists, their origin, 12
Anacharsis, reputed contriver of the
potter's wheel, 438
Analysis, critical not applicable with
exactness to poetry, 325 ; but grows
more accurate as criticism improves, 321
Anaverdy Khan, governor of tlie
Carnatic, 211
Angria, his fortress of Gheriah reduced
by Clive, 228
Anne, Queen, her political and
religious inclinations, 130 ; changes in
her government in 1710, 130 ; relative
estimation bv the Whigs and the Tories
of her reign, 133 140 ; state of parties at
her accession, v. 352, 352 353 ;
dismisses the Whigs, 381 382 ; change

in the conduct of public affairs
consequent on her death, 397 ; touches
Johnson for the king's evil, 173 ; her
cabinet during the Seven Years' War, 410
Antijacobin Review, (the new), vi. 405;
contrasted with the Antijacobin, 400 407
Antioch, Grecian eloquence at, 301
Anytus, 420
Apostolical succession, Mr. Gladstone
claims it for the Church of England, 100 ;
to 178. 178
Apprentices, negro, in the West Indies,
307 374 370 378 383
Aquinas, Thomas, 478
Arab fable of the Great Pyramid, 347
Arbuthnot's Satirical Works, 377
Archimedes, his slight estimate of his
inventions, 450
Archytas, rebuked by Plato, 449
Arcot, Nabob of, his relations with
England, 211 219 ; his claims recognized
by the English, 213
Areopagitiea, Milton's allusion to, 204
Argyle, Duke of, secedes from
Walpole's administration, 204
Arimant, Dryden's, 357

Ariosto, 60
Aristodemus, 2 303
Aristophanes, 352 ; his clouds a true
picture of the change in his countrymen's
character, 383
Aristotle, his authority impaired by the
Reformation, 440 ; the most profound
critic of antiquity, 140 141 ; his doctrine
in regard to poetry, 40 ; the
superstructure of his treatise on poetry
not equal to its plan, 140
Arithmetic, comparative estimate of, by
Plato and by Bacon, 448
Arlington, Lord, his character, 30 ; his
coldness for the Triple Alliance, 37 ; his
impeachment, 50
Armies in the middle ages, how
constituted, 282 478
a powerful restraint on the regal
power, 478 ; subsequent change in this
respect, 479
Arms, British, successes of, against the
French in 1758, 244 247
Army, (the) control of, by Charles I., or
by the Parliament, 489 ; its triumph over
both, 497 ; danger of a standing army
becoming an instrument of despotism,
487

Arne, Dr., set to music Addison's opera
of Rosamund, 361
Arragon and Castile, their old
institutions favorable to public liberty iii.
80. 80
Arrian, 395
Art of War, Machiavelli's, 306
Arundel, Earl of, iii. 434
Asia, Central, its people, 28
Asiatic Society, commencement of its
career under Warren Hastings, 98
Assemblies, deliberative, 2 40
Assembly, National, the French, 46 48
68 71 443 446
Astronomy, comparative estimate of by
Socrates and by Bacon, 452
Athenian jurymen, stipend of, 33 ;
note; police, name of, i. 34, 34 ; note;
magistrates, name of, who took
cognisance of offences against religion, i.
53, 139 ; note.; orators, essay on, 139
157 ; oratory unequalled, 145 ; causes of
its excellence, 145 ; its quality, 151 153
156
Johnson's ignorance of Athenian
character, 146 418 ; intelligence of the
populace, and its causes, 140 149 ;
books the least part of their education,
147 ; what it consisted in, 148 ; their

knowledge necessarily defective, 148 ;
and illogical from its conversational
character, 149 ; eloquence, history of,
151 153 ; when at its height, 153 154 ;
coincidence between their progress in
the art of war and the art of oratory, 155
; steps by which Athenian oratory
approached to finished excellence
extemporaneous with those by which its
character sank, 153 ; causes of this
phenomenon, 154 ; orators, in
proportion as they became more expert,
grew less respectable in general
character, 155 ; their vast abilities, 151 ;
statesmen, their decline and its causes,
155 ; ostracism, 182 ; comedies,
impurity of, 182 2 ; reprinted at the two
Universities, 182 ; iii. 2. 2
"Athenian Revels," Scenes from, 30 ;
to: 54
Athenians (the) grew more sceptical
with the progress of their civilization, 383
; the causes of their deficiencies in
logical accuracy, 383 384
Johnson's opinion of them, 384 418
Athens, the most disreputable part of,
i. 31, note ; favorite epithet of, i. 30, 30 ;
note; her decline and its characteristics,
153 154 Mr. Clifford's preference of
Sparta over, 181 ; contrasted with
Sparta, 185 187 ; seditions in, 188 ;
effect of slavery in, 181 ; her liturgic
system, 190 ; period of minority in, 191

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