2.2.2 Disadvantages of Vector Graphics

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Contents 

Acknowledgements...........................................................................................................i 

1. Introduction ...............................................................................................................1 

2. What are Computer Graphics?..................................................................................3 

2.1. The Nature of Computer Graphics...........................................................3 

2.2. Vector Graphics .......................................................................................3 

2.2.1. Advantages of Vector Graphics...................................................4 

2.2.2. Disadvantages of Vector Graphics ..............................................4 

2.2.3. Uses of Vector Graphics ..............................................................5 

2.3. Bitmapped Graphics.................................................................................5 

2.3.1. Resolution and Colour in Bitmaps...............................................5 

2.3.2. Advantages of Bitmaps................................................................5 

2.3.3. Disadvantages of Bitmaps............................................................6 

2.3.4. Lookup Tables, Colour and Greyscale ........................................6 

2.4. File Compression .....................................................................................7 

2.4.1. Run Length Encoding (RLE).......................................................8 

2.4.2. Huffman Coding ..........................................................................8 

2.4.3. Other Compression Methods .......................................................9 

2.4.4. Lossless v Lossy Compression ....................................................9 

2.5. Graphics File Formats and Standards ......................................................9 

3. Graphics Hardware ...................................................................................................13 

3.1. Memory Issues.........................................................................................13 

3.1.1. Memory Requirements of Vector v Bitmapped Graphics ...........13 

3.1.2. Disk Storage.................................................................................14 

3.1.3. Computer Memory (RAM)..........................................................15 

3.2. Monitors...................................................................................................15 

3.2.1. CRT Displays...............................................................................15 

3.2.2. Liquid Crystal Displays ...............................................................16 

3.2.3. Video Display Standards .............................................................17 

3.3. Video Cards .............................................................................................18 

3.4. Colour Printers.........................................................................................19 

3.4.1. Problems of Colour Printing ........................................................19 

3.4.2. Types of Colour Printer ...............................................................20 

3.5. Limitations of Colour Output ..................................................................21 

3.6. Colour Scanners.......................................................................................22 

4. Colour ........................................................................................................................23 

4.1. What is Colour? .......................................................................................23 

4.2. The Human Visual System ......................................................................24 

4.3. The Perception of Colour and Brightness................................................25 

4.4. Colour Models .........................................................................................25 

4.4.1. Additive and Subtractive Colours................................................25 

4.4.2. The CIE Diagram.........................................................................26 

4.4.3. Red, Green and Blue (RGB) ........................................................27 

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Understanding IT: Computer Graphics University of Hull

Contents 

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4.4.4. Hue, Light and Saturation (HLS).................................................27 

4.4.5. Hue, Saturation and Value (HSV) ...............................................28 

4.4.6. Cyan, Magenta, Yellow and Black (CMYK) ..............................29 

4.4.7. Other Colour Models ...................................................................29 

4.5. The Use of Colour....................................................................................30 

4.5.1. Lighting and Backgrounds...........................................................30 

4.5.2. Warm and Cool Colours ..............................................................30 

4.5.3. Colour Deficiency........................................................................31 

5. Graphics Packages ....................................................................................................33 

5.1. Popular Microcomputer Graphics Packages............................................33 

5.1.1. Painting and Drawing ..................................................................33 

5.1.2. Presentation..................................................................................34 

5.1.3. Photography .................................................................................35 

5.1.4. Graphics Utilities .........................................................................36 

5.1.5. Animation ....................................................................................36 

5.2. Incorporating Graphics into Applications and Documents......................37 

5.2.1. Programming Languages and Authoring Tools...........................37 

5.2.2. Desktop Publishing (DTP)...........................................................38 

5.2.3. Placing Graphics into Non-Graphics Files ..................................39 

6. Computer Graphics in Higher Education..................................................................41 

Glossary ...........................................................................................................................43 

Annotated Bibliography...................................................................................................47 

Index.................................................................................................................................51 

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Understanding IT: Computer Graphics University of Hull

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Acknowledgements 

This book was written at the University of Hull under the auspices of the 

Information Technology Training Initiative (ITTI) project Multimedia-based IT 

Training for the Humanities. ITTI is an initiative of the Information Systems 

Committee of the Higher Education Funding Councils. 

I would like to thank all the people who have helped me, directly or indirectly, 

in this project, in particular Dr Lorraine Warren for her role in the genesis of this 

book, Richard Hicks for his technical advice, James Willmott for introducing me to 

cyberspace (and providing landmarks), and Jenny Parsons upon whom I inflicted 

early drafts for review and criticism. I also wish to thank all the staff at the University 

of Hull Computer Centre for all the help and invaluable expertise they have given me 

in this and other projects, and the staff at the Language Centre for giving me a home 

and cups of tea. 

Registered Trademarks 

The following table lists the registered trademarks used in this work: 

Trademark Company 

PostScript Adobe 

MacPaint Apple 

Macintosh Apple 

GIF CompuServe 

Corel Draw! Corel Corporation 

Windows Microsoft 

Pantone Pantone Inc 

Harvard Graphics Software Publishing Corporation 

Targa Truevision Inc. 

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Understanding IT: Computer Graphics i University of Hull

Acknowledgements 

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Understanding IT: Computer Graphics ii University of Hull

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Chapter One 

Introduction 

We are all familiar with computer graphics, in the sense that we see them 

everyday: on television, in films, in books and magazines, on posters, and - of course 

- on our computer monitors. They are so ubiquitous that we no longer pay them any 

special heed. 

Yet, even though computer graphics surround us, most of us know very little 

about them. What are they? How can they be created and edited? How can we use 

them? Why should we use them? Even people who use graphics packages are often 

unaware of the nature of the graphics they are manipulating and are unable to 

understand, for example, the difference between metafiles and bitmaps, or why 

resizing a picture can distort and degrade it, or how to use the vast range of image 

effects that are supplied with today's packages, and so on. 

This publication attempts to answer such common questions and to thus enable 

the reader to understand computer graphics and use them (more) effectively. It is not 

a text for programmers who want to write Assembler routines to decode PCX files, or 

for people looking for Bezier curve algorithms. Rather, it is aimed at the 'average' 

user with at least a basic level of computer literacy - that is, you should know the 

meanings of terms such as processor and operating system - and no previous 

knowledge of computer graphics is assumed. Instead, the information within is 

biased towards the practical, so that the reader can learn about, say, colour models, 

then attempt to apply that knowledge in their favourite graphics package. The 

emphasis is also upon generic information which can be applied whichever package 

you use, rather than specific instructions as to how to carry out operations in a 

particular application. 

This work is slanted heavily towards microcomputers, in particular PCs and 

Apple Macintoshes. There are two main reasons for this. Firstly, the ordinary user is 

much more likely to have access to a humble micro than a Sparc workstation; and 

secondly, it is highly probable that users of high-end graphics workstations are 

familiar with computer graphics concepts already and will gain little from a 

beginners' text. Nevertheless, much of the information in the following pages is 

applicable whatever your platform. 

Chapter Two, What are Computer Graphics?, answers that basic question by 

looking at the two different types of graphic - vector and bitmap (raster) - and the 

associated pros and cons. Much of the Chapter is devoted to bitmapped graphics, 

particularly the issues of memory, disk storage and file compression. 

Chapter Three, Graphics Hardware, looks at the hardware requirements of 

computer graphics. It explores the topics of memory (including video memory) and 

disk storage in further detail than Chapter Two, explains how graphics are output on 

both monitors and colour printers, and finally considers colour scanners. 

Chapter Four, Colour, looks at the human visual system and the way we 

perceive colour, goes on to consider some of the colour models in common use, and 

ends with some brief guidelines on how colour should be used. 

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Understanding IT: Computer Graphics 1 University of Hull

Introduction 

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Chapter Five, Graphics Packages, looks at the types of applications available in 

the micro market both to create graphics and to incorporate them within documents 

and applications. 

Chapter Six, Computer Graphics in Higher Education, is a short section which 

considers some of the possible educational uses to which graphics can be put. 

At the end of the work there is a glossary, an annotated bibliography, and an 

index to help the interested reader dig deeper into the field of computer graphics. 

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Chapter Two 

What are Computer Graphics? 

This Chapter looks at the nature of computer graphics and considers the issues 

related to the storage of computer graphics in memory and on disk, including the 

important topics of image compression and graphics file formats. 

2.1 The Nature of Computer Graphics 

A computer graphic is really nothing more than an image represented by a 

computer, usually on screen and sometimes on a printout. The image may come from 

the real world - such as a photograph or a drawing that has been digitised (converted 

into computer-readable form) - or it may have been generated in a computer using 

graphics software. In essence, a computer graphic is no different from an ordinary 

picture on paper, at least in appearance; however, being stored in digital form bestows 

many advantages on an image. It can be: 

• copied freely and stored safely on disk 

• distributed with ease, either on disk or by data transmission along 

communications lines 

• manipulated in literally hundreds of different ways by software 

• incorporated into documents such as reports and publications (Desktop 

Publishing, or DTP) 

• archived in image libraries 

• output to a wide variety of devices, particularly monitors, TVs and printers 

The vast majority of the images we see today - in books or magazines, on 

advertising hoardings, on television - are, or at some time have been, computer 

graphics. All graphics fall into two broad categories: vector graphics and bitmapped 

graphics, the difference between the two being the method of storing the image data. 

2.2 Vector Graphics 

Vector images are composed of objects. All objects are built up from primitives 

- basic drawing instructions such as line, rectangle and ellipse - and objects may be 

grouped together to form new composite objects to form an object hierarchy. 

Consider, for example, a picture of an aircraft in vector format (Figure 2.1). 

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Figure 2.1 Figure 2.1a 

Vector image of aircraft Partially disassembled vector image 

At its simplest level it is composed of primitives such as circles, lines and 

rectangles; however, it can also - and more usefully - be represented as a collection of 

objects (wheels, engines, wings, doors, etc - Figure 2.1a) which can be composed of 

primitives and objects. So, for instance, the wing object is composed of component 

objects (ailerons, flaps, engine) which in turn contain other objects (the engine 

contains a propeller, fuel feed, etc) and so on down the hierarchy until primitives are 

reached. The wing object itself, of course, is part of the overall aircraft object. In a 

drawing package objects can be aggregated and disassembled at will, giving the 

designer of the graphic considerable flexibility. 

2.2.1 Advantages of Vector Graphics 

Vector graphics are highly flexible in terms of image manipulation: they can be 

resized in any direction and to any magnitude without loss of quality (although if 

scaled by different amounts in the horizontal and vertical directions some distortion 

of proportion will occur) and their constituent objects and primitives may also be 

scaled or moved at will. Vector images are also very cheap in terms of memory as the 

image data is simply a set of graphical instructions to the computer - eg 

line(x1,y1,x2,y2), circle(x,y,radius) - together with their parameters (or operands) - 

x1, y1, radius - and any associated colour data, all of which is coded as a small set of 

numbers which take up very little disk space, enabling a complex image to be stored 

in a file only a few tens of kilobytes in size. 

2.2.2 Disadvantages of Vector Graphics 

All vector graphics are computer-generated - by definition - and thus are rarely, 

if ever, truly accurate representations of real-world objects. It would be extremely 

impracticable - and in all likelihood impossible - to draw a vector image of, say, an 

oak tree which incorporated all the whorls and knarls within the trunk and the 

intricate structure of branches. It would certainly be possible to draw a schematic oak 

to show that all oaks have similar overall shapes, the same leaf and the same fruit and 

often this would be all that was necessary, but it would contain only a fraction of the 

visual information present in a photograph of a particular oak 1 . Similarly, whilst a 

vector image of the Mona Lisa would be recognisable as a representation of 

1 Advanced graphics packages allow the 'vectorisation' of bitmapped images so that the photo of the oak could be 

traced into a vector form. Nevertheless, the real-world image is still created outside the computer. 

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Leonardo's work it would not be capable of storing the level of detail that exists in the 

original portrait. 

2.2.3 Uses of Vector Graphics 

Vector graphics find their metier in technical areas such as CAD/CAM 

(Computer-Aided Design/Manufacture), scientific modelling, and architecture where 

the ability to manipulate parts of a graphic - moving, copying, deleting, resizing, etc - 

is of high importance. Vector graphics are also increasingly being used in the 'graphic 

art' world, with the advent of sophisticated off-the-shelf vector-based graphics 

applications such as Corel Draw! and Harvard Graphics. 

2.3 Bitmapped Graphics 

Bitmaps - sometimes known as raster graphics - are images composed of 

discrete dots known as picture elements or pixels (Plates 1, 1a), each of which can be 

any colour within a specified range of colours. Bitmaps can be created on the 

computer but most are real-world images in digital form, such as satellite 

photographs. 

2.3.1 Resolution and Colour in Bitmaps 

The resolution of a bitmap is determined by its horizontal and vertical 

dimensions measured in pixels. Thus a 640 x 480 bitmap displayed on a standard 

VGA monitor will look better than a 320 x 200 bitmap displayed in the same area, 

although it will be inferior to a 1024 x 768 bitmap. The simple principle is that the 

greater the number of pixels per unit area the better the resolution and the fewer 

visual imperfections in the picture. 

The colour depth2 of a bitmap is determined by the amount of memory allocated 

to each pixel. Once again, there is a simple principle, which is that the number of 

colours that can be displayed is given by 2 to the power of the number of bits 

available per pixel. With 4 bits (a ‘nibble’!) per pixel there are a possible 24 = 16 

colours, with 8 bits (a byte) a possible 28 = 256 colours, and the most natural-looking 

results are obtained with 24-bit colour which can display up to 224 = 16.7 million 

colours per pixel. Naturally, high-resolution bitmaps can only be displayed in their 

full glory on appropriate hardware with sufficient video card memory (video RAM, or 

VRAM - see Chapter 3). However, it is usually possible to display any resolution of 

bitmap on the humblest 16-colour systems (such as the base VGA standard) although 

of course the greater the gap between image resolution and system capability the 

greater the degradation in image quality. 

2 Sometimes known more technically as the number of bit planes. 

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2.3.2 Advantages of Bitmaps 

The bitmap format is ideal for detailed art and real-world images. Bitmaps can 

potentially store immense amounts of information (as reflected by the large file sizes) 

and can be edited in great detail, even to adjusting the colour of individual pixels. 

Artistic brush and smear effects can be applied to simulate 'real' painting. 

Quite sophisticated effects can be achieved on bitmaps with the right software. 

Image Processing (IP) techniques can be applied to sharpen or smooth details, adjust 

contrast and brightness, apply different coloured filters to the image, detect edges and 

thresholds, remove noise, and so on. This sort of image enhancement is frequently 

used by, say, geologists or archaeologists on aerial or satellite photos in order to 

reveal structures on the ground, and such advanced IP tools are now available to 

ordinary users of graphics packages. 

2.3.3 Disadvantages of Bitmaps 

It is difficult to resize bitmaps without image degradation or information loss. 

Enlarging a bitmap means that new pixels have to be created so as not to leave blank 

spots, and the colour of each new pixel is commonly based on the colour of its 

neighbours - this usually results in a blocky, unnatural appearance (Plate 2). 

Reducing a bitmap involves discarding pixels, which necessarily results in loss 

of information and detail. As with vector images distortion will also occur if the 

image is not scaled equally in the horizontal and vertical dimensions because the 

elements of the picture will no longer be in the proper proportion to each other (Plate 

2a). 

Bitmaps can be quite expensive in terms of memory as the value of each 

individual pixel is recorded: the size of the bitmap in bytes is the product of the 

horizontal and vertical dimensions (in pixels) and the number of bytes per pixel. For 

example, a 256-colour (ie 1 byte/pixel) bitmap of dimensions 640 x 480 pixels (the 

same size as a standard VGA monitor) will take up 640 x 480 x 1 = 307,200 bytes. 

As the colour depth of the image is improved so does the memory requirement: a 'true 

colour', 24-bit (ie 3 bytes/pixel) image (such as a photograph) of the same size will 

take up 3 times as much memory, or 921,600 bytes. Moreover, 640 x 480 is quite a 

low resolution for the human eye, and truly realistic images can require resolutions in 

the region of 3000 x 2000 x 24-bit ≈ 18MB of data. 

2.3.4 Lookup Tables, Colour and Greyscale 

Pixels have numerical values associated with them, and when these numbers are 

interpreted by a display system - a monitor, or a printer, say - particular colours are 

produced. The values are interpreted as different intensity levels for each of the three 

electron guns of a monitor - red, green and blue. With a 24-bit system one byte (8 

bits) is allocated to each gun, allowing 2 8 = 256 intensity levels for each primary 

colour, and three bytes - representing the levels for each gun - make up a pixel. 

The full range of colours available on a display system is known as its palette, 

but often only subsets of the palette are desirable or even possible. In high-end truecolour 

systems the user may wish to restrict the range of colours available for 

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different types of work - pastel shades for artistic work, or saturated colours for 

presentations, say - and in less expensive systems the realisable colour range is 

usually smaller than the palette. To enable these subsets to be used a Lookup Table 

(LUT) is kept in memory comprising as many entries as there are pixel values, each 

entry containing a value corresponding to a screen colour (Figure 2.2). 

Pixel in Video Memory 

(Framestore) 

47 

255 

48 

47 

46 

Figure 2.2 

Colour Lookup Table in 256-colour (8-bit) display system 

0 

Lookup Table Monitor 

137 

136 

135 

Pixel of colour 136 

The pixel value in the image is used as an index into the LUT, so that a pixel of 

value 47 would cause the colour in the 47th entry in the LUT to be displayed rather 

than colour number 47 - in the example shown in Figure 2.2 this would be 136. Both 

the values in the entries and the entry point to the LUT can be changed at will which 

can result in very different colours being displayed with the same pixel values: in this 

way the visual appearance of an image can be changed without altering the actual 

image data. LUTs are used extensively in graphics applications both to enable userdefined 

colour subsets (which, confusingly, are usually called 'palettes') and for 

special effects. 

Because pixels are only represented by values, there is no reason why they 

should display colour at all when it is not necessary. Often, for scientific purposes, 

colour is an artificial distraction, and it can be more informative to see an image in 

greyscale, where the different pixel values are interpreted as lying on a monochrome 

scale ranging from pure white to pure black; satellite data is often recorded and 

manipulated in greyscale. Whilst colours are sometimes added to enhance certain 

features of the image to the human eye these are purely artificial and the result is 

known as a false colour image. 

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2.4 File Compression 

As bitmaps followed hardware advances in becoming more colourful and 

detailed, so the mushrooming memory requirements for these images spurred the 

development of file compression methods. Compression is the process of eliminating 

redundant data in a file so as to reduce its size, and advances in compression 

techniques have taken on crucial - if not determining - importance as the field of 

computer graphics has advanced into true-colour, high-resolution images and moving 

video. It would, for instance, be practically impossible to store video sequences on 

disk without sophisticated file compression techniques being applied to the data 3 . 

There are a wide range of compression algorithms in use today, with more 

being developed every year, which are not only used for graphics files but also for 

'ordinary' data and program files. It is now commonplace for commercial applications 

to be distributed on disk in compressed form. The subject of data compression is large 

and often highly technical and is of little interest to ordinary users of graphics, who 

are only concerned that their images be reduced to a manageable size. However, 

consideration of two of the simplest and most common compression methods used in 

the field of computer graphics will illustrate the redundancy to be found in many 

bitmapped images and the advantages of compression. 

2.4.1 Run Length Encoding (RLE) 

Consider a - 256-colour, for convenience - bitmap, large areas of which are the 

same colour. The 'raw' method of storing such an image is to allocate one byte per 

pixel, that byte containing the numerical value of the pixel, (58, say); thus, a 640 x 

480 pixel bitmap would be stored as 640 x 480 x 1 = 307,200 bytes. However, in 

images with uniform areas of colour - that is, adjacent pixels having the same value - 

a more efficient storage method is to find sets of adjacent pixels of the same value 

and store each set as a pair of bytes, one byte of the pair being the number of pixels in 

the set and the other the pixel value. For example, consider the following sequence of 

pixels from a bitmap: 

57 57 57 57 57 57 110 110 110 132 55 200 200 200 200 200 200 200 200 200 200 

Ordinarily, these 22 values would require 22 bytes for storage; however, using 

RLE they can be encoded as follows: 

{5,57} {2,110} {0,132} {0,55} {9,200} 

That is, 6 - computers count from zero! - bytes of 57, then 3 of 110, then 1 of 

132, then 1 of 55, then 10 of 200, and so on. Note that RLE has reduced the data from 

21 to 10 bytes, a reduction of over 50%, and reductions of 90%+ are possible with 

3 A 100MB hard disk would only be able to hold 13 seconds of full-screen, full-motion video, and a 600MB 

CDROM would be filled by 78 seconds. 

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suitable images. The downside to RLE is that, when used on complex bitmaps where 

adjacent pixels are rarely the same colour, it can actually produce a larger file. 4 

2.4.2 Huffman Coding 

This is a statistical compression method based on the frequency of occurrence 

of pixel values. The bitmap is analysed to produce a table consisting of each value 

and the number of times it occurs, then binary codes are allotted to each value such 

that the shortest codes belong to the most frequently occurring values. For example, 

the following table might represent the first four rows of a frequency analysis of a 

bitmap: 

Pixel Value Frequency of Occurrence Code (binary) 

54 132 0 

22 84 01 

112 57 10 

243 33 11 

Once the analysis is complete, the pixel values are replaced by the binary codes, 

so the following sequence might be replaced by the bit sequence below it: 

112 243 243 54 54 54 22 22 

10 11 11 0 0 0 01 01 

thus replacing an 8-byte sequence with a stream of 13 bits (1011110000101), an 

80% compression. Huffman coding works best with bitmaps with a small range of 

values; the larger the range the larger the binary code to represent each value, until 

the situation is reached where the code is bigger than the value. 

2.4.3 Other Compression Methods 

More complex methods include LZW (Lempel-Ziv-Welch), DCT (Discrete 

Cosine Transform) and the intriguing Fractal Compression which reduces complex, 

real-world images (such as photos) by decomposing them into 'fractals' (objects with 

'fractional dimensions') which can be described by matrices of numbers called affine 

transformations 5 . 

2.4.4 Lossless v Lossy Compression 

All of the compression methods described thus far are lossless - that is, no 

image data is lost during compression. Lossy methods also exist, which rely on the 

inability of human eyes to spot slight image degradation, and thus trade some slight 

data loss for efficiency and greater compression; the JPEG method (see next section) 

4 RLE is used by the PCX file format, and an interesting experiment in the efficacy of the compression can be 

carried out by opening BMP files - which contain raw, uncompressed image data - in Windows Paintbrush, 

saving them as PCX files, then comparing the different file sizes for the same image. 

5 A detailed description of fractal compression can be found in Peterson[1988], pp 128-132 

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allows for 'lossy' compression of images which are to be viewed by people, rather 

than processed by computers. 

2.5 Graphics File Formats and Standards 

There are a bewildering plethora of methods by which graphics may be stored 

and/or compressed, and this is reflected in the large number of graphics file formats 

available. This used to be a major headache for computer users who wished to 

incorporate graphics into their work, whether they were applications programmers or 

secretaries, because in the early days of computer graphics these formats were 

incompatible. A graphic produced in one drawing package couldn't be imported into 

another drawing package which didn't support the particular format that the creating 

package used unless an - often temperamental - conversion program was employed. 

This situation created great pressure both for the development of common 

graphics standards and for applications programmers to build a wide variety of 

graphics filters (conversion utilities) into their applications. Whilst there are still a 

large number of formats tied to particular products (eg Corel Draw! .CDR files) Table 

2.1 below lists the most common formats which are largely independent of particular 

packages: 

Bitmap Vector 

BMP (Windows Paintbrush) CGM (Computer Graphics Metafile) 

GIF (Compuserve Graphics Interchange 

Format) 

DXF (Computer-Aided Design) 

MAC (MacPaint - monochrome only) WMF (Windows Metafile) 6 

PCX (PC Paintbrush) 

PICT (Macintosh) 

TGA (Targa - 24-bit) 

TIFF (Tagged Image File Format) 

JPEG (Joint Photographic Experts Group) 

Table 2.1 

Common graphics file formats 

Although formats such as PCX and TGA were designed for particular products 

(PC Paintbrush and the Targa video card respectively) they have gained widespread 

acceptance in the marketplace; others, such as CGM and TIFF, were designed by 

groups independent of particular firms, and have been approved by the ISO 

(International Standards Organisation). New formats are constantly being developed 

by both companies and bodies to cater for the rapid advances in graphics-related 

hardware and software, an example being MPEG (Motion Picture Experts Group), 

designed for digital moving video 7 . 

6 Whilst this format is dependent upon the Microsoft Windows environment it is independent of any particular 

Windows application. 

7 Indeed, the whole area of graphics standards and formats is being increasingly regulated and formalised under 

the umbrella of the ISO. Now, developers of graphics packages are under pressure to build their applications 

according to international standards: GKS (Graphical Kernel System) and its successor PHIGS (Programmer's 

Hierarchical Interactive Graphics System) are prominent examples of these standards. 

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Graphics can also sometimes be found in Postscript (.PS or .EPS) format. 

Postscript is a page description language created by Adobe which has rapidly become 

a printer standard. Each Postscript file is essentially a computer program which tells a 

printer fitted with a Postscript reader how to print the page. A Postscript file created 

in any application can be printed on any output device with a Postscript reader, and 

this independence from particular hardware or software makes it an ideal general 

format. Postscript comes in a number of variants - Level 1, Level 2, Encapsulated, 

and Display - and is most often used for documents mixing text and graphics; it is not 

the ideal format for storing image data alone, particularly in bitmap format. 

The user might wonder what differentiates these formats. This book is not 

intended to cover the highly technical differences between formats (the interested 

reader is directed towards Rimmer[1990] and Kay and Levine [1992]) which are 

normally invisible to the user: often, it will make little odds whether, say, a bitmap is 

stored as PCX or TIFF. There are, however, general points which should be born in 

mind: 

• BMP files, unlike nearly all other bitmapped formats, are not compressed so in 

the majority of cases a BMP file will be larger - often substantially so - than, 

say, a PCX file containing the same image. This is obviously disadvantageous 

in terms of disk space, but can be a plus when viewing bitmaps on a slow 

computer, as viewing a compressed file requires the computer's main processor 

to work on the decompression. 

• Of the above formats, only PICT and DXF are unable to support 24-bit colour. 

• TIFF files can come in different 'flavours' - that is, some applications add little 

'tweaks' to the TIFF files they generate which can make them unreadable to 

other applications - this is inherent in the design of TIFF, which contains a 

number of 'hooks' onto which new methods of encapsulation or compression of 

graphic data can be attached. Applications written prior to any new methods 

may thus be unable to read more up-to-date TIFF files. Similarly, the JPEG 

format contains a number of options which can result in readability and 

compatibility problems 

This plethora of standards has spawned a large number of file conversion 

utilities, enabling users to convert, say, a PCX file to TIFF: although many of these 

utilities are of dubious quality, they do have the advantage of usually being in the 

public domain, either as freeware or shareware. 

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Chapter Three 

Graphics Hardware 

This Chapter considers the hardware issues involved in computer graphics. It 

explores the sort of hardware required to display, print and scan graphics in 

microcomputer systems, after having first looked at the question of memory and 

storage. 

Hardware is of crucial importance to graphics, probably more so than software: 

the cleverest algorithms in the world are of no use if the hardware is incapable of 

displaying the results. Indeed, it is primarily technical advances in hardware that have 

brought ordinary users into the field of high quality computer graphics which was 

previously the sole province of professional designers. 

3.1 Memory Issues 

Generating computer graphics places great demands on a system in terms of 

memory, particularly in the case of bitmapped graphics. Until relatively recently, 

dynamic memory - that is, random access memory (RAM) - has been at a premium in 

the world of the desktop computer; the original PC XT, for example, only had 1M of 

RAM, of which only 640k was available for application programs. Similarly, only 

during the second half of the 1980s did microcomputers acquire large amounts of disk 

storage space as hard disks became larger, faster, and above all cheaper. A positive 

feedback cycle exists whereby hardware advances in memory allow more complex 

software to be written which then encourages the hardware manufacturers to enhance 

their product, and so on. 

The reason why graphical images take up a lot of memory is, simply, because 

they contain a lot of information. The old saying that "a picture says a thousand 

words" probably underestimates the information content of a detailed image such as a 

photograph by one or two orders of magnitude. For example, a colour picture of a 

landscape may contain information relating to the geology and geomorphology of the 

scene, the weather and season when the photo was taken, and past and present social, 

economic and cultural uses of the land. To transfer all the information in an image of, 

say, Dentdale in the Yorkshire Dales to text would take many pages, even in the most 

concise writing. 

3.1.1 Memory Requirements of Vector v Bitmapped 

Graphics 

As noted in the previous Chapter, vector-based images require much less 

memory than bitmaps because they are stored as sets of objects, each object 

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containing other objects and/or graphical primitives, such as lines and rectangles: in 

effect, a vector graphic is no more than a collection of drawing instructions which can 

be stored in a relatively small number set. So, the graphic does not store the 

information about the individual elements of the screen - pixels - but rather the 

instructions to reconstitute objects: the Olympic flag, for example, would be stored in 

vector format as: 

a rectangle 

the top left and bottom right rectangle coordinates 

five circle instructions 

the centre and radius of each circle 

the colour of each circle 

A bitmap of the flag, however, would contain the value of each pixel in the 

image, even though much of that data is redundant. In an experiment such an image 

was drawn and saved in vector and bitmapped formats: the vector file was a mere 315 

bytes in size, compared to the 65k the bitmap took up. Of course, the bitmap can be 

compressed - a PCX file of the same image was only 9k in size - but nevertheless this 

example illustrates the point that, for images composed of objects the vector graphic 

format is most appropriate. 

3.1.2 Disk Storage 

Plainly bitmaps require large amounts of disk space. This is - and has always 

been - a problem in the field of graphics, which is addressed on two fronts: hardware 

and software. 

On the hardware side, the amount of data that can be stored on magnetic, floppy 

disk has steadily increased over time until, at the time of writing, 1.44 MB floppy 

disks are standard and 2.88 MB disks are just emerging. However, magnetic media 

are no longer sufficient to hold the vast amounts of data required by modern 

graphically-intensive applications and true colour 8 bitmaps, and thus optical media 

are rapidly coming into their own. 

Optical disks use visible laser light to read and write data from and to the disk. 

The most common optical disk is the CDROM (Compact Disk Read-Only Memory) 

with a data capacity of around 600MB. CDROMs are rapidly becoming the standard 

for the distribution of large applications and graphics because of their low production 

costs, robustness, and the increasing availability of cheap CDROM drives which will 

certainly become standard on all desktop computers in the very near future. Another 

read-only optical disk is the WORM (Write Once Read Many times): this is usually 

used for large-scale archiving, and is unlikely to impact the desktop market for quite 

some time. Very recently a number of read/write optical and magneto-optical disks 

have arrived promising astonishing data densities - 1.3 Gigabytes on a 5.25" disk! 9 - 

and one of these new optical technologies may well become a standard in the future 10 . 

8 This term usually refers to 24-bit colour. 

9 "A New Phase for the Floppy", Personal Computer World, Vol 16 No 4, April 1993, pp 457-8. 

10 An early and quite common form of optical disk, still in use today, is the Videodisc. However, this is used to 

hold video in analogue form and is unsuitable for storing digital data. 

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Software solutions to the problem of large bitmaps take the form of 

compression algorithms (see the section on File Compression in the previous 

Chapter). These are methods of removing redundant data from the bitmap in order to 

reduce its size and on the whole are very successful: nearly all the common bitmap 

file formats utilise one or more compression methods. However, the efficacy of 

compression declines as the complexity of the image increases, unless one is willing 

to accept some degradation of the picture; nevertheless, compression is an invaluable 

tool for reducing the majority of bitmapped images to manageable sizes. 

3.1.3 Computer Memory (RAM) 

For the same reason that they place great demands on disk space, bitmapped 

graphics also require large amounts of working memory, or RAM (Random Access 

Memory), both to display the image and to manipulate it. Display memory is known 

as Video RAM (VRAM), a special fast type of RAM purely dedicated to the video 

display; this will be discussed in section 3.3 on Video Cards. 

For manipulation purposes - editing, adding special effects, etc - the image, or 

portions of it, are loaded into RAM from disk. Until relatively recently this was a 

distinct problem as not only was RAM quite expensive in relation to the cost of disk 

space but many microcomputers had a low limit to the amount of RAM that could be 

added; however, the cost of memory chips has plummeted sharply in recent years and 

microcomputer manufacturers now incorporate substantial memory expansion 

capacity into their products. The average PC or Mac is now capable of handling highresolution 

8-bit (256 colour) images, and for relatively little extra cost can be turned 

into a full-blown true-colour graphics workstation. 

3.2 Monitors 

The computer monitor is the most commonly used output device for computer 

graphics. Virtually all modern monitors in general use build up the picture from 

horizontal lines running from the top to the bottom of the screen, each line consisting 

of a series of individual dots. They fall into two categories: Cathode Ray Tubes 

(CRTs) and Liquid Crystal Displays (LCDs) 

3.2.1 CRT Displays 

The CRT is probably the most ubiquitous electronic display device of modern 

times, being at the heart of every television set. Beams of electrons ('cathode rays') 

from three electron guns are fired through a shadow mask - a sheet of metal with 

regular apertures which focus the beam - to strike phosphor dots on the screen 

surface. There are three types of phosphor, as there are three electron guns, one for 

each primary colour - red, green and blue - and when a phosphor is struck by the 

electron beam from 'its' gun it emits its characteristic colour. The strength of this 

emission, that is the luminance of the phosphor, is proportional to the power of the 

beam, and the combination of the three phosphors at their different intensities 

produces the colour of the picture element, or pixel. The electron guns build up the 

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picture, or frame, line by line from top to bottom, and each line - or scan - is 

composed of many pixels. As the dots only phosphoresce for a short time after being 

struck by the electron beam the screen has to be redrawn, or refreshed, many times a 

second: for a flicker-free display a refresh rate of 25-30 frames per second is 

required. 

Blue 

Green 

Red 

Electron Guns 

Figure 3.1 

Colour CRT system 

R 

R 

G R B G 

G 

G 

B 

B 

R 

R 

G 

G 

B 

B 

R 

R 

G 

G 

B 

B 

Shadow Mask 

Pixel 

Figure 3.2 

Arrangement of phosphors on CRT screen 

Screen 

Red phosphor 

Green phosphor 

Blue phosphor 

Different colours are produced by assigning different voltages to each electron 

gun thus lighting phosphors to different intensities, and the colour range a monitor is 

capable of is determined by how many voltage levels can be supported by the guns. 

For example, the now obsolete EGA (Enhanced Graphics Adaptor) standard for PC 

monitors had only 4 levels per gun which could produce 64 colours (4x4x4), although 

in practice the standard dictated that only 16 colours were available at any one time 11 . 

In contrast, SVGA (Super VGA) allows for 256 voltage levels per gun producing a 

maximum 16.7 million (256 3 ) colours. Greyscale images - where the colour of a pixel 

lies in a range from white to black - are produced by assigning the same voltage to 

each gun so that each colour phosphor shines at the same intensity, resulting in as 

many shades of grey as there are gun levels. 

3.2.2 Liquid Crystal Displays 

LCD systems use long crystalline molecules ('liquid crystals') which change 

their position when an electric field is applied. An LCD display consists of a thin 

layer of liquid crystal sandwiched between two densely-packed sets of thin wires, one 

horizontal and one vertical (Figure 3.3). Together these wires form an interlocking 

grid, each intersection representing a dot on the display. This sandwich is in turn 

sandwiched by two polarising filters, again one horizontal and one vertical. The 

display is created by matrix addressing whereby each dot is addressed in turn by 

passing a current through each horizontal and vertical wire in sequence, and 

11 EGA allocated only 4 bits per pixel which meant that, at any one time, one gun had 2 bits (4 levels) and the 

others had 1 bit (2 levels) apiece, allowing only 4x2x2=16 colours. 

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whenever the combined currents at an intersection are sufficiently strong the resulting 

electric field moves the crystals at that point so that when seen through the polarising 

filters they are opaque - that is, the dot becomes dark. 

Figure 3.3 

Elements of a Liquid Crystal Display. (Adapted from Foley et al [1990].) 

Modern liquid crystal displays are backlit by an integral light source as displays 

which depend upon incident light perform poorly in low light environments, as well 

as being prone to reflection in bright environments which obscures the display. 

Colour LCDs operate on the same principle of molecules changing their 

orientation under electric fields but use three liquid crystal layers - one each for red, 

green and blue - and coloured polarising filters to generate a palette of colours. 

LCDs are capable of very high resolutions and of course require very little 

power, and are used in devices where a high-voltage CRT is inappropriate, such as 

portable computers and hand-held televisions. 

3.2.3 Video Display Standards 

Whilst there have been a plethora of display standards in the PC world, at the 

time of writing the most common are VGA (Video Graphics Adaptor) and SVGA. 

VGA allows for 16 colours at 640 x 480 resolution, or 256 at 320 x 200. The SVGA 

standard encompasses any display that is superior to VGA and allows for a number of 

permutations of resolution and colour, from the maximum 1280 x 1024 x 16.7M to 

the minimum of 640 x 480 x 256. It should be added, however, that many of the 

cheaper monitors achieve the maximum SVGA resolution by a technique known as 

interlacing, whereby every other line on the display is drawn in each frame - rather 

than every consecutive line, as is normal - meaning that it takes two complete scans to 

create a picture. The disadvantage of this is that the frame rate - the number of frames 

per second - is halved and this can lead to perceptible screen flickering. 

It is important to note that these standards are independent of the physical size 

of the monitor: SVGA has the same resolution and colour depth regardless of whether 

the monitor is 14" or 21". For this reason it is often a good idea for users who 

constantly work at high resolutions (800 x 600 or better) to obtain large monitors, 

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particularly if their work involves Desktop Publishing: a letter which is, say, 10 pixels 

vertically will look illegibly small on a 1024 x 768 display on a 14" monitor but will 

be perfectly readable at the same resolution on a 21". 

PC monitors operate in either text mode or graphics mode. In text mode - the 

default - graphics cannot be output and text is displayed on screen in a standard font 

using a hardware character generator. In order to display graphics the monitor has to 

be switched by software into graphics mode. These modes are normally invisible to 

the user as graphics applications switch into graphics mode upon startup, but they do 

need to be borne in mind by programmers. 

There is no equivalent in the Apple Macintosh world of the discrete PC 

graphics standards. Macs were designed from the outset to be graphical, unlike PCs 

which were originally text-only displays, and have consistently led PCs in terms of 

graphics capabilities; only very recently has the PC monitor attained parity with that 

of the Mac. 

3.3 Video Cards 

Given that present-day monitors are capable of high-resolution true colour, the 

only restricting factors for graphical displays are: 

• the amount of memory available for the display 

• the speed at which the display 'redraws' itself. 

These factors are controlled by video cards. These are add-ons to the 

microcomputer, printed circuit boards with on-board memory (VRAM) and/or 

processors which go into expansion slots in the machine and enhance its existing 

graphics capability. Indeed, in the early days of microcomputing, video cards were 

necessary to display graphics at all, but as time went on the function of the cards was 

built into the motherboard (the main circuit board of the computer) so the add-ons 

were purely for enhancement of the built-in graphics standard. (At the time of writing, 

PCs come with on-board VGA capability, but this will surely become SVGA in the 

near future.) 

The extra VRAM makes it possible to display wider colour ranges and 

resolutions than the on-board graphics hardware is capable of. Memory is crucial in 

determining these ranges because even though the monitor may be capable of 

displaying high-quality images it will only do so if enough memory is allocated to the 

display to hold the required picture information. 

However, extra VRAM alone does not always make a satisfactory graphics 

workstation as memory takes time to be read by the computer. Watching a highresolution 

true-colour image being read from the framestore (a synonym for VRAM) 

and drawn on screen is a tedious experience at the best of times. When the screen has 

to be redrawn frequently - as is the case with a Graphical User Interface such as 

Microsoft Windows, where simply switching from one window to another forces a 

redraw - then the computer becomes unacceptably slow and users become frustrated 

and lose time. The solution to this problem is to place another processor on to the 

video card itself, thus relieving the computer's processor of the burden of the graphics 

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display. An accelerator card, as it is known, speeds up high quality displays 

considerably, although of course it costs rather more than a standard video card. 

3.4 Colour Printers 

3.4.1 Problems of Colour Printing 

Putting a colour image on screen is somewhat easier and cheaper than placing it 

on paper. There are a number of technical problems associated with colour printing 

which have to do with colorimetry (colour science), physics, chemistry and even 

human physiology and the psychology of perception. Fortunately, it is not necessary 

to go into tedious technical detail in order to outline the major problems that beset the 

transfer of colour screen images to hard copy. These include: 

Different colour coding schemes 

Screen colours are additive, the colour of a pixel depending upon the differing 

intensities emitted by its red, green and blue phosphors: the screen shines by 

its own light. Printed colours are subtractive as the colour of an ink depends 

upon the wavelengths it absorbs from the incident light: red ink appears red 

because it absorbs - subtracts - light of all the wavelengths other than those in 

the red part of the visible spectrum, which it reflects. Additive and subtractive 

colour schemes use different colour models, so colours have to be translated 

from one scheme to the other for faithful rendition from screen to page. (See 

the following Chapter on Colour.) 

Different display resolutions 

Printer resolutions are usually better than screen resolutions: a typical colour 

printer will be capable of 300 dpi (dots per inch) horizontal resolution whereas 

the monitor might only be capable of, say, 64 dpi. This means that a printed 

image of the same resolution is smaller than the screen image, so to print at 

screen size the image has to be enlarged. This can be a problem with bitmaps 

as extra pixels have to be created by a process called interpolation which can 

lead to blockiness and 'staircasing' of lines. 

Dithering 

Basic mixing of the three primary colours in the CMYK scheme - Cyan, 

Magenta and Yellow (black is not used for mixing) - used in colour printing 

only produces 8 colours, so with the exception of high-end products colour 

printers have to use dithering to achieve a wide colour range. Dithering 

involves printing varying proportions of dots of the 8 colours in a square 

dither matrix - usually 4 x 4 or 2 x 2 dots - to give the appearance, from a 

normal viewing distance, of another colour. The main disadvantages of this 

method - aside from the inherent complexity of the various dithering 

algorithms - are increased memory and processing overhead and a reduction in 

resolution; a 300 dpi printer using a 4 x 4 matrix only has an effective 

resolution of 75 pixels per inch. 

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Chemistry 

The chemical composition and physical properties of inks and papers are of 

crucial importance in colour printing. The formulation of the ink is most 

important as users increasingly demand to be able to use normal office paper 

rather than special papers. Among the properties of the ink that need to be 

carefully controlled are its miscibility, viscosity, surface tension, pH, 

dielectric properties and optical density. Synthesising printer inks is not an 

easy task. 

It is not really necessary to outline the other problems in colour printing in 

order to make the point that it is an expensive and complicated business. Whilst these 

problems are mostly solved before the final product hits the market an understanding 

of them goes a long way to explaining why the user can never count on the printed 

image looking exactly as it did on screen and why colour printers are so dear. 

High-quality colour hard copy can also be achieved photographically, images 

from the screen being placed directly on to film. This method bypasses the problems 

of ink and paper chemistry by using familiar and well-tried photographic technology, 

but has the disadvantages of relatively long development times - in comparison to the 

few minutes it takes to print an image to ordinary paper - and expense. Colour film, 

however, enables much higher resolution and colour depth than can currently be 

achieved on plain paper and is the preferred method for those users requiring the 

highest quality colour hard copy. 

3.4.2 Types of Colour Printer 

Colour printers come in a number of types. These are, in rough order of cost 

from cheapest to dearest: 

• dot matrix 

• inkjet 

• thermal wax 

• dye sublimation 

• laser 

In dot matrix printers tiny pins strike the paper through a multicoloured ribbon 

containing the CMYK primaries to produce a coloured dot. The pins are gathered 

together in a matrix inside the printhead, which can consist of 9 or 24 pins, a 24-pin 

dot matrix producing better results than a 9-pin. Such printers are really only suitable 

for coloured text: printing fills - blocks of solid colour - results in horizontal streaks 

and considerable paper distortion from the multiple pin impacts. They are also very 

slow, as the printhead takes four passes to print a line, and very noisy, but they have 

the saving grace of being cheap. 

Inkjet printers produce an image by spraying individual, very fine drops of ink 

at the paper from inkwells of the four primaries. Such devices are capable of 

producing good quality, low-cost colour prints quickly and quietly, and at a not 

excessive cost. The main disadvantages of inkjets are that colours can sometimes look 

'muddy' or 'washed out' because of inks mixing at dot edges. 

Thermal wax technology uses a thermal print head to melt wax from a 

multicoloured ribbon on to the paper. Ironically, because this eliminates the flow of 

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ink over dot edges it can often produce a grainy picture, although the colour quality is 

superb. Many thermal wax printers can work with good quality office paper, by the 

simple expedient of placing a special coating on the sheet during the printing process 

to produce a smooth printing surface. 

Dye sublimation printers also use coloured ribbons with a thermal print head, 

but instead of melting ink on to paper the print head vaporises the ink which then 

condenses on to special paper very close to the ribbon. By this method the size of the 

dots can be controlled and the primary colours can be blended together, doing away 

with the need for dithering. This is a qualitative improvement over other printing 

technologies and produces very high quality continuous tone (not composed of dots) 

output, its main disadvantage being the high cost of consumables (ink and paper). 

Colour laser printing works by using a laser and a photosensitive drum to place 

electrostatic charges on the paper corresponding to the printing positions: when the 

paper is taken through a colour toner reservoir the magnetically-charged toner is 

attracted to the paper as dust and is later heat-bonded to fix it. The results, on the very 

best printers, are indistinguishable from photographs, and unsurprisingly such 

machines are very expensive; however, because they can print to ordinary office 

paper the cost per print is low. 

Less common devices are colour plotters - both pen and electrostatic - which 

are extensively used in Computer-Aided Design (CAD) and scientific disciplines such 

as meteorology where a relatively small colour range is sufficient and large printouts 

are required; the average office or home user is unlikely to come across these devices. 

3.5 Limitations of Colour Output 

Computer graphics technology is still far from the point where the full range of 

visible colours can be rendered either on screen or on paper. The main reason for this 

is that neither monitors or printers are yet capable of producing totally pure primary 

colours. Figure 3.4 shows the approximate colour range of current monitors within 

the CIE chromaticity diagram (see the following Chapter on Colour). 

Figure 3.4 

Realisable colours on a typical colour monitor, in relation to the full range of visible colours. 

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3.6 Colour Scanners 

Scanners are used to digitise an image on paper, photographic negative, or slide 

into a bitmap. This is accomplished by passing light containing a known set of 

wavelengths - usually similar to daylight - over the image and receiving that light in 

photosensitive semiconductors known as Charged Coupled Devices (CCDs) which 

emit voltages proportional to the intensity of the light falling upon them. The image is 

scanned using red, green and blue filters either using three CCDs - one per primary 

colour - or one, in which case each scan requires three passes to view the image under 

the three different filters. Naturally, scanners with three CCDs are more expensive 

than those with only one, but as they only require one pass to scan an image are 

obviously faster. 

There are two types of hard copy scanner: hand-held and flatbed. Hand-held 

scanners contain a light source and CCDs and pick up the image by being swept down 

the page; if the image to be scanned is wider than the scanner head the process is 

repeated across the page and the resulting strips are 'stitched together' by software 

inside the computer. With flatbed scanners the paper is placed on a glass surface and 

the light is moved over it in a similar way to a photocopier. Hand-held scanners are 

cheaper than flatbeds but the latter come into their own when most of the images to 

be scanned are page-sized. Scanning resolution and colour depth varies from 256 

colours (8-bit) at 300 dpi to 16.7 million colours (24-bit) at 1200 dpi for high-end 

products. The crucial factor, as ever, is cost. 

Flatbed scanners can be used to scan photographic slides and negatives, but this 

is usually performed with specialised scanners known as Film Recorders which scan 

at higher resolutions than hard copy scanners, typically up to 3048 x 2072 pixels in 

24-bit colour (≈ 3000 dpi). 

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4.1 What is Colour? 

Chapter Four 

Colour 

Colour is the human perception of the visible portion of the electromagnetic 

spectrum, the full range of which stretches from low frequency radio waves to gamma 

rays (Figure 4.1). This visible portion of the spectrum comprises radiation of 

wavelengths from roughly 380 to 700 nanometres (1 nm = 10 -9 m, or 1 billionth of a 

metre), which we see as a range of colours from violet (360 nm) through blue (480 

nm) and yellow (580 nm) to red (700 nm). 

Wavelength 

3km 

3cm 

0.3 mm 

3000 nm 

300 nm 

30 nm 

0.003 nm 

0.0003 nm 

0.00003 nm 

Radio waves 

Microwave 

Infrared 

Ultraviolet 

Xrays 

Gamma rays 

Cosmic rays 

Figure 4.1 

The Electromagnetic spectrum. 

Frequency (Hz) 

10 

10 

10 

10 

10 

10 

10 

10 

10 

5 

10 

12 

14 

15 

16 

20 

21 

22 

Red 

Violet 

380 nm 

Visible Spectrum 

700 nm 

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Colour 

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4.2 The Human Visual System 

The human eye is an extraordinarily sensitive and sophisticated visual system 

capable of distinguishing very subtle differences in shade and hue. In essence it's a 

simple mechanism (Figure 4.2). 

Muscle 

Iris 

Cornea 

Lens 

Figure 4.2 

Simplified cross-section of the human eye 

Vitreous Humor 

Retina 

Fovea 

(focal point) 

Light is focussed on to the sensitive retina at the back of the eye by the flexible 

lens, the shape of which is controlled by muscles. The retina is composed of two 

types of light-sensitive cells, rods and cones. Rods function in dim light and are 

concentrated in the peripheries of the retina: they are responsible for our night sight 

and are not colour sensitive. Cones contain photopigments rendering them sensitive to 

colour, but only operate in good light. Rods vastly outnumber the cones over most of 

the retina with the exception of the focal point of the eye, the fovea (Figure 4.3). 

Figure 4.3 

Schematic cross-section of retina near fovea, showing distribution of rods and cones. 

Blind spot 

Optic nerve 

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Colour 

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4.3 The Perception of Colour and Brightness 

Within the visible spectrum our eyes are more sensitive to some wavelengths 

than others. Broadly speaking we can see yellows and reds rather better than blues 

and violets, longer wavelengths better than shorter. There are a number of reasons for 

this: 

• the lens and vitreous humor absorb more long wavelengths than short 

• there are many more red-sensitive cones than green and blue (roughly 65% red, 

32% green, 3% blue) 

• the lens cannot adjust sufficiently to properly focus the shorter wavelength blues 

on to the fovea (this is why blues can often appear blurred) 

For these physical reasons our perception of colour differences varies according 

to the position of the colours in the visible spectrum. We find it easier to perceive 

changes in colour at the red end than at the blue. Moreover, we see reds and yellows 

as inherently brighter than blues and violets, so that to give the appearance of equal 

brightness a blue area has to have a greater intensity than a red. 

Our perception of a colour's brightness is similarly non-linear. Brightness is our 

perception of intensity, which represents the energy in the light (the peak, or 

amplitude of its waveform). Although we can distinguish in the order of 10 billion 

intensity levels from near-darkness to unbearable glare changes in intensity at low 

levels result in much greater increases in brightness than the same changes at higher 

levels. Replacing a 50W bulb with a 100W will create a greater brightness increase 

than changing the 100W for a 200W even though each increase represents a doubling 

of intensity. 

4.4 Colour Models 

A colour classification system based on the physical properties of light is not 

really suitable to measure the human perception of colour. A colour described as 'EM 

radiation of wavelength 700nm' would make little sense to most of us, to whom the 

word 'red' is rather easier to envisage. Moreover, colour mixtures with no one 

wavelength would be very difficult to describe. Instead, colour models (sometimes 

known as colour spaces) have been devised based on the concept of colour mixing, 

whereby 3 primary colours can combine to produce the full colour range. Not only 

are these more intuitive than purely physical descriptions but they also provide a 

numerical description of colour that can be used for displays and printing. 

4.4.1 Additive and Subtractive Colours 

We perceive the colours of objects either by the light they emit or the light they 

reflect. A tomato on television appears red because its image emits red light, whereas 

a real tomato gets its colour by absorbing all other colours in white light apart from 

red, which it reflects. 

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Computer monitors and televisions shine by their own light and produce colours 

by the addition of differing amounts of primary colours - this is known as additive 

mixing. On the other hand colour printing uses the concept of subtractive mixing 

whereby primary coloured inks subtract colour from the incident light. Thus, in 

additive schemes the combination of the three primaries at full intensity produces 

white, whereas in subtractive schemes it produces black. 

4.4.2 The CIE Diagram 

In 1931 the Commission Internationale de L'Eclairage developed the CIE 

chromaticity diagram (Figure 4.4) based on empirical data gathered on human colour 

perception, and this diagram has been the baseline for colour modelling since. All of 

the observable colours are placed in a horseshoe shape between x-y axes so that each 

colour has unique coordinates. All the visible pure colours (hues, or spectral colours 

because they occur in the visible spectrum) are to be found on the edge of the curve. 

The colours occurring inside the curve and on the straight 'purple line' which 

completes the curve are mixtures of hues. Being 2-dimensional the CIE Diagram 

cannot show intensity/luminance, but this factor can be derived using simple 

mathematics. 

Figure 4.4 

The CIE chromaticity diagram. 

The diagram has a number of useful properties. For example, a line drawn 

between any two points on the curve defines all the colours that can be derived from 

mixing those two colours; similarly, finding all the possible mixes of three colours is 

done by drawing a triangle connecting the three points. 

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Colour 

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Whilst the CIE Diagram is used extensively by television engineers, more 

suitable colour models - some of which have been derived from it - have been 

invented for use in computer graphics. 

4.4.3 Red, Green and Blue (RGB) 

As monitors and televisions use red, green and blue for primary colours, it is 

unsurprising that the RGB model is widely used for visual display. This is often 

visualised as a cube (Figure 4.5). 

Cyan 

(0,G,B) 

Blue 

(0,B,0) 

Green 

(0,G,0) 

Black 

(0,0,0) 

White 

(R,G,B) 

Magenta 

(R,0,B) 

Figure 4.5 

The RGB cube. (Adapted from Burger and Gillies [1989].) 

Yellow 

(G,R,0) 

Red 

(R,0,0) 

From a hardware point of view, the RGB model is easy to work with as the 

different percentages of red, green and blue in a particular colour can be directly 

mapped to electron gun intensities. It is, though, unintuitive to use: given a particular 

colour it is not always easy to decide how to adjust the RGB balance in the colour to 

make it darker or lighter or to add tints of a non-primary colour. 

4.4.4 Hue, Light and Saturation (HLS) 

HLS uses the more friendly concepts of hue and saturation. Hue is the pure 

colour as people see it - red, yellow, green - and saturation expresses the relative 

amounts of pure hue and white in a colour, ranging from 0% (no hue at all, ie grey) to 

100% (pure hue). The third factor - light - can be thought of as the brightness of the 

colour. HLS is easiest to visualise as a diagram (Figure 4.6). 

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Colour 

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Yellow 

(180º) 

Green 

(120º) 

Red 

(240º) 

L=1 

(white) 

L=0 

(black) 

Cyan 

(60º) 

Magenta 

(300º) 

Blue 

(0º) 

Figure 4.6 

The HLS cone. Saturation increases away from the central axis. (Adapted from Burger and Gillies [1989].) 

The hues are defined by their counter-clockwise angle from the benchmark blue 

line, saturation is the distance from the central axis with 100% saturation (pure colour 

or hue) at the circle edge, and light is the distance of the circle along the vertical axis. 

The central axis also represents the greyscale, being the line of 0% saturation. 

4.4.5 Hue, Saturation and Value (HSV) 

The visualisation of HSV is similar to HLS insofar as hue is measured as an 

angle and Saturation and Value conceptually correspond with Saturation and Light in 

the HLS model. However, HSV differs from HLS in both the visual analogy - HSV is 

conceived as a single cone (Figure 4.7) the top of which marks V = 1 - and in the 

method of calculating Saturation and Value 12 . 

Green Yellow 

Cyan Red 

Blue 

V 

Magenta 

V = 0 

Figure 4.7 

The HSV hexagonal cone. As with HLS, saturation increases away from the central axis. (Adapted from Burger and 

Gillies [1989].) 

HSV and HLS can be simply related arithmetically to RGB values so it is not 

difficult to translate a shade represented in either of these systems into electron gun 

intensities for screen display. At least two - RGB and HLS - and often all three can be 

found in modern graphics packages so that the user has a choice of colour models. 

12 See Burger & Gillies [1989], pp 333-338, for details of the calculation method. 

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Colour 

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4.4.6 Cyan, Magenta, Yellow and Black (CMYK) 

For colour hard copy the subtractive colour model in use is the CMYK system 

(Figure 4.8). This uses Cyan, Magenta and Yellow as the three primary mixing 

colours. Whilst theoretically these colours can create black, in practice a pure black 

pigment is usually included in the scheme in order to print text and to produce deep 

black fills. 

Green 

Yellow 

Figure 4.8 

The CMY cube. 

Cyan 

White 

Black 

Red 

4.4.7 Other Colour Models 

Blue 

Magenta 

There are a number of variations on the HLS theme, including HSI (Hue, 

Saturation, Intensity) and HVC (Hue, Value, Chroma). HSI and HVC are based upon 

the non-linear human perception of colour in contrast to HLS and HSV which are 

based on the linear manner in which the computer produces colour. Whilst they are 

more complex mathematically than linear models and therefore make the computer 

work harder such perception-based models are easier to use from a human standpoint 

and are likely to supersede the machine-based models over time. 

Although not a colour model as such it is worth mentioning the Pantone 

Matching System used in colour printing. This is a set of standard shades, each shade 

having a unique shade number and consisting of specified percentages of each of the 

CMY primaries. This allows colours to be printed consistently by disparate hardware. 

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Colour 

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4.5 The Use of Colour 

4.5.1 Lighting and Backgrounds 

Different coloured lighting changes the colours of objects which we see by 

reflected light - this can easily be verified by standing under a sodium street lamp and 

looking at a colour magazine. This is not a problem in the field of computer graphics 

as virtually all colour printouts are intended to be viewed in white light, and of course 

colour monitors luminesce so the display is unaffected by any incident light. 

However, the colour of backgrounds affects the appearance of foreground 

objects. Obviously blue text will have much less contrast against a violet background 

than against a red (and none at all against a blue!). Less obvious, however, is that a 

colour on a light background appears to be more saturated than if it were on a dark 

background. It's also interesting to note that a large area of a colour appears more 

saturated than a small area of the same colour. 

4.5.2 Warm and Cool Colours 

As a result both of our non-linear colour perception and of colour associations 

found in nature it is possible to develop rules for the use of colour. Blue appears 

distant to our perceptions for physiological reasons (see The Perception of Colour and 

Brightness above) and is also the colour of the sky and sea. To our eyes, then, it has 

properties of distance and tranquillity. On the other hand, we perceive red very 

strongly in comparison to blue and it is also the colour of blood and fire. Not 

unnaturally we associate it with passion, heat and activity. 

A rough division of the visible spectrum can be made into 'warm' and 'cool' 

colours, warm colours corresponding to long wavelengths (reds, yellows) and cool to 

short wavelengths (blues, violets). This scheme is usually conceived as a colour 

wheel (Plate 3). 

Colours opposite each other on the wheel (180° apart) are known as direct 

complements and provide the most vivid and vibrant contrasts. The two colours 30° 

either side of a direct complement are known as the split complements; using split 

complements of a colour results in greater harmony than the use of the direct 

complement, at the loss of some vibrancy and contrast. 

The use of colour is a large and complex topic which is dealt with ably in a 

wide range of works (see the Bibliography). There are, however, a number of simple 

guidelines that should be borne in mind when using colour, whatever the purpose: 

• cool colours are distancing and should generally be used in backgrounds, 

whereas warm colours grab the attention and are better placed in the 

foreground 

• the eye finds it difficult to focus on short wavelengths, so avoid the use of 

blues and violets for text and where edges are important 

• be sparing in the use of saturated colours - the eye tires quickly when faced 

with vibrant shades 

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Colour 

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• bear in mind that colours often have psychological and symbolic associations; 

this can be particularly important when addressing an international audience 

as these associations are often very culture-specific. 

4.5.3 Colour Deficiency 

Sometimes incorrectly termed 'colour blindness', colour deficiency is the result 

of a minor genetic error which mainly affects men. Roughly 8% of men, and 9% of 

the population as a whole, exhibit some form of colour deficiency. This ranges from 

complete colour blindness, which is quite rare, to mild colour insensitivity. The most 

common deficiency is dichromatism in which the person's retina lacks the green or 

red photopigment so that they are unable to distinguish red or orange from green or 

yellow. 

Colour deficiency is not usually a problem for the people affected, and indeed 

some are completely unaware that their colour vision is impaired. They still see a 

tomato as a shade which they call 'red' even though they may be lacking a red 

photopigment; problems only arise when they are required to distinguish between 

colours to which they are insensitive, usually red and green. It is therefore important 

for designers who use colour to be aware of this and to avoid, for example, using red 

text on a green background. 

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Colour 

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Chapter Five 

Graphics Packages 

This Chapter looks at the graphics programs available to the 'ordinary' user - 

that is, the sort of package that can be bought from the dealers that advertise in the 

mainstream computer press. It lists some of the leading product in each application 

category and explains how graphics are incorporated into useful applications, either 

by using programming and/or authoring or by employing the inherent features of the 

Windows and Macintosh Graphical User Interfaces (GUIs). However, the relative 

merits of packages are not discussed as this is a survey of the market rather than a 

review of individual products. 

5.1 Popular Microcomputer Graphics Packages 

5.1.1 Painting and Drawing 

The most popular types of graphics application are undoubtedly general purpose 

painting and drawing packages. Such a package allows the user to create original 

artwork, or to edit and manipulate existing computer graphic images. These graphics 

can then be used in a number of ways, including: 

• desktop publishing 

• enhancing documents with diagrams and pictures 

• business presentations 

• training and education 

• databases 

and so on - the list is potentially endless. Although often used interchangeably, 

the terms painting package and drawing package refer to bitmapped and vector-based 

(sometimes called object-oriented) applications respectively; a generic term that 

includes both might be 'art package'. Tables 5.1 and 5.2 list some of the best-known 

computer art packages available at the time of writing 13 for both Macintosh and PC. 

13 It must be emphasised that these tables are neither exhaustive nor necessarily indicate the best packages on 

the market: new packages emerge seemingly daily and any list is no better than a snapshot of a dynamic 

market. 

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Package PC Mac Notes 

Claris MacPaint √ Monochrome only. 

Electronic Arts Studio/8 √ Also available as Studio/32, for 32-bit colour 

Aldus SuperPaint √ 

Windows Paintbrush √ Comes free with Microsoft Windows. 

Table 5.1 

Popular bitmapped computer art packages 

Package PC Mac 

Adobe Illustrator √ √ 

Aldus Freehand √ √ 

CA CricketDraw √ √ 

Claris MacDraw √ 

Corel Draw! √ 

Harvard Graphics √ 

Micrografx Designer √ 

Table 5.2 

Popular object-oriented (vector) computer art packages 

Most modern commercial applications now include both bitmapped and objectoriented 

editors in the full package to enable the user to handle any graphic they may 

come across. For example, the Corel Draw! suite of applications includes the objectoriented 

Draw! itself and the bitmap editor PhotoPaint! 

It is quite common for large Clip Art libraries to be distributed with graphics 

applications. Clip Art is the generic term applied to computer graphics images that are 

in the public domain and can be used freely, without copyright restrictions. Both 

vector and bitmapped Clip Art is available in a wide variety of file formats and in an 

astonishing range of subjects. Given the size of modern programs and the amount of 

Clip Art bundled with them the preferred distribution medium for commercial 

applications is rapidly becoming the CDROM. 

5.1.2 Presentation 

The use of slide shows to present, promote and sell ideas, services and products 

has a long history predating the advent of computers. Data is often much more 

comprehensible when presented visually as an image (such as a pie chart or bar 

graph), and across all fields people whose job it is to make presentations to audiences 

know that slides can complement and enhance their performance. Modern micros 

with their rapidly improving graphics capabilities are ideal vehicles for creating and 

displaying 'slide shows'. Each 'slide' is simply a computer graphic - usually vectorbased 

- created using standard techniques which can then be printed to film, paper or 

acetate, or displayed direct from the computer. 

Computer-generated slide shows have a number of significant advantages over 

traditional methods: 

• distribution: a large presentation can be saved to disk and copied or downloaded 

cheaply and quickly 

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• ease and cost of production: anyone with a micro and presentation software can 

create a presentation, eliminating the cost and delay of using professional bureaux 

• ease of editing: changes can be made to slides with little effort or waste 

• special effects: using the computer for the presentation means that sophisticated 

visual effects can be included in a 'show', including sound, video and animation 

As a consequence, a large and lucrative market in presentation software has 

exploded into being in recent years. In addition, it is now quite common for large 

drawing packages to include slide show 'modules' within the package. Table 5.3 is a 

sample of the specialist presentation packages currently on the market. 


Aldus Persuasion √ √ 

CA Cricket Presents √ √ 

MacroMedia Action √ √ 

Micrografx Charisma √ 

Microsoft Powerpoint √ 

Symantec More √ 

Table 5.3 

Presentation Packages 

5.1.3 Photography 

As hardware becomes more and more powerful so the field of photography has 

been brought into computing. Photographs - whether negative, print or slide - can 

now be digitised by scanners and saved as bitmaps, despite the enormous memory 

overheads of high-resolution true-colour photos. This service, which was once the 

exclusive province of professionals, has been brought on to the high street by Kodak 

with their proprietary PhotoCD technology: ordinary members of the public can walk 

into a retail photographic shop and have their negatives digitised on to a CDROM. 

In recent years a number of packages have come on the market that are capable 

of manipulating digitised photographs. These programs are essentially sophisticated 

bitmap editors tailored to the requirements of photographic editing which allow an 

impressive array of image processing effects to be applied to the photographic image, 

such as changing its colour balance or smoothing or enhancing certain features, as 

well as detailed editing down to the level of individual pixels. Indeed, in skilful hands 

the image can be retouched drastically without any sign of these changes appearing in 

the final print; the old saying that 'the camera never lies' has never been less true than 

today. Because of the high memory requirements of photographic bitmaps, and the 

amount of processing involved in applying image processing techniques to them, 

photographic applications require high-end machines to run efficiently. 

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Adobe Photoshop √ √ 

Aldus PhotoStyler √ 

Corel PHOTO-PAINT √ Supplied with Corel Draw! 

Kodak PhotoCD Access √ √ Just used for decoding PhotoCD files, and only 

supports simple operations such as clipping and 

scaling. 

Micrografx Photo Magic √ Supplied with Micrografx Graphics Works 

Table 5.4 

Photographic editing applications 

5.1.4 Graphics Utilities 

Traditionally the field of computer graphics has been beset by a profusion of 

incompatible file formats and this has been a fertile ground for the growth of utility 

programs which convert images from one format to another. Increasing user demand 

for sophisticated bitmap editing facilities, including the application of special effects 

to the image, also spawned a large number of utilities, as until the recent past both 

painting and drawing packages were relatively basic. Modern packages, however, 

incorporate graphics file conversion filters for a wide range of file formats, and an 

often bewildering armoury of image processing tools to create special effects, so the 

niches occupied by graphics utilities are gradually shrinking. 

The vast majority of these utilities are in the public domain, either as shareware 

- software which can be copied freely and used on a 'try before you buy' basis - or 

freeware. There are so many utilities, and they vary so much in quality (although all 

are cheap), that it would be a fruitless and inaccurate exercise to compile a list of the 

'best-known' or 'most common'. The best advice for users who feel they may need one 

or more utilities - possibly because their art package has only rudimentary facilities - 

is to consult sources of shareware, usually either a dealer or a software archive. 

5.1.5 Animation 

Animation is a very recent arrival on the microcomputer scene primarily 

because it requires a fast processor and a large amount of memory. Essentially, 

animation is no more than a series of frames flashed before our eyes so quickly that 

they give the appearance of movement. In this respect, computer animation is no 

more sophisticated than 'what-the-butler-saw' machines. However, computers do 

vastly ease the animator's job by being able to extrapolate frames from a start point 

and an end point. If, for example, the start scene is a car at the left edge of the screen 

and the end scene the same car at the right edge the computer can use in-betweening 

to create the intermediate frames. When the scene is played from start to end it 

appears that the car is moving across the screen. In traditional animation these 

intermediate scenes would have to be drawn by hand. 

Animation packages are vector-based, object-oriented applications. Each figure 

in an animation is an object (an actor, in animation jargon) the properties and 

movements of which are defined by the user. This means that all the information 

about a particular animation - objects, backgrounds, movement - can be stored quite 

compactly, in the same way that vector graphics produce compact files. 

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Because animation is such a new field in the microcomputer world, and because 

its market isn't as well defined as that of art or presentation packages, there are 

relatively few packages on the market. Table 5.5 shows the most common. 


Autodesk Animator √ 

Corel Move! √ Supplied with Corel Draw! 

Gold Disk Animation Works √ 

Interactive 

√ 

MacroMind Director √ 

Table 5.5 

Animation packages 

5.2 Incorporating Graphics into Applications and 

Documents 

Graphics often need to be used within programs to have the most effect. For 

example, a series of photographs of archeological sites may be quite nice to look at in 

a drawing package, but their impact and educational content is exponentially 

increased by placing them within a program which contains text about the sites, uses 

image processing techniques to bring out hidden features (old field boundaries, for 

instance), and cross-references sites according to age and period to show germane 

features of the civilisations that created them. 

We have already looked at one method of using graphics in a program, in the 

previous section on Presentation Packages, and in the following sections we explore 

other methods of using the informational power of computer graphics within 

applications. 

5.2.1 Programming Languages and Authoring Tools 

These are the 'traditional' methods of creating applications. Programming 

languages allow the programmer to have close control over the computer, but of 

course require programming skills to use and have a steep learning curve . Their use 

is thus restricted to IT professionals and enthusiastic users with a lot of time. These 

days programming languages incorporate such a vast array of graphics-handling 

functions that it's often easier to list what programmers can't do with graphics than 

what they can. 

Authoring tools allow authors without programming skills to produce 

applications, usually in particular subject areas. Using simple actions such as 

selecting text and applying menu commands complex structures can be created, the 

'pre-programming' of these structures already having been performed by the 

developer. Usually a small amount of simple programming in a language specific to 

the package is required to get the most out of the system but full-blown programming 

skills are never called for. All authoring packages have graphics-handling capabilities 

which can range from the rudimentary to the sophisticated often depending, 

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unsurprisingly, on the cost of the package. The main drawbacks of authoring tools 

are: 

• Proprietary software. An authoring package is linked inextricably with the 

company that produced it, unlike general-purpose programming languages 

which are in the public domain. This usually results in a severe lack of 

portability because an application produced in a particular package can only be 

run in that one package (or a cut-down runtime version of it used for 

distributing the application). 

• Efficiency. Applications produced by an authoring system are usually slower 

and less efficient than if they had been written in a programming language. 

• Specialisation. Although most tools attempt to be general-purpose application 

generators, in practice they tend to be very good in certain areas and poor in 

others. 

On the other hand ordinary users - even computer novices - can produce usable 

applications with authoring tools after only a short learning period, and it is this 

productivity together with ease of use that are the main selling points of authoring 

packages and the reasons for their enduring popularity. Table 5.6 lists some of the 

most common. 

Package Company PC Mac Notes 

Authorware 

Professional 

Authorware Inc √ √ 

Guide Info-Access √ Primarily for hypertext applications. 

Hypercard Claris √ The most popular application generator in the 

Mac world. 

IconAuthor AimTech Corp. √ 

Toolbook Asymetrix √ Sometimes called 'Hypercard for the PC' 

because of its similarities to the Mac program. 

Table 5.6 

Authoring packages. 

5.2.2 Desktop Publishing (DTP) 

Desktop Publishing is the name given to the process of producing documents 

containing both text and graphics - newspapers, magazines, etc - using a computer. 

The phenomenal growth in the power and affordability of DTP packages for micros 

has put publishing into the hands of anyone with a micro and a laser printer. DTP 

applications allow very sophisticated text formatting and the placing of a wide variety 

of graphics within the publication. Their graphics-handling capabilities are usually 

restricted to cropping and resizing, so the editing of a graphic has to be performed in 

a specialist graphics package. 

DTP owes its very existence to the rapid advances in the graphics capabilities of 

computers, particularly micros. Not only is its raison d'etre the incorporation of 

graphics into text, but the WYSIWYG (What You See Is What You Get) display of 

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DTP programs, whereby the document appears on the screen exactly as it will print, is 

extremely graphically intensive. 

Despite being a relatively mature field - in microcomputing terms - DTP is 

dominated by just a few packages (Table 5.7), although given the abilities of modern 

Word Processing programs the line between WP and DTP is becoming ever more 

tenuous. 


Adobe Illustrator √ 

Aldus PageMaker √ √ 

Quark Xpress √ 

Ventura Publisher √ 

Table 5.7 

Leading DTP packages 

5.2.3 Placing Graphics into Non-Graphics Files 

Graphics can also be placed into files generated by non-graphics packages by 

cut and paste techniques. If both packages work within the same operating 

environment, such as the Macintosh System 7 which uses a common data format 

regardless of the type of file, then data can simply be copied from the source to the 

destination via the clipboard (an area of memory set aside for temporary storage). For 

example, you could create a drawing in MacDraw, select that drawing, copy it to the 

clipboard, and paste it into a MacWrite document. 

A qualitative improvement on simple cut and paste is Object Linking and 

Embedding (OLE) introduced by Microsoft in Windows version 3.1. Central to OLE 

is the object, defined as data created by a Windows application that can be placed - 

either by linking or embedding - into another Windows application. 

The capacity to use OLE has to be specifically built into a Windows application 

by its programmers, although this is now the case with the vast majority of 

commercial Windows programs. Programs that support OLE can be either, or both: 

a server, which can supply objects to other applications 

a client, which can accept objects from server applications 

Each object is created in its own specific server application - say, Windows 

Paintbrush - and is then placed as an object in the client file. This object can be either 

a link to the server file or a copy of the server file embedded into the client. 

The main qualitative difference between OLE and simple cut and paste is that 

the object can be edited from within the destination file. To edit a graphic placed into 

a database record, say, the user simply double-clicks the mouse over the graphic and 

the originating graphics application is opened. 

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Chapter Six 

Computer Graphics in Higher Education 

If a picture is worth a thousand words, there can be few more obvious uses for it 

than in the area of imparting knowledge to others. Text is not a very natural teaching 

medium for us. Not only does it have a low information density but it also results in 

processing overheads for the brain, insofar as it is a symbolic representation of 

information that has to be decoded. Still and moving images, on the other hand, 

usually require no decoding (although they need a lot of interpretation) and have high 

information densities. Moreover, the human visual system has evolved to be highly 

efficient at information gathering and processing, so the presentation of information 

as graphics takes advantage of this natural ability. (This is by no means to deride text 

- plainly it would not be possible to put across the information in this book purely 

graphically - but simply to recognise its limitations.) 

It would not be accurate to say as yet that computer graphics has qualitatively 

changed the delivery of education and training, although it has certainly significantly 

enhanced the quality of teaching. In contrast to science, where new technologies 

open up new avenues of research and change the ways that science is carried out, the 

practice of teaching changes slowly and is driven by theories of learning as well as 

more contingent socio-economic factors. Primary of these factors, of course, is the 

availability of funding, and this has meant that the use of computer graphics in public 

education has lagged behind private training. Corporations are willing to spend large 

sums on high-end training facilities because of the real productivity gains that can be 

realised and measured in cash terms. This contrasts with public education where 

'productivity' is a more slippery concept and where the information being conveyed is 

of a different nature from training: broadly speaking education is about concepts and 

techniques - the Why and How - whereas training concentrates narrowly on the How. 

This concentration upon technique makes training much more amenable to the 

use of computer graphics than education. It's a fairly simple matter to use graphics to 

illustrate the assembly of an electronic circuit, but rather harder to explain the 

quantum mechanics that enable the semiconductors in the circuit to work. 

However, there are areas of education where graphics can, and are, being put to 

good use. In the sciences visualisation - the graphical representation of data - is used 

extensively for both data analysis and teaching, and in many fields images are 

essential to grasp fundamental concepts - the structure of a molecule in chemistry, or 

the process of cell division in biology, for example. The teaching of history, 

particularly of the 20th century, could benefit from the use of the vast amount of still 

and moving images in the archives, as could the teaching of Art by the use of image 

databases. 

There are a few subjects which would not benefit substantially from the use of 

computer graphics in their teaching: it's difficult to see how courses in such areas as 

English Literature, Philosophy, Music, or Theology, could be much enhanced by 

images whether moving or still. These, though, are the exceptions rather than the rule, 

and in most subjects the judicious use of graphics can only improve the teaching 

material. If nothing else, the inclusion of interesting images can make learning fun, 

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Computer Graphics in Higher Education 

_________________________________________________________________________________ 

which of course has a positive effect on the student's attitude and thus on knowledge 

uptake. 

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Glossary 

Actor An active object in an animation. 

ADC Analogue to Digital Converter. A specialised chip which converts an analogue 

waveform into digital data. The reverse process is carried out by a Digital to 

Analogue Converter (DAC). 

Additive Colours Colours produced by the addition of light from luminescent 

primary sources, which in the case of computer monitors and televisions are red, 

green and blue. See also RGB. 

Aliasing Image imperfections whereby jagged edges and staircasing appear on lines 

and edges due to the limitations of raster systems in the representation of lines and 

curves. These imperfections may be removed by anti-aliasing, a technique which 

varies the intensities of pixels along the line. 

Authoring Tool A program which enables the user (author) to create applications 

without writing computer progrms. 

Bandwidth The range of frequencies in which a signal is transmitted. The greater the 

bandwidth the more information the signal carries. 

Bitmap A computer graphic composed of individual dots known as pixels. 

CCD Charged Coupled Device. A photosensitive semiconductor which emits a 

voltage proportional to the intensity of the light falling upon it. Used in scanners. 

CDROM An acronym for Compact Disk Read Only Memory. A CDROM is 

physically identical to an audio CD, and is used to store large amounts of data (up to 

600MB). This makes it particularly useful for distributing large graphical or video 

files, and for reference works like dictionaries. A CDROM is read by a CDROM 

drive which can be either inside or outside the computer. 

CGA Colour Graphics Adaptor. An obsolete PC monitor standard, capable of 

displaying low-resolution graphics (640x200 pixels) in 16 colours. 

Character Generator A device implemented in display system hardware which 

creates standard text characters on screen. A PC uses its character generator when 

operating in text mode. 

CIE Diagram A conceptual colour space empirically derived from data on human 

colour perception by the Commission Internationale de L'Eclairage in 1931. 

Clip Art Digital images in the public domain which can be used without copyright 

restrictions. 

CMYK Standing for Cyan, Magenta, Yellow and blacK, the colour scheme used for 

printing. Although in theory cyan, magenta and yellow inks can combine to form 

black, in practice the mixture is often not sufficiently dark so additional black ink is 

used. 

Colorimetry The science of colours, or more explicitly - given that colour is a purely 

human perception of the visible part of the electromagnetic spectrum - the science of 

the human perception of colour. 

Colour Model Empirical colour description scheme based on the concept of colour 

mixing, whereby each shade is specified by a unique combination of primary factors 

such as hue, saturation and brightness. 

Colour Wheel Spectral hues arranged in a circle so that opposite colours (180º apart) 

are complementary. 

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Glossary 

_________________________________________________________________________________ 

Compression The process of removing redundant data from a file to reduce its size. 

CRT Cathode Ray Tube. A CRT produces an image by firing electrons at 

phosphorescent dots on the inside of the screen. In a colour CRT three electron guns 

and three colours of phosphor dot - red, green and blue - are used. 

DAC Digital to Analogue Converter. A specialised chip which converts digital data 

into an analogue waveform. The reverse process is carried out by an Analogue to 

Digital Converter (ADC). 

Desktop Publishing (DTP) The use of computer software to produce printed 

publications which can incorporate both text and graphics. 

Digitise To convert a real-world image or sound clip into binary data so that it can be 

read by computers. 

Direct Complement The direct complement of a colour is the colour which lies 

opposite it on the colour wheel; it can also be thought of as its 'negative'. 

Dithering On systems with a limited number of colours dithering is a method of 

simulating out-of-range colours by mixing pixels/dots of existing colours. For 

example, a 16-colour system can display 256-colour images using dithering. If there 

are only two colours available, such as is the case with monochrome printers, the 

process is known as half-toning; this is how black-and-white newspaper pictures are 

produced. 

DPI Dots per inch, a measure of image resolution on output devices such as monitors 

and printers. 

EGA Enhanced Graphics Adaptor. PC monitor mode capable of displaying 16 

colours at 640x350 resolution. 

Electron Gun Used in CRTs (such as televisions and desktop computer monitors). 

An electron gun fires a beam of electrons at phosphorescent dots on the inside of the 

screen. See CRT. 

False Colour Artificial colour applied to a greyscale image in image processing in 

order to enhance salient features of the image. 

Film Recorder A hardware device which prints a computer graphic to a 

photographic medium, usually 35mm slides. Film recorders are used by slide bureaux 

to convert computer-based presentations into slides. 

Framestore See Video RAM. 

Graphical User Interface (GUI) An interface between an application or operating 

system and the user which communicates with the user by employing graphical 

elements, such as windows and icons. Sometimes known as a WIMP Interface 

(Windows, Icons, Menus, Pointer). Examples include the Macintosh System 7, 

Microsoft Windows, and Sun OpenWindows. 

Greyscale The range of grey shades available on a system. 

Greyscale Image A monochrome bitmap where pixel values are interpreted as 

different shades of grey on a scale from pure white to pure black. 

HLS Hue, Light and Saturation. An additive colour model appropriate for visual 

displays, developed by Tektronix. 

HSV Hue, Saturation and Value. An additive colour scheme for visual displays. 

Hue A pure - 100% saturated - colour. A hue is a spectral colour, one that occurs in 

the visible spectrum. 

Hue, Light and Saturation See HLS. 

Hue, Saturation and Value See HSV. 

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Glossary 

_________________________________________________________________________________ 

Interlacing A method of producing high-resolution displays by scanning every 

alternate line in a frame, thus taking two frames to display the whole picture. 

Normally found in less expensive systems. 

Interpolation A method of enlarging bitmaps by adding extra pixels, substituting 

each pixel in the original image by a n x n square of pixels where n is the degree of 

enlargement. 

Liquid Crystal Display A visual display which exploits the light polarisation 

properties of special molecules (liquid crystals) when small voltages are applied to 

them. LCD displays are used in portable computers because of their lightness and low 

power consumption. 

Lookup Table (LUT) A subset of the full palette available on a system. The LUT is 

a set of index values which point to colour values in the full palette. 

Megaflop A million floating-point operations per second. Often used as a measure of 

processor speed. 

Object A constituent part of a vector graphic composed of other objects and/or 

primitives. 

Object Linking and Embedding (OLE) A feature introduced by Microsoft with 

Windows 3.1 allowing data from any OLE-aware Windows application file to be 

placed into another similar file, even if they are of different application types (eg a 

sound file being placed into a word processing document). 

Object-Oriented In graphics terms, referring to a vector-based drawing package. 

Operand See Parameter. 

Optical Media Storage media in which laser light is used to read and write disk data, 

such as CDROMs and Laserdisks. Optical media are generally read-only, as the laser 

used in the writing process burns pits into the disk surface, but some expensive 

optical disks are read/write. 

Page Description Language Commonly used in Desktop Publishing, PDLs are 

effectively programming languages which instruct output devices how to output 

documents. One of the most common PDLs is Adobe Postscript. 

PAL The main analogue video standard used in the world outside North America 

and Japan. 

Palette The full range of colours available on a system. 

Pantone A standard colour matching scheme for printing which specifies the 

percentage of each primary colour used to produce a particular shade. Each shade has 

a unique shade number. The scheme was created to achieve uniformity and 

consistency in colour printing. 

Parameter Data supplied to a command, usually in parentheses. For example, the 

common graphical primitive rectangle(x1,y1,x2,y2) takes the four parameters x1, y1, 

x2, y2. 

Phosphor Dot A dot coloured red, green or blue on the screen surface, which 

phosphoresces when struck by electrons. An RGB triplet of phosphors comprises a 

pixel. 

Pixel Picture Element. A discrete dot on a television or monitor, or the smallest 

element in a bitmap. 

Primary Colours Pure hues which when mixed can produce the full range of visible 

shades. 

Primitive The lowest level vector graphics object, such as a line or a rectangle. All 

objects are ultimately composed of primitives. 

Raster Scan Device See Cathode Ray Tube. 

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Glossary 

_________________________________________________________________________________ 

Ray Tracing An important form of rendering which produces more 'realistic' scenes 

by taking into account factors such as the lighting of the scene and the reflectivity or 

opacity of the scene objects. 

Refresh Rate The number of times a second a display is redrawn. To give a flickerfree 

display this needs to be at least 25 times a second. 

Rendering The generation of artificial 3-D scenes using geometrical and lighting 

data. 

RGB Standing for Red, Green and Blue, the colour scheme used by colour monitors 

and televisions. Different intensities of each colour combine to produce the full 

colour range. 

Saturation The amount of pure hue in a colour, on a linear scale from 0% saturation 

- no hue at all, resulting in a white or grey - to 100% saturation which represents the 

pure hue itself. Pastel colours lie within this range. 

Scanner A hardware device for digitising (converting to binary data) printed 

material, such as pictures on paper or 35 mm slide. 

Shadow Mask In a CRT, a sheet of metal with regular apertures which focusses the 

beams from the electron guns onto their corresponding phosphors. 

Spectral Colour A colour of the visible spectrum, a pure fully-saturated hue. 

Split Complement The split complements of a colour are those which lie either side 

of its complement on the colour wheel. 

Subtractive Colours Colours produced by the subtraction of colours from incident 

light. A tomato appears red in daylight because it absorbs all other colours in the 

visible spectrum other than red, which it reflects. See also CMYK. 

Super VGA A wide term, covering a number of screen devices with superior 

resolution and/or colour capability to VGA. 

VGA Video Graphics Adaptor 14 . A PC monitor standard of at least 640 x 480 x 16 

colours. 

Video Card A slot-in expansion card containing video RAM and - usually - a 

processor, which improves the resolution and colour depth of the display. Some video 

cards, known as accelerator cards, also speed up the display by reducing the screen 

redraw time. 

Video RAM (VRAM) The amount of dynamic memory devoted to screen display, 

usually resident on the video card. Also known as the framestore. 

Window A rectangular viewing area which is processed separately from the rest of 

the screen. 

WORM Write Once Read Many times. A type of read-only optical disk usually 

employed for archiving. 

WYSIWYG "What You See Is What You Get", pronounced "wizziwig". Normally 

applied to Word Processing and Desk Top Publishing packages which operate in a 

graphical mode. What appears on the screen is what will appear on hard copy. 

14 Some textbooks translate the acronym as 'Video Gate Array'. 

__________________________________________________________________________________________________________ 


_________________________________________________________________________________ 

Author Title 

Annotated Bibliography 

Ammeraal, Leendert Graphics Programming in Turbo C. 1989. Wiley & 

Sons, Chichester. 

Guide to graphics programming using Borland Turbo C 

Arnold, D B 

Duce, D A 

Baker, M P 

Hearn, D 

Barlow, Horace 

Blakemore, Colin 

Weston-Smith, Miranda 

version 2.0 

ISO Standards for Computer Graphics: The First 

Generation. 1990. Butterworths, London. 

A detailed treatment of the ISO-approved graphics 

standards - PHIGS, GKS, CGM, etc - and the 

standardisation and approval procedures. 

Computer Graphics. 1986. Prentice-Hall, London. 

Theoretical and practical study of underlying concepts of 

computer graphics, including maths (algebra, matrices) 

and Pascal code, well-diagrammed throughout. 

Images and Understanding. 1990. Cambridge 

University Press, Cambridge. 

Collection of papers from an international conference on 

the title subject, held in 1986. 

Beard, Nick Visualisation - a series of 4 articles in Personal 

Computer World, Vol 15 (11 & 12) & Vol 16 (1&2) (Nov 

1992 - Feb 1993). A non-technical look at data 

Browne, Jimmie 

McMahon, Chris 

Burger, Peter 

Gillies, Duncan 

visualisation. 

CADCAM: From Principles to Practice. 1993. 

Addison-Wesley, Wokingham. 

A standard introductory technical texbook on the subject. 

Interactive Computer Graphics. 1989. Addison- 

Wesley, Wokingham. 

Comprehensive and advanced treatment of computer 

graphics with mathematical concepts and algorithms. 

Carlson, W E A Survey of Computer Graphics Encoding and 

Storage Formats. Computer Graphics, April 1991, Vol 

25(2), pp 67-75. 

A survey of several graphics file formats, including 

descriptions of common compression methods. 

Durrett, H J (ed) Color and the Computer. 1987. Academic Press Inc, 

Orlando, Florida. 

A collection of papers on the title subject, discussing 

topics such as Colour Science, colour and humancomputer 

interaction, and colour hardware. 

__________________________________________________________________________________________________________ 



_________________________________________________________________________________ 

Foley, J 

van Damm, A 

Feiner, S 

Hughes, J 

Gonzalez, Rafael C 

Woods, Richard E 

Computer Graphics Principles & Practice. 1990. 

Addison-Wesley, Reading, Mass. 

Very large and all-embracing text on the theory and 

practice of computer graphics, particularly mathematical 

concepts and algorithms. 

Digital Image Processing. 1992. Addison-Wesley, 

Reading, Mass. 

Comprehensive introductory text to the field of Image 

Processing. 

Hopgood, F R A Using Colour in Computer Graphics. 1991. Advisory 

Group on Computer Graphics, Loughborough University. 

A short and concise paper from ACOCG's Technical 

Hopgood, F R A 

Duce, D A 

Johnston, D J 

Report Series. 

A Primer for PHIGS: C Programmer's Edition. 1992. 

Wiley & Sons, Chichester. 

Technical desciption of the PHIGS standard from a C 

programmer's perspective, with full details of the C 

language binding and example source code. 

Jute, André Colour for Professional Communicators. 1993. 

Batsford, London. 

A friendly and colourful 'how to' guide to the use of 

Kay, David C 

Levine, John R 

colour for communication. 

Graphics File Formats. 1992. Windcrest/McGraw-Hill, 

NY. 

Excellent reference on all the graphics file formats in 

current use, giving detailed technical information about 

each format. 

Latham, Roy The Dictionary of Computer Graphics Technology 

and Applications. 1991. Springer-Verlag, NY. 

A comprehensive dictionary of computer graphics 

terminology. 

Low, Adrian Introductory Computer Vision and Image 

Processing. 1991. McGraw-Hill, Maidenhead. 

Mealing, Stuart The Art and Science of Computer Animation. 1992. 

Intellect Books, Oxford. 

Comprehensive and accessible textbook covering all 

aspects of the field of animation. 

Peterson, Ivars The Mathematical Tourist. 1988. W H Freeman, NY. 

A non-mathematical look at some aspects of modern 

mathematics, particularly mathematics which can be 

expressed graphically, such as fractals. 

Pickover, Clifford Computers, Pattern, Chaos and Beauty.1990. St 

Martin's Press, New York. 

An eclectic work on the images that can be generated on 

computers using non-linear mathematical functions . 

__________________________________________________________________________________________________________ 



_________________________________________________________________________________ 

Rimmer, Steve Bit-Mapped Graphics. 1990. Windcrest Books, Blue 

Ridge, PA. 

A highly technical book for C and Assembler 

programmers on common bitmap formats and how to 

display and manipulate them. Contains plenty of source 

Rogers, D F 

Adams, J A 

code. 

Mathematical Elements for Computer Graphics. 

1990. McGraw-Hill, NY. 

Heavily theoretical mathematical treatment of computer 

graphics concepts, from simple 2D and 3D 

transformations to surfaces and splines. 

Rowell, Jan Picture Perfect: Color Output for Computer 

Graphics. 1991. Tektronix Inc, Beaverton, Oregon. 

A glossy, friendly booklet on colour and colour printing 

with an unsurprising bias towards Tektronix products. 

Well written and informative, though. 

Smith, W J 

Using Computer Color Effectively: An Illustrated 

Thorell, L G 

Reference. 1990. Prentice-Hall, New Jersey. 

Stonier, Tom Information and the Internal Structure of the 

Universe. 1990. Springer-Verlag, London. 

Subtitled "An Exploration into Information Physics" the 

book outlines the author's view that information is an 

integral part of the natural world. 

Warren, Lorraine Understanding IT: Computer-based Presentations. 

1994. University of Hull. 

A highly readable guide to producing presentations with 

presentation packages. 

Watt, Alan Fundamentals of Three-Dimensional Computer 

Graphics. 1989. Addison-Wesley, Reading, Mass. 

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_________________________________________________________________________________ 

__________________________________________________________________________________________________________ 


_________________________________________________________________________________ 

animation, 36-37 

authoring packages, 37-38 

bitmapped graphics, 5-7 

advantages, 6 

colour depth, 5 

disadvantages, 6 

memory, 5, 6, 13-14 

resizing, 6 

resolution, 5 

brightness, 25 

cathode ray tube (CRT), 16 

charged coupled device (CCD), 22 

Clip Art, 34 

colour 

additive, 19, 26 

definition, 23 

depth, 5 

dithering, 19 

greyscale, 7, 16 

perception, 24-25 

spectral, 26 

subtractive, 19, 26 

warm and cool, 30-31 

colour models 

CIE diagram, 26-27 

CMYK, 19, 29 

HLS, 27-28 

HSI, 29 

HSV, 28-29 

HVC, 29 

RGB, 27 

colour printers, 18-21 

dot matrix, 20 

dye sublimation, 21 

inkjet, 20 

laser, 21 

limitations, 21 

plotters, 21 

resolution, 19 

thermal wax, 21 

colour wheel, 30 

computer memory (RAM), 15 

desktop publishing (DTP), 38-39 

dichromatism, 31 

direct complements, 30 

disk storage, 14-15 

Index 

__________________________________________________________________________________________________________ 


Index 

_________________________________________________________________________________ 

optical media, 14 

drawing package, 4 

electron guns, 6, 15 

eye 

colour perception, 24-25 

diagram, 24 

retina, 24 

rods and cones, 24 

false colour, 7 

file compression, 8-10, 15 

fractal, 9 

Huffman coding, 9 

lossless and lossy, 9 

run length encoding (RLE), 8 

file formats, 10-13 

conversion between, 36 

graphics packages 

animation, 36-37 

desktop publishing, 38-39 

photo editing, 35-36 

presentation, 34-35 

utilities, 36 

hue, 27 

image processing, 6 

image processing, 37 

intensity, 25 

interlacing, 17 

interpolation, 19 

liquid crystal display (LCD), 16-17 

lookup table, 6 

matrix addressing, 16 

Microsoft Windows, 39 

monitors, 15-18 

CRT, 16 

LCD, 17 

object, 3, 13 

object linking and embedding (OLE), 39 

Pantone, 29 

phosphor dot, 15 

photographs, digitised, 35 

pixel, 5, 14 

primitive, 3, 14 

saturation, 27 

shadow mask, 15 

split complements, 30 

SVGA, 16 

System 7, 39 

vector graphics, 3-5 

vector graphics 

__________________________________________________________________________________________________________ 


Index 

_________________________________________________________________________________ 

advantages, 4 

disadvantages, 4 

memory, 13 

uses, 5 

video card, 15 

video cards, 18 

video ram (VRAM), 5, 15, 18 

visual system, human, 23-24 

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2.2.2 Disadvantages of Vector Graphics

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