2.2.2 Disadvantages of Vector Graphics
2.2.2 Disadvantages of Vector Graphics
2.2.2 Disadvantages of Vector Graphics
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
___________________________________________________________________________________<br />
Contents<br />
Acknowledgements...........................................................................................................i<br />
1. Introduction ...............................................................................................................1<br />
2. What are Computer <strong>Graphics</strong>?..................................................................................3<br />
2.1. The Nature <strong>of</strong> Computer <strong>Graphics</strong>...........................................................3<br />
2.2. <strong>Vector</strong> <strong>Graphics</strong> .......................................................................................3<br />
2.2.1. Advantages <strong>of</strong> <strong>Vector</strong> <strong>Graphics</strong>...................................................4<br />
<strong>2.2.2</strong>. <strong>Disadvantages</strong> <strong>of</strong> <strong>Vector</strong> <strong>Graphics</strong> ..............................................4<br />
2.2.3. Uses <strong>of</strong> <strong>Vector</strong> <strong>Graphics</strong> ..............................................................5<br />
2.3. Bitmapped <strong>Graphics</strong>.................................................................................5<br />
2.3.1. Resolution and Colour in Bitmaps...............................................5<br />
2.3.2. Advantages <strong>of</strong> Bitmaps................................................................5<br />
2.3.3. <strong>Disadvantages</strong> <strong>of</strong> Bitmaps............................................................6<br />
2.3.4. Lookup Tables, Colour and Greyscale ........................................6<br />
2.4. File Compression .....................................................................................7<br />
2.4.1. Run Length Encoding (RLE).......................................................8<br />
2.4.2. Huffman Coding ..........................................................................8<br />
2.4.3. Other Compression Methods .......................................................9<br />
2.4.4. Lossless v Lossy Compression ....................................................9<br />
2.5. <strong>Graphics</strong> File Formats and Standards ......................................................9<br />
3. <strong>Graphics</strong> Hardware ...................................................................................................13<br />
3.1. Memory Issues.........................................................................................13<br />
3.1.1. Memory Requirements <strong>of</strong> <strong>Vector</strong> v Bitmapped <strong>Graphics</strong> ...........13<br />
3.1.2. Disk Storage.................................................................................14<br />
3.1.3. Computer Memory (RAM)..........................................................15<br />
3.2. Monitors...................................................................................................15<br />
3.2.1. CRT Displays...............................................................................15<br />
3.2.2. Liquid Crystal Displays ...............................................................16<br />
3.2.3. Video Display Standards .............................................................17<br />
3.3. Video Cards .............................................................................................18<br />
3.4. Colour Printers.........................................................................................19<br />
3.4.1. Problems <strong>of</strong> Colour Printing ........................................................19<br />
3.4.2. Types <strong>of</strong> Colour Printer ...............................................................20<br />
3.5. Limitations <strong>of</strong> Colour Output ..................................................................21<br />
3.6. Colour Scanners.......................................................................................22<br />
4. Colour ........................................................................................................................23<br />
4.1. What is Colour? .......................................................................................23<br />
4.2. The Human Visual System ......................................................................24<br />
4.3. The Perception <strong>of</strong> Colour and Brightness................................................25<br />
4.4. Colour Models .........................................................................................25<br />
4.4.1. Additive and Subtractive Colours................................................25<br />
4.4.2. The CIE Diagram.........................................................................26<br />
4.4.3. Red, Green and Blue (RGB) ........................................................27<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> University <strong>of</strong> Hull
Contents<br />
___________________________________________________________________________________<br />
4.4.4. Hue, Light and Saturation (HLS).................................................27<br />
4.4.5. Hue, Saturation and Value (HSV) ...............................................28<br />
4.4.6. Cyan, Magenta, Yellow and Black (CMYK) ..............................29<br />
4.4.7. Other Colour Models ...................................................................29<br />
4.5. The Use <strong>of</strong> Colour....................................................................................30<br />
4.5.1. Lighting and Backgrounds...........................................................30<br />
4.5.2. Warm and Cool Colours ..............................................................30<br />
4.5.3. Colour Deficiency........................................................................31<br />
5. <strong>Graphics</strong> Packages ....................................................................................................33<br />
5.1. Popular Microcomputer <strong>Graphics</strong> Packages............................................33<br />
5.1.1. Painting and Drawing ..................................................................33<br />
5.1.2. Presentation..................................................................................34<br />
5.1.3. Photography .................................................................................35<br />
5.1.4. <strong>Graphics</strong> Utilities .........................................................................36<br />
5.1.5. Animation ....................................................................................36<br />
5.2. Incorporating <strong>Graphics</strong> into Applications and Documents......................37<br />
5.2.1. Programming Languages and Authoring Tools...........................37<br />
5.2.2. Desktop Publishing (DTP)...........................................................38<br />
5.2.3. Placing <strong>Graphics</strong> into Non-<strong>Graphics</strong> Files ..................................39<br />
6. Computer <strong>Graphics</strong> in Higher Education..................................................................41<br />
Glossary ...........................................................................................................................43<br />
Annotated Bibliography...................................................................................................47<br />
Index.................................................................................................................................51<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> University <strong>of</strong> Hull
___________________________________________________________________________________<br />
Acknowledgements<br />
This book was written at the University <strong>of</strong> Hull under the auspices <strong>of</strong> the<br />
Information Technology Training Initiative (ITTI) project Multimedia-based IT<br />
Training for the Humanities. ITTI is an initiative <strong>of</strong> the Information Systems<br />
Committee <strong>of</strong> the Higher Education Funding Councils.<br />
I would like to thank all the people who have helped me, directly or indirectly,<br />
in this project, in particular Dr Lorraine Warren for her role in the genesis <strong>of</strong> this<br />
book, Richard Hicks for his technical advice, James Willmott for introducing me to<br />
cyberspace (and providing landmarks), and Jenny Parsons upon whom I inflicted<br />
early drafts for review and criticism. I also wish to thank all the staff at the University<br />
<strong>of</strong> Hull Computer Centre for all the help and invaluable expertise they have given me<br />
in this and other projects, and the staff at the Language Centre for giving me a home<br />
and cups <strong>of</strong> tea.<br />
Registered Trademarks<br />
The following table lists the registered trademarks used in this work:<br />
Trademark Company<br />
PostScript Adobe<br />
MacPaint Apple<br />
Macintosh Apple<br />
GIF CompuServe<br />
Corel Draw! Corel Corporation<br />
Windows Micros<strong>of</strong>t<br />
Pantone Pantone Inc<br />
Harvard <strong>Graphics</strong> S<strong>of</strong>tware Publishing Corporation<br />
Targa Truevision Inc.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> i University <strong>of</strong> Hull
Acknowledgements<br />
____________________________________________________________________________________<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> ii University <strong>of</strong> Hull
_________________________________________________________________________________<br />
Chapter One<br />
Introduction<br />
We are all familiar with computer graphics, in the sense that we see them<br />
everyday: on television, in films, in books and magazines, on posters, and - <strong>of</strong> course<br />
- on our computer monitors. They are so ubiquitous that we no longer pay them any<br />
special heed.<br />
Yet, even though computer graphics surround us, most <strong>of</strong> us know very little<br />
about them. What are they? How can they be created and edited? How can we use<br />
them? Why should we use them? Even people who use graphics packages are <strong>of</strong>ten<br />
unaware <strong>of</strong> the nature <strong>of</strong> the graphics they are manipulating and are unable to<br />
understand, for example, the difference between metafiles and bitmaps, or why<br />
resizing a picture can distort and degrade it, or how to use the vast range <strong>of</strong> image<br />
effects that are supplied with today's packages, and so on.<br />
This publication attempts to answer such common questions and to thus enable<br />
the reader to understand computer graphics and use them (more) effectively. It is not<br />
a text for programmers who want to write Assembler routines to decode PCX files, or<br />
for people looking for Bezier curve algorithms. Rather, it is aimed at the 'average'<br />
user with at least a basic level <strong>of</strong> computer literacy - that is, you should know the<br />
meanings <strong>of</strong> terms such as processor and operating system - and no previous<br />
knowledge <strong>of</strong> computer graphics is assumed. Instead, the information within is<br />
biased towards the practical, so that the reader can learn about, say, colour models,<br />
then attempt to apply that knowledge in their favourite graphics package. The<br />
emphasis is also upon generic information which can be applied whichever package<br />
you use, rather than specific instructions as to how to carry out operations in a<br />
particular application.<br />
This work is slanted heavily towards microcomputers, in particular PCs and<br />
Apple Macintoshes. There are two main reasons for this. Firstly, the ordinary user is<br />
much more likely to have access to a humble micro than a Sparc workstation; and<br />
secondly, it is highly probable that users <strong>of</strong> high-end graphics workstations are<br />
familiar with computer graphics concepts already and will gain little from a<br />
beginners' text. Nevertheless, much <strong>of</strong> the information in the following pages is<br />
applicable whatever your platform.<br />
Chapter Two, What are Computer <strong>Graphics</strong>?, answers that basic question by<br />
looking at the two different types <strong>of</strong> graphic - vector and bitmap (raster) - and the<br />
associated pros and cons. Much <strong>of</strong> the Chapter is devoted to bitmapped graphics,<br />
particularly the issues <strong>of</strong> memory, disk storage and file compression.<br />
Chapter Three, <strong>Graphics</strong> Hardware, looks at the hardware requirements <strong>of</strong><br />
computer graphics. It explores the topics <strong>of</strong> memory (including video memory) and<br />
disk storage in further detail than Chapter Two, explains how graphics are output on<br />
both monitors and colour printers, and finally considers colour scanners.<br />
Chapter Four, Colour, looks at the human visual system and the way we<br />
perceive colour, goes on to consider some <strong>of</strong> the colour models in common use, and<br />
ends with some brief guidelines on how colour should be used.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 1 University <strong>of</strong> Hull
Introduction<br />
_________________________________________________________________________________<br />
Chapter Five, <strong>Graphics</strong> Packages, looks at the types <strong>of</strong> applications available in<br />
the micro market both to create graphics and to incorporate them within documents<br />
and applications.<br />
Chapter Six, Computer <strong>Graphics</strong> in Higher Education, is a short section which<br />
considers some <strong>of</strong> the possible educational uses to which graphics can be put.<br />
At the end <strong>of</strong> the work there is a glossary, an annotated bibliography, and an<br />
index to help the interested reader dig deeper into the field <strong>of</strong> computer graphics.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 2 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
Chapter Two<br />
What are Computer <strong>Graphics</strong>?<br />
This Chapter looks at the nature <strong>of</strong> computer graphics and considers the issues<br />
related to the storage <strong>of</strong> computer graphics in memory and on disk, including the<br />
important topics <strong>of</strong> image compression and graphics file formats.<br />
2.1 The Nature <strong>of</strong> Computer <strong>Graphics</strong><br />
A computer graphic is really nothing more than an image represented by a<br />
computer, usually on screen and sometimes on a printout. The image may come from<br />
the real world - such as a photograph or a drawing that has been digitised (converted<br />
into computer-readable form) - or it may have been generated in a computer using<br />
graphics s<strong>of</strong>tware. In essence, a computer graphic is no different from an ordinary<br />
picture on paper, at least in appearance; however, being stored in digital form bestows<br />
many advantages on an image. It can be:<br />
• copied freely and stored safely on disk<br />
• distributed with ease, either on disk or by data transmission along<br />
communications lines<br />
• manipulated in literally hundreds <strong>of</strong> different ways by s<strong>of</strong>tware<br />
• incorporated into documents such as reports and publications (Desktop<br />
Publishing, or DTP)<br />
• archived in image libraries<br />
• output to a wide variety <strong>of</strong> devices, particularly monitors, TVs and printers<br />
The vast majority <strong>of</strong> the images we see today - in books or magazines, on<br />
advertising hoardings, on television - are, or at some time have been, computer<br />
graphics. All graphics fall into two broad categories: vector graphics and bitmapped<br />
graphics, the difference between the two being the method <strong>of</strong> storing the image data.<br />
2.2 <strong>Vector</strong> <strong>Graphics</strong><br />
<strong>Vector</strong> images are composed <strong>of</strong> objects. All objects are built up from primitives<br />
- basic drawing instructions such as line, rectangle and ellipse - and objects may be<br />
grouped together to form new composite objects to form an object hierarchy.<br />
Consider, for example, a picture <strong>of</strong> an aircraft in vector format (Figure 2.1).<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 3 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
Figure 2.1 Figure 2.1a<br />
<strong>Vector</strong> image <strong>of</strong> aircraft Partially disassembled vector image<br />
At its simplest level it is composed <strong>of</strong> primitives such as circles, lines and<br />
rectangles; however, it can also - and more usefully - be represented as a collection <strong>of</strong><br />
objects (wheels, engines, wings, doors, etc - Figure 2.1a) which can be composed <strong>of</strong><br />
primitives and objects. So, for instance, the wing object is composed <strong>of</strong> component<br />
objects (ailerons, flaps, engine) which in turn contain other objects (the engine<br />
contains a propeller, fuel feed, etc) and so on down the hierarchy until primitives are<br />
reached. The wing object itself, <strong>of</strong> course, is part <strong>of</strong> the overall aircraft object. In a<br />
drawing package objects can be aggregated and disassembled at will, giving the<br />
designer <strong>of</strong> the graphic considerable flexibility.<br />
2.2.1 Advantages <strong>of</strong> <strong>Vector</strong> <strong>Graphics</strong><br />
<strong>Vector</strong> graphics are highly flexible in terms <strong>of</strong> image manipulation: they can be<br />
resized in any direction and to any magnitude without loss <strong>of</strong> quality (although if<br />
scaled by different amounts in the horizontal and vertical directions some distortion<br />
<strong>of</strong> proportion will occur) and their constituent objects and primitives may also be<br />
scaled or moved at will. <strong>Vector</strong> images are also very cheap in terms <strong>of</strong> memory as the<br />
image data is simply a set <strong>of</strong> graphical instructions to the computer - eg<br />
line(x1,y1,x2,y2), circle(x,y,radius) - together with their parameters (or operands) -<br />
x1, y1, radius - and any associated colour data, all <strong>of</strong> which is coded as a small set <strong>of</strong><br />
numbers which take up very little disk space, enabling a complex image to be stored<br />
in a file only a few tens <strong>of</strong> kilobytes in size.<br />
<strong>2.2.2</strong> <strong>Disadvantages</strong> <strong>of</strong> <strong>Vector</strong> <strong>Graphics</strong><br />
All vector graphics are computer-generated - by definition - and thus are rarely,<br />
if ever, truly accurate representations <strong>of</strong> real-world objects. It would be extremely<br />
impracticable - and in all likelihood impossible - to draw a vector image <strong>of</strong>, say, an<br />
oak tree which incorporated all the whorls and knarls within the trunk and the<br />
intricate structure <strong>of</strong> branches. It would certainly be possible to draw a schematic oak<br />
to show that all oaks have similar overall shapes, the same leaf and the same fruit and<br />
<strong>of</strong>ten this would be all that was necessary, but it would contain only a fraction <strong>of</strong> the<br />
visual information present in a photograph <strong>of</strong> a particular oak 1 . Similarly, whilst a<br />
vector image <strong>of</strong> the Mona Lisa would be recognisable as a representation <strong>of</strong><br />
1 Advanced graphics packages allow the 'vectorisation' <strong>of</strong> bitmapped images so that the photo <strong>of</strong> the oak could be<br />
traced into a vector form. Nevertheless, the real-world image is still created outside the computer.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 4 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
Leonardo's work it would not be capable <strong>of</strong> storing the level <strong>of</strong> detail that exists in the<br />
original portrait.<br />
2.2.3 Uses <strong>of</strong> <strong>Vector</strong> <strong>Graphics</strong><br />
<strong>Vector</strong> graphics find their metier in technical areas such as CAD/CAM<br />
(Computer-Aided Design/Manufacture), scientific modelling, and architecture where<br />
the ability to manipulate parts <strong>of</strong> a graphic - moving, copying, deleting, resizing, etc -<br />
is <strong>of</strong> high importance. <strong>Vector</strong> graphics are also increasingly being used in the 'graphic<br />
art' world, with the advent <strong>of</strong> sophisticated <strong>of</strong>f-the-shelf vector-based graphics<br />
applications such as Corel Draw! and Harvard <strong>Graphics</strong>.<br />
2.3 Bitmapped <strong>Graphics</strong><br />
Bitmaps - sometimes known as raster graphics - are images composed <strong>of</strong><br />
discrete dots known as picture elements or pixels (Plates 1, 1a), each <strong>of</strong> which can be<br />
any colour within a specified range <strong>of</strong> colours. Bitmaps can be created on the<br />
computer but most are real-world images in digital form, such as satellite<br />
photographs.<br />
2.3.1 Resolution and Colour in Bitmaps<br />
The resolution <strong>of</strong> a bitmap is determined by its horizontal and vertical<br />
dimensions measured in pixels. Thus a 640 x 480 bitmap displayed on a standard<br />
VGA monitor will look better than a 320 x 200 bitmap displayed in the same area,<br />
although it will be inferior to a 1024 x 768 bitmap. The simple principle is that the<br />
greater the number <strong>of</strong> pixels per unit area the better the resolution and the fewer<br />
visual imperfections in the picture.<br />
The colour depth2 <strong>of</strong> a bitmap is determined by the amount <strong>of</strong> memory allocated<br />
to each pixel. Once again, there is a simple principle, which is that the number <strong>of</strong><br />
colours that can be displayed is given by 2 to the power <strong>of</strong> the number <strong>of</strong> bits<br />
available per pixel. With 4 bits (a ‘nibble’!) per pixel there are a possible 24 = 16<br />
colours, with 8 bits (a byte) a possible 28 = 256 colours, and the most natural-looking<br />
results are obtained with 24-bit colour which can display up to 224 = 16.7 million<br />
colours per pixel. Naturally, high-resolution bitmaps can only be displayed in their<br />
full glory on appropriate hardware with sufficient video card memory (video RAM, or<br />
VRAM - see Chapter 3). However, it is usually possible to display any resolution <strong>of</strong><br />
bitmap on the humblest 16-colour systems (such as the base VGA standard) although<br />
<strong>of</strong> course the greater the gap between image resolution and system capability the<br />
greater the degradation in image quality.<br />
2 Sometimes known more technically as the number <strong>of</strong> bit planes.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 5 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
2.3.2 Advantages <strong>of</strong> Bitmaps<br />
The bitmap format is ideal for detailed art and real-world images. Bitmaps can<br />
potentially store immense amounts <strong>of</strong> information (as reflected by the large file sizes)<br />
and can be edited in great detail, even to adjusting the colour <strong>of</strong> individual pixels.<br />
Artistic brush and smear effects can be applied to simulate 'real' painting.<br />
Quite sophisticated effects can be achieved on bitmaps with the right s<strong>of</strong>tware.<br />
Image Processing (IP) techniques can be applied to sharpen or smooth details, adjust<br />
contrast and brightness, apply different coloured filters to the image, detect edges and<br />
thresholds, remove noise, and so on. This sort <strong>of</strong> image enhancement is frequently<br />
used by, say, geologists or archaeologists on aerial or satellite photos in order to<br />
reveal structures on the ground, and such advanced IP tools are now available to<br />
ordinary users <strong>of</strong> graphics packages.<br />
2.3.3 <strong>Disadvantages</strong> <strong>of</strong> Bitmaps<br />
It is difficult to resize bitmaps without image degradation or information loss.<br />
Enlarging a bitmap means that new pixels have to be created so as not to leave blank<br />
spots, and the colour <strong>of</strong> each new pixel is commonly based on the colour <strong>of</strong> its<br />
neighbours - this usually results in a blocky, unnatural appearance (Plate 2).<br />
Reducing a bitmap involves discarding pixels, which necessarily results in loss<br />
<strong>of</strong> information and detail. As with vector images distortion will also occur if the<br />
image is not scaled equally in the horizontal and vertical dimensions because the<br />
elements <strong>of</strong> the picture will no longer be in the proper proportion to each other (Plate<br />
2a).<br />
Bitmaps can be quite expensive in terms <strong>of</strong> memory as the value <strong>of</strong> each<br />
individual pixel is recorded: the size <strong>of</strong> the bitmap in bytes is the product <strong>of</strong> the<br />
horizontal and vertical dimensions (in pixels) and the number <strong>of</strong> bytes per pixel. For<br />
example, a 256-colour (ie 1 byte/pixel) bitmap <strong>of</strong> dimensions 640 x 480 pixels (the<br />
same size as a standard VGA monitor) will take up 640 x 480 x 1 = 307,200 bytes.<br />
As the colour depth <strong>of</strong> the image is improved so does the memory requirement: a 'true<br />
colour', 24-bit (ie 3 bytes/pixel) image (such as a photograph) <strong>of</strong> the same size will<br />
take up 3 times as much memory, or 921,600 bytes. Moreover, 640 x 480 is quite a<br />
low resolution for the human eye, and truly realistic images can require resolutions in<br />
the region <strong>of</strong> 3000 x 2000 x 24-bit ≈ 18MB <strong>of</strong> data.<br />
2.3.4 Lookup Tables, Colour and Greyscale<br />
Pixels have numerical values associated with them, and when these numbers are<br />
interpreted by a display system - a monitor, or a printer, say - particular colours are<br />
produced. The values are interpreted as different intensity levels for each <strong>of</strong> the three<br />
electron guns <strong>of</strong> a monitor - red, green and blue. With a 24-bit system one byte (8<br />
bits) is allocated to each gun, allowing 2 8 = 256 intensity levels for each primary<br />
colour, and three bytes - representing the levels for each gun - make up a pixel.<br />
The full range <strong>of</strong> colours available on a display system is known as its palette,<br />
but <strong>of</strong>ten only subsets <strong>of</strong> the palette are desirable or even possible. In high-end truecolour<br />
systems the user may wish to restrict the range <strong>of</strong> colours available for<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 6 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
different types <strong>of</strong> work - pastel shades for artistic work, or saturated colours for<br />
presentations, say - and in less expensive systems the realisable colour range is<br />
usually smaller than the palette. To enable these subsets to be used a Lookup Table<br />
(LUT) is kept in memory comprising as many entries as there are pixel values, each<br />
entry containing a value corresponding to a screen colour (Figure 2.2).<br />
Pixel in Video Memory<br />
(Framestore)<br />
47<br />
255<br />
48<br />
47<br />
46<br />
Figure 2.2<br />
Colour Lookup Table in 256-colour (8-bit) display system<br />
0<br />
Lookup Table Monitor<br />
137<br />
136<br />
135<br />
Pixel <strong>of</strong> colour 136<br />
The pixel value in the image is used as an index into the LUT, so that a pixel <strong>of</strong><br />
value 47 would cause the colour in the 47th entry in the LUT to be displayed rather<br />
than colour number 47 - in the example shown in Figure 2.2 this would be 136. Both<br />
the values in the entries and the entry point to the LUT can be changed at will which<br />
can result in very different colours being displayed with the same pixel values: in this<br />
way the visual appearance <strong>of</strong> an image can be changed without altering the actual<br />
image data. LUTs are used extensively in graphics applications both to enable userdefined<br />
colour subsets (which, confusingly, are usually called 'palettes') and for<br />
special effects.<br />
Because pixels are only represented by values, there is no reason why they<br />
should display colour at all when it is not necessary. Often, for scientific purposes,<br />
colour is an artificial distraction, and it can be more informative to see an image in<br />
greyscale, where the different pixel values are interpreted as lying on a monochrome<br />
scale ranging from pure white to pure black; satellite data is <strong>of</strong>ten recorded and<br />
manipulated in greyscale. Whilst colours are sometimes added to enhance certain<br />
features <strong>of</strong> the image to the human eye these are purely artificial and the result is<br />
known as a false colour image.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 7 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
2.4 File Compression<br />
As bitmaps followed hardware advances in becoming more colourful and<br />
detailed, so the mushrooming memory requirements for these images spurred the<br />
development <strong>of</strong> file compression methods. Compression is the process <strong>of</strong> eliminating<br />
redundant data in a file so as to reduce its size, and advances in compression<br />
techniques have taken on crucial - if not determining - importance as the field <strong>of</strong><br />
computer graphics has advanced into true-colour, high-resolution images and moving<br />
video. It would, for instance, be practically impossible to store video sequences on<br />
disk without sophisticated file compression techniques being applied to the data 3 .<br />
There are a wide range <strong>of</strong> compression algorithms in use today, with more<br />
being developed every year, which are not only used for graphics files but also for<br />
'ordinary' data and program files. It is now commonplace for commercial applications<br />
to be distributed on disk in compressed form. The subject <strong>of</strong> data compression is large<br />
and <strong>of</strong>ten highly technical and is <strong>of</strong> little interest to ordinary users <strong>of</strong> graphics, who<br />
are only concerned that their images be reduced to a manageable size. However,<br />
consideration <strong>of</strong> two <strong>of</strong> the simplest and most common compression methods used in<br />
the field <strong>of</strong> computer graphics will illustrate the redundancy to be found in many<br />
bitmapped images and the advantages <strong>of</strong> compression.<br />
2.4.1 Run Length Encoding (RLE)<br />
Consider a - 256-colour, for convenience - bitmap, large areas <strong>of</strong> which are the<br />
same colour. The 'raw' method <strong>of</strong> storing such an image is to allocate one byte per<br />
pixel, that byte containing the numerical value <strong>of</strong> the pixel, (58, say); thus, a 640 x<br />
480 pixel bitmap would be stored as 640 x 480 x 1 = 307,200 bytes. However, in<br />
images with uniform areas <strong>of</strong> colour - that is, adjacent pixels having the same value -<br />
a more efficient storage method is to find sets <strong>of</strong> adjacent pixels <strong>of</strong> the same value<br />
and store each set as a pair <strong>of</strong> bytes, one byte <strong>of</strong> the pair being the number <strong>of</strong> pixels in<br />
the set and the other the pixel value. For example, consider the following sequence <strong>of</strong><br />
pixels from a bitmap:<br />
57 57 57 57 57 57 110 110 110 132 55 200 200 200 200 200 200 200 200 200 200<br />
Ordinarily, these 22 values would require 22 bytes for storage; however, using<br />
RLE they can be encoded as follows:<br />
{5,57} {2,110} {0,132} {0,55} {9,200}<br />
That is, 6 - computers count from zero! - bytes <strong>of</strong> 57, then 3 <strong>of</strong> 110, then 1 <strong>of</strong><br />
132, then 1 <strong>of</strong> 55, then 10 <strong>of</strong> 200, and so on. Note that RLE has reduced the data from<br />
21 to 10 bytes, a reduction <strong>of</strong> over 50%, and reductions <strong>of</strong> 90%+ are possible with<br />
3 A 100MB hard disk would only be able to hold 13 seconds <strong>of</strong> full-screen, full-motion video, and a 600MB<br />
CDROM would be filled by 78 seconds.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 8 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
suitable images. The downside to RLE is that, when used on complex bitmaps where<br />
adjacent pixels are rarely the same colour, it can actually produce a larger file. 4<br />
2.4.2 Huffman Coding<br />
This is a statistical compression method based on the frequency <strong>of</strong> occurrence<br />
<strong>of</strong> pixel values. The bitmap is analysed to produce a table consisting <strong>of</strong> each value<br />
and the number <strong>of</strong> times it occurs, then binary codes are allotted to each value such<br />
that the shortest codes belong to the most frequently occurring values. For example,<br />
the following table might represent the first four rows <strong>of</strong> a frequency analysis <strong>of</strong> a<br />
bitmap:<br />
Pixel Value Frequency <strong>of</strong> Occurrence Code (binary)<br />
54 132 0<br />
22 84 01<br />
112 57 10<br />
243 33 11<br />
Once the analysis is complete, the pixel values are replaced by the binary codes,<br />
so the following sequence might be replaced by the bit sequence below it:<br />
112 243 243 54 54 54 22 22<br />
10 11 11 0 0 0 01 01<br />
thus replacing an 8-byte sequence with a stream <strong>of</strong> 13 bits (1011110000101), an<br />
80% compression. Huffman coding works best with bitmaps with a small range <strong>of</strong><br />
values; the larger the range the larger the binary code to represent each value, until<br />
the situation is reached where the code is bigger than the value.<br />
2.4.3 Other Compression Methods<br />
More complex methods include LZW (Lempel-Ziv-Welch), DCT (Discrete<br />
Cosine Transform) and the intriguing Fractal Compression which reduces complex,<br />
real-world images (such as photos) by decomposing them into 'fractals' (objects with<br />
'fractional dimensions') which can be described by matrices <strong>of</strong> numbers called affine<br />
transformations 5 .<br />
2.4.4 Lossless v Lossy Compression<br />
All <strong>of</strong> the compression methods described thus far are lossless - that is, no<br />
image data is lost during compression. Lossy methods also exist, which rely on the<br />
inability <strong>of</strong> human eyes to spot slight image degradation, and thus trade some slight<br />
data loss for efficiency and greater compression; the JPEG method (see next section)<br />
4 RLE is used by the PCX file format, and an interesting experiment in the efficacy <strong>of</strong> the compression can be<br />
carried out by opening BMP files - which contain raw, uncompressed image data - in Windows Paintbrush,<br />
saving them as PCX files, then comparing the different file sizes for the same image.<br />
5 A detailed description <strong>of</strong> fractal compression can be found in Peterson[1988], pp 128-132<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 9 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
allows for 'lossy' compression <strong>of</strong> images which are to be viewed by people, rather<br />
than processed by computers.<br />
2.5 <strong>Graphics</strong> File Formats and Standards<br />
There are a bewildering plethora <strong>of</strong> methods by which graphics may be stored<br />
and/or compressed, and this is reflected in the large number <strong>of</strong> graphics file formats<br />
available. This used to be a major headache for computer users who wished to<br />
incorporate graphics into their work, whether they were applications programmers or<br />
secretaries, because in the early days <strong>of</strong> computer graphics these formats were<br />
incompatible. A graphic produced in one drawing package couldn't be imported into<br />
another drawing package which didn't support the particular format that the creating<br />
package used unless an - <strong>of</strong>ten temperamental - conversion program was employed.<br />
This situation created great pressure both for the development <strong>of</strong> common<br />
graphics standards and for applications programmers to build a wide variety <strong>of</strong><br />
graphics filters (conversion utilities) into their applications. Whilst there are still a<br />
large number <strong>of</strong> formats tied to particular products (eg Corel Draw! .CDR files) Table<br />
2.1 below lists the most common formats which are largely independent <strong>of</strong> particular<br />
packages:<br />
Bitmap <strong>Vector</strong><br />
BMP (Windows Paintbrush) CGM (Computer <strong>Graphics</strong> Metafile)<br />
GIF (Compuserve <strong>Graphics</strong> Interchange<br />
Format)<br />
DXF (Computer-Aided Design)<br />
MAC (MacPaint - monochrome only) WMF (Windows Metafile) 6<br />
PCX (PC Paintbrush)<br />
PICT (Macintosh)<br />
TGA (Targa - 24-bit)<br />
TIFF (Tagged Image File Format)<br />
JPEG (Joint Photographic Experts Group)<br />
Table 2.1<br />
Common graphics file formats<br />
Although formats such as PCX and TGA were designed for particular products<br />
(PC Paintbrush and the Targa video card respectively) they have gained widespread<br />
acceptance in the marketplace; others, such as CGM and TIFF, were designed by<br />
groups independent <strong>of</strong> particular firms, and have been approved by the ISO<br />
(International Standards Organisation). New formats are constantly being developed<br />
by both companies and bodies to cater for the rapid advances in graphics-related<br />
hardware and s<strong>of</strong>tware, an example being MPEG (Motion Picture Experts Group),<br />
designed for digital moving video 7 .<br />
6 Whilst this format is dependent upon the Micros<strong>of</strong>t Windows environment it is independent <strong>of</strong> any particular<br />
Windows application.<br />
7 Indeed, the whole area <strong>of</strong> graphics standards and formats is being increasingly regulated and formalised under<br />
the umbrella <strong>of</strong> the ISO. Now, developers <strong>of</strong> graphics packages are under pressure to build their applications<br />
according to international standards: GKS (Graphical Kernel System) and its successor PHIGS (Programmer's<br />
Hierarchical Interactive <strong>Graphics</strong> System) are prominent examples <strong>of</strong> these standards.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 10 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
<strong>Graphics</strong> can also sometimes be found in Postscript (.PS or .EPS) format.<br />
Postscript is a page description language created by Adobe which has rapidly become<br />
a printer standard. Each Postscript file is essentially a computer program which tells a<br />
printer fitted with a Postscript reader how to print the page. A Postscript file created<br />
in any application can be printed on any output device with a Postscript reader, and<br />
this independence from particular hardware or s<strong>of</strong>tware makes it an ideal general<br />
format. Postscript comes in a number <strong>of</strong> variants - Level 1, Level 2, Encapsulated,<br />
and Display - and is most <strong>of</strong>ten used for documents mixing text and graphics; it is not<br />
the ideal format for storing image data alone, particularly in bitmap format.<br />
The user might wonder what differentiates these formats. This book is not<br />
intended to cover the highly technical differences between formats (the interested<br />
reader is directed towards Rimmer[1990] and Kay and Levine [1992]) which are<br />
normally invisible to the user: <strong>of</strong>ten, it will make little odds whether, say, a bitmap is<br />
stored as PCX or TIFF. There are, however, general points which should be born in<br />
mind:<br />
• BMP files, unlike nearly all other bitmapped formats, are not compressed so in<br />
the majority <strong>of</strong> cases a BMP file will be larger - <strong>of</strong>ten substantially so - than,<br />
say, a PCX file containing the same image. This is obviously disadvantageous<br />
in terms <strong>of</strong> disk space, but can be a plus when viewing bitmaps on a slow<br />
computer, as viewing a compressed file requires the computer's main processor<br />
to work on the decompression.<br />
• Of the above formats, only PICT and DXF are unable to support 24-bit colour.<br />
• TIFF files can come in different 'flavours' - that is, some applications add little<br />
'tweaks' to the TIFF files they generate which can make them unreadable to<br />
other applications - this is inherent in the design <strong>of</strong> TIFF, which contains a<br />
number <strong>of</strong> 'hooks' onto which new methods <strong>of</strong> encapsulation or compression <strong>of</strong><br />
graphic data can be attached. Applications written prior to any new methods<br />
may thus be unable to read more up-to-date TIFF files. Similarly, the JPEG<br />
format contains a number <strong>of</strong> options which can result in readability and<br />
compatibility problems<br />
This plethora <strong>of</strong> standards has spawned a large number <strong>of</strong> file conversion<br />
utilities, enabling users to convert, say, a PCX file to TIFF: although many <strong>of</strong> these<br />
utilities are <strong>of</strong> dubious quality, they do have the advantage <strong>of</strong> usually being in the<br />
public domain, either as freeware or shareware.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 11 University <strong>of</strong> Hull
What are Computer <strong>Graphics</strong>?<br />
_________________________________________________________________________________<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 12 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
Chapter Three<br />
<strong>Graphics</strong> Hardware<br />
This Chapter considers the hardware issues involved in computer graphics. It<br />
explores the sort <strong>of</strong> hardware required to display, print and scan graphics in<br />
microcomputer systems, after having first looked at the question <strong>of</strong> memory and<br />
storage.<br />
Hardware is <strong>of</strong> crucial importance to graphics, probably more so than s<strong>of</strong>tware:<br />
the cleverest algorithms in the world are <strong>of</strong> no use if the hardware is incapable <strong>of</strong><br />
displaying the results. Indeed, it is primarily technical advances in hardware that have<br />
brought ordinary users into the field <strong>of</strong> high quality computer graphics which was<br />
previously the sole province <strong>of</strong> pr<strong>of</strong>essional designers.<br />
3.1 Memory Issues<br />
Generating computer graphics places great demands on a system in terms <strong>of</strong><br />
memory, particularly in the case <strong>of</strong> bitmapped graphics. Until relatively recently,<br />
dynamic memory - that is, random access memory (RAM) - has been at a premium in<br />
the world <strong>of</strong> the desktop computer; the original PC XT, for example, only had 1M <strong>of</strong><br />
RAM, <strong>of</strong> which only 640k was available for application programs. Similarly, only<br />
during the second half <strong>of</strong> the 1980s did microcomputers acquire large amounts <strong>of</strong> disk<br />
storage space as hard disks became larger, faster, and above all cheaper. A positive<br />
feedback cycle exists whereby hardware advances in memory allow more complex<br />
s<strong>of</strong>tware to be written which then encourages the hardware manufacturers to enhance<br />
their product, and so on.<br />
The reason why graphical images take up a lot <strong>of</strong> memory is, simply, because<br />
they contain a lot <strong>of</strong> information. The old saying that "a picture says a thousand<br />
words" probably underestimates the information content <strong>of</strong> a detailed image such as a<br />
photograph by one or two orders <strong>of</strong> magnitude. For example, a colour picture <strong>of</strong> a<br />
landscape may contain information relating to the geology and geomorphology <strong>of</strong> the<br />
scene, the weather and season when the photo was taken, and past and present social,<br />
economic and cultural uses <strong>of</strong> the land. To transfer all the information in an image <strong>of</strong>,<br />
say, Dentdale in the Yorkshire Dales to text would take many pages, even in the most<br />
concise writing.<br />
3.1.1 Memory Requirements <strong>of</strong> <strong>Vector</strong> v Bitmapped<br />
<strong>Graphics</strong><br />
As noted in the previous Chapter, vector-based images require much less<br />
memory than bitmaps because they are stored as sets <strong>of</strong> objects, each object<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 13 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
containing other objects and/or graphical primitives, such as lines and rectangles: in<br />
effect, a vector graphic is no more than a collection <strong>of</strong> drawing instructions which can<br />
be stored in a relatively small number set. So, the graphic does not store the<br />
information about the individual elements <strong>of</strong> the screen - pixels - but rather the<br />
instructions to reconstitute objects: the Olympic flag, for example, would be stored in<br />
vector format as:<br />
a rectangle<br />
the top left and bottom right rectangle coordinates<br />
five circle instructions<br />
the centre and radius <strong>of</strong> each circle<br />
the colour <strong>of</strong> each circle<br />
A bitmap <strong>of</strong> the flag, however, would contain the value <strong>of</strong> each pixel in the<br />
image, even though much <strong>of</strong> that data is redundant. In an experiment such an image<br />
was drawn and saved in vector and bitmapped formats: the vector file was a mere 315<br />
bytes in size, compared to the 65k the bitmap took up. Of course, the bitmap can be<br />
compressed - a PCX file <strong>of</strong> the same image was only 9k in size - but nevertheless this<br />
example illustrates the point that, for images composed <strong>of</strong> objects the vector graphic<br />
format is most appropriate.<br />
3.1.2 Disk Storage<br />
Plainly bitmaps require large amounts <strong>of</strong> disk space. This is - and has always<br />
been - a problem in the field <strong>of</strong> graphics, which is addressed on two fronts: hardware<br />
and s<strong>of</strong>tware.<br />
On the hardware side, the amount <strong>of</strong> data that can be stored on magnetic, floppy<br />
disk has steadily increased over time until, at the time <strong>of</strong> writing, 1.44 MB floppy<br />
disks are standard and 2.88 MB disks are just emerging. However, magnetic media<br />
are no longer sufficient to hold the vast amounts <strong>of</strong> data required by modern<br />
graphically-intensive applications and true colour 8 bitmaps, and thus optical media<br />
are rapidly coming into their own.<br />
Optical disks use visible laser light to read and write data from and to the disk.<br />
The most common optical disk is the CDROM (Compact Disk Read-Only Memory)<br />
with a data capacity <strong>of</strong> around 600MB. CDROMs are rapidly becoming the standard<br />
for the distribution <strong>of</strong> large applications and graphics because <strong>of</strong> their low production<br />
costs, robustness, and the increasing availability <strong>of</strong> cheap CDROM drives which will<br />
certainly become standard on all desktop computers in the very near future. Another<br />
read-only optical disk is the WORM (Write Once Read Many times): this is usually<br />
used for large-scale archiving, and is unlikely to impact the desktop market for quite<br />
some time. Very recently a number <strong>of</strong> read/write optical and magneto-optical disks<br />
have arrived promising astonishing data densities - 1.3 Gigabytes on a 5.25" disk! 9 -<br />
and one <strong>of</strong> these new optical technologies may well become a standard in the future 10 .<br />
8 This term usually refers to 24-bit colour.<br />
9 "A New Phase for the Floppy", Personal Computer World, Vol 16 No 4, April 1993, pp 457-8.<br />
10 An early and quite common form <strong>of</strong> optical disk, still in use today, is the Videodisc. However, this is used to<br />
hold video in analogue form and is unsuitable for storing digital data.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 14 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
S<strong>of</strong>tware solutions to the problem <strong>of</strong> large bitmaps take the form <strong>of</strong><br />
compression algorithms (see the section on File Compression in the previous<br />
Chapter). These are methods <strong>of</strong> removing redundant data from the bitmap in order to<br />
reduce its size and on the whole are very successful: nearly all the common bitmap<br />
file formats utilise one or more compression methods. However, the efficacy <strong>of</strong><br />
compression declines as the complexity <strong>of</strong> the image increases, unless one is willing<br />
to accept some degradation <strong>of</strong> the picture; nevertheless, compression is an invaluable<br />
tool for reducing the majority <strong>of</strong> bitmapped images to manageable sizes.<br />
3.1.3 Computer Memory (RAM)<br />
For the same reason that they place great demands on disk space, bitmapped<br />
graphics also require large amounts <strong>of</strong> working memory, or RAM (Random Access<br />
Memory), both to display the image and to manipulate it. Display memory is known<br />
as Video RAM (VRAM), a special fast type <strong>of</strong> RAM purely dedicated to the video<br />
display; this will be discussed in section 3.3 on Video Cards.<br />
For manipulation purposes - editing, adding special effects, etc - the image, or<br />
portions <strong>of</strong> it, are loaded into RAM from disk. Until relatively recently this was a<br />
distinct problem as not only was RAM quite expensive in relation to the cost <strong>of</strong> disk<br />
space but many microcomputers had a low limit to the amount <strong>of</strong> RAM that could be<br />
added; however, the cost <strong>of</strong> memory chips has plummeted sharply in recent years and<br />
microcomputer manufacturers now incorporate substantial memory expansion<br />
capacity into their products. The average PC or Mac is now capable <strong>of</strong> handling highresolution<br />
8-bit (256 colour) images, and for relatively little extra cost can be turned<br />
into a full-blown true-colour graphics workstation.<br />
3.2 Monitors<br />
The computer monitor is the most commonly used output device for computer<br />
graphics. Virtually all modern monitors in general use build up the picture from<br />
horizontal lines running from the top to the bottom <strong>of</strong> the screen, each line consisting<br />
<strong>of</strong> a series <strong>of</strong> individual dots. They fall into two categories: Cathode Ray Tubes<br />
(CRTs) and Liquid Crystal Displays (LCDs)<br />
3.2.1 CRT Displays<br />
The CRT is probably the most ubiquitous electronic display device <strong>of</strong> modern<br />
times, being at the heart <strong>of</strong> every television set. Beams <strong>of</strong> electrons ('cathode rays')<br />
from three electron guns are fired through a shadow mask - a sheet <strong>of</strong> metal with<br />
regular apertures which focus the beam - to strike phosphor dots on the screen<br />
surface. There are three types <strong>of</strong> phosphor, as there are three electron guns, one for<br />
each primary colour - red, green and blue - and when a phosphor is struck by the<br />
electron beam from 'its' gun it emits its characteristic colour. The strength <strong>of</strong> this<br />
emission, that is the luminance <strong>of</strong> the phosphor, is proportional to the power <strong>of</strong> the<br />
beam, and the combination <strong>of</strong> the three phosphors at their different intensities<br />
produces the colour <strong>of</strong> the picture element, or pixel. The electron guns build up the<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 15 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
picture, or frame, line by line from top to bottom, and each line - or scan - is<br />
composed <strong>of</strong> many pixels. As the dots only phosphoresce for a short time after being<br />
struck by the electron beam the screen has to be redrawn, or refreshed, many times a<br />
second: for a flicker-free display a refresh rate <strong>of</strong> 25-30 frames per second is<br />
required.<br />
Blue<br />
Green<br />
Red<br />
Electron Guns<br />
Figure 3.1<br />
Colour CRT system<br />
R<br />
R<br />
G R B G<br />
G<br />
G<br />
B<br />
B<br />
R<br />
R<br />
G<br />
G<br />
B<br />
B<br />
R<br />
R<br />
G<br />
G<br />
B<br />
B<br />
Shadow Mask<br />
Pixel<br />
Figure 3.2<br />
Arrangement <strong>of</strong> phosphors on CRT screen<br />
Screen<br />
Red phosphor<br />
Green phosphor<br />
Blue phosphor<br />
Different colours are produced by assigning different voltages to each electron<br />
gun thus lighting phosphors to different intensities, and the colour range a monitor is<br />
capable <strong>of</strong> is determined by how many voltage levels can be supported by the guns.<br />
For example, the now obsolete EGA (Enhanced <strong>Graphics</strong> Adaptor) standard for PC<br />
monitors had only 4 levels per gun which could produce 64 colours (4x4x4), although<br />
in practice the standard dictated that only 16 colours were available at any one time 11 .<br />
In contrast, SVGA (Super VGA) allows for 256 voltage levels per gun producing a<br />
maximum 16.7 million (256 3 ) colours. Greyscale images - where the colour <strong>of</strong> a pixel<br />
lies in a range from white to black - are produced by assigning the same voltage to<br />
each gun so that each colour phosphor shines at the same intensity, resulting in as<br />
many shades <strong>of</strong> grey as there are gun levels.<br />
3.2.2 Liquid Crystal Displays<br />
LCD systems use long crystalline molecules ('liquid crystals') which change<br />
their position when an electric field is applied. An LCD display consists <strong>of</strong> a thin<br />
layer <strong>of</strong> liquid crystal sandwiched between two densely-packed sets <strong>of</strong> thin wires, one<br />
horizontal and one vertical (Figure 3.3). Together these wires form an interlocking<br />
grid, each intersection representing a dot on the display. This sandwich is in turn<br />
sandwiched by two polarising filters, again one horizontal and one vertical. The<br />
display is created by matrix addressing whereby each dot is addressed in turn by<br />
passing a current through each horizontal and vertical wire in sequence, and<br />
11 EGA allocated only 4 bits per pixel which meant that, at any one time, one gun had 2 bits (4 levels) and the<br />
others had 1 bit (2 levels) apiece, allowing only 4x2x2=16 colours.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 16 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
whenever the combined currents at an intersection are sufficiently strong the resulting<br />
electric field moves the crystals at that point so that when seen through the polarising<br />
filters they are opaque - that is, the dot becomes dark.<br />
Figure 3.3<br />
Elements <strong>of</strong> a Liquid Crystal Display. (Adapted from Foley et al [1990].)<br />
Modern liquid crystal displays are backlit by an integral light source as displays<br />
which depend upon incident light perform poorly in low light environments, as well<br />
as being prone to reflection in bright environments which obscures the display.<br />
Colour LCDs operate on the same principle <strong>of</strong> molecules changing their<br />
orientation under electric fields but use three liquid crystal layers - one each for red,<br />
green and blue - and coloured polarising filters to generate a palette <strong>of</strong> colours.<br />
LCDs are capable <strong>of</strong> very high resolutions and <strong>of</strong> course require very little<br />
power, and are used in devices where a high-voltage CRT is inappropriate, such as<br />
portable computers and hand-held televisions.<br />
3.2.3 Video Display Standards<br />
Whilst there have been a plethora <strong>of</strong> display standards in the PC world, at the<br />
time <strong>of</strong> writing the most common are VGA (Video <strong>Graphics</strong> Adaptor) and SVGA.<br />
VGA allows for 16 colours at 640 x 480 resolution, or 256 at 320 x 200. The SVGA<br />
standard encompasses any display that is superior to VGA and allows for a number <strong>of</strong><br />
permutations <strong>of</strong> resolution and colour, from the maximum 1280 x 1024 x 16.7M to<br />
the minimum <strong>of</strong> 640 x 480 x 256. It should be added, however, that many <strong>of</strong> the<br />
cheaper monitors achieve the maximum SVGA resolution by a technique known as<br />
interlacing, whereby every other line on the display is drawn in each frame - rather<br />
than every consecutive line, as is normal - meaning that it takes two complete scans to<br />
create a picture. The disadvantage <strong>of</strong> this is that the frame rate - the number <strong>of</strong> frames<br />
per second - is halved and this can lead to perceptible screen flickering.<br />
It is important to note that these standards are independent <strong>of</strong> the physical size<br />
<strong>of</strong> the monitor: SVGA has the same resolution and colour depth regardless <strong>of</strong> whether<br />
the monitor is 14" or 21". For this reason it is <strong>of</strong>ten a good idea for users who<br />
constantly work at high resolutions (800 x 600 or better) to obtain large monitors,<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 17 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
particularly if their work involves Desktop Publishing: a letter which is, say, 10 pixels<br />
vertically will look illegibly small on a 1024 x 768 display on a 14" monitor but will<br />
be perfectly readable at the same resolution on a 21".<br />
PC monitors operate in either text mode or graphics mode. In text mode - the<br />
default - graphics cannot be output and text is displayed on screen in a standard font<br />
using a hardware character generator. In order to display graphics the monitor has to<br />
be switched by s<strong>of</strong>tware into graphics mode. These modes are normally invisible to<br />
the user as graphics applications switch into graphics mode upon startup, but they do<br />
need to be borne in mind by programmers.<br />
There is no equivalent in the Apple Macintosh world <strong>of</strong> the discrete PC<br />
graphics standards. Macs were designed from the outset to be graphical, unlike PCs<br />
which were originally text-only displays, and have consistently led PCs in terms <strong>of</strong><br />
graphics capabilities; only very recently has the PC monitor attained parity with that<br />
<strong>of</strong> the Mac.<br />
3.3 Video Cards<br />
Given that present-day monitors are capable <strong>of</strong> high-resolution true colour, the<br />
only restricting factors for graphical displays are:<br />
• the amount <strong>of</strong> memory available for the display<br />
• the speed at which the display 'redraws' itself.<br />
These factors are controlled by video cards. These are add-ons to the<br />
microcomputer, printed circuit boards with on-board memory (VRAM) and/or<br />
processors which go into expansion slots in the machine and enhance its existing<br />
graphics capability. Indeed, in the early days <strong>of</strong> microcomputing, video cards were<br />
necessary to display graphics at all, but as time went on the function <strong>of</strong> the cards was<br />
built into the motherboard (the main circuit board <strong>of</strong> the computer) so the add-ons<br />
were purely for enhancement <strong>of</strong> the built-in graphics standard. (At the time <strong>of</strong> writing,<br />
PCs come with on-board VGA capability, but this will surely become SVGA in the<br />
near future.)<br />
The extra VRAM makes it possible to display wider colour ranges and<br />
resolutions than the on-board graphics hardware is capable <strong>of</strong>. Memory is crucial in<br />
determining these ranges because even though the monitor may be capable <strong>of</strong><br />
displaying high-quality images it will only do so if enough memory is allocated to the<br />
display to hold the required picture information.<br />
However, extra VRAM alone does not always make a satisfactory graphics<br />
workstation as memory takes time to be read by the computer. Watching a highresolution<br />
true-colour image being read from the framestore (a synonym for VRAM)<br />
and drawn on screen is a tedious experience at the best <strong>of</strong> times. When the screen has<br />
to be redrawn frequently - as is the case with a Graphical User Interface such as<br />
Micros<strong>of</strong>t Windows, where simply switching from one window to another forces a<br />
redraw - then the computer becomes unacceptably slow and users become frustrated<br />
and lose time. The solution to this problem is to place another processor on to the<br />
video card itself, thus relieving the computer's processor <strong>of</strong> the burden <strong>of</strong> the graphics<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 18 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
display. An accelerator card, as it is known, speeds up high quality displays<br />
considerably, although <strong>of</strong> course it costs rather more than a standard video card.<br />
3.4 Colour Printers<br />
3.4.1 Problems <strong>of</strong> Colour Printing<br />
Putting a colour image on screen is somewhat easier and cheaper than placing it<br />
on paper. There are a number <strong>of</strong> technical problems associated with colour printing<br />
which have to do with colorimetry (colour science), physics, chemistry and even<br />
human physiology and the psychology <strong>of</strong> perception. Fortunately, it is not necessary<br />
to go into tedious technical detail in order to outline the major problems that beset the<br />
transfer <strong>of</strong> colour screen images to hard copy. These include:<br />
Different colour coding schemes<br />
Screen colours are additive, the colour <strong>of</strong> a pixel depending upon the differing<br />
intensities emitted by its red, green and blue phosphors: the screen shines by<br />
its own light. Printed colours are subtractive as the colour <strong>of</strong> an ink depends<br />
upon the wavelengths it absorbs from the incident light: red ink appears red<br />
because it absorbs - subtracts - light <strong>of</strong> all the wavelengths other than those in<br />
the red part <strong>of</strong> the visible spectrum, which it reflects. Additive and subtractive<br />
colour schemes use different colour models, so colours have to be translated<br />
from one scheme to the other for faithful rendition from screen to page. (See<br />
the following Chapter on Colour.)<br />
Different display resolutions<br />
Printer resolutions are usually better than screen resolutions: a typical colour<br />
printer will be capable <strong>of</strong> 300 dpi (dots per inch) horizontal resolution whereas<br />
the monitor might only be capable <strong>of</strong>, say, 64 dpi. This means that a printed<br />
image <strong>of</strong> the same resolution is smaller than the screen image, so to print at<br />
screen size the image has to be enlarged. This can be a problem with bitmaps<br />
as extra pixels have to be created by a process called interpolation which can<br />
lead to blockiness and 'staircasing' <strong>of</strong> lines.<br />
Dithering<br />
Basic mixing <strong>of</strong> the three primary colours in the CMYK scheme - Cyan,<br />
Magenta and Yellow (black is not used for mixing) - used in colour printing<br />
only produces 8 colours, so with the exception <strong>of</strong> high-end products colour<br />
printers have to use dithering to achieve a wide colour range. Dithering<br />
involves printing varying proportions <strong>of</strong> dots <strong>of</strong> the 8 colours in a square<br />
dither matrix - usually 4 x 4 or 2 x 2 dots - to give the appearance, from a<br />
normal viewing distance, <strong>of</strong> another colour. The main disadvantages <strong>of</strong> this<br />
method - aside from the inherent complexity <strong>of</strong> the various dithering<br />
algorithms - are increased memory and processing overhead and a reduction in<br />
resolution; a 300 dpi printer using a 4 x 4 matrix only has an effective<br />
resolution <strong>of</strong> 75 pixels per inch.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 19 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 20 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
Chemistry<br />
The chemical composition and physical properties <strong>of</strong> inks and papers are <strong>of</strong><br />
crucial importance in colour printing. The formulation <strong>of</strong> the ink is most<br />
important as users increasingly demand to be able to use normal <strong>of</strong>fice paper<br />
rather than special papers. Among the properties <strong>of</strong> the ink that need to be<br />
carefully controlled are its miscibility, viscosity, surface tension, pH,<br />
dielectric properties and optical density. Synthesising printer inks is not an<br />
easy task.<br />
It is not really necessary to outline the other problems in colour printing in<br />
order to make the point that it is an expensive and complicated business. Whilst these<br />
problems are mostly solved before the final product hits the market an understanding<br />
<strong>of</strong> them goes a long way to explaining why the user can never count on the printed<br />
image looking exactly as it did on screen and why colour printers are so dear.<br />
High-quality colour hard copy can also be achieved photographically, images<br />
from the screen being placed directly on to film. This method bypasses the problems<br />
<strong>of</strong> ink and paper chemistry by using familiar and well-tried photographic technology,<br />
but has the disadvantages <strong>of</strong> relatively long development times - in comparison to the<br />
few minutes it takes to print an image to ordinary paper - and expense. Colour film,<br />
however, enables much higher resolution and colour depth than can currently be<br />
achieved on plain paper and is the preferred method for those users requiring the<br />
highest quality colour hard copy.<br />
3.4.2 Types <strong>of</strong> Colour Printer<br />
Colour printers come in a number <strong>of</strong> types. These are, in rough order <strong>of</strong> cost<br />
from cheapest to dearest:<br />
• dot matrix<br />
• inkjet<br />
• thermal wax<br />
• dye sublimation<br />
• laser<br />
In dot matrix printers tiny pins strike the paper through a multicoloured ribbon<br />
containing the CMYK primaries to produce a coloured dot. The pins are gathered<br />
together in a matrix inside the printhead, which can consist <strong>of</strong> 9 or 24 pins, a 24-pin<br />
dot matrix producing better results than a 9-pin. Such printers are really only suitable<br />
for coloured text: printing fills - blocks <strong>of</strong> solid colour - results in horizontal streaks<br />
and considerable paper distortion from the multiple pin impacts. They are also very<br />
slow, as the printhead takes four passes to print a line, and very noisy, but they have<br />
the saving grace <strong>of</strong> being cheap.<br />
Inkjet printers produce an image by spraying individual, very fine drops <strong>of</strong> ink<br />
at the paper from inkwells <strong>of</strong> the four primaries. Such devices are capable <strong>of</strong><br />
producing good quality, low-cost colour prints quickly and quietly, and at a not<br />
excessive cost. The main disadvantages <strong>of</strong> inkjets are that colours can sometimes look<br />
'muddy' or 'washed out' because <strong>of</strong> inks mixing at dot edges.<br />
Thermal wax technology uses a thermal print head to melt wax from a<br />
multicoloured ribbon on to the paper. Ironically, because this eliminates the flow <strong>of</strong><br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 21 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
ink over dot edges it can <strong>of</strong>ten produce a grainy picture, although the colour quality is<br />
superb. Many thermal wax printers can work with good quality <strong>of</strong>fice paper, by the<br />
simple expedient <strong>of</strong> placing a special coating on the sheet during the printing process<br />
to produce a smooth printing surface.<br />
Dye sublimation printers also use coloured ribbons with a thermal print head,<br />
but instead <strong>of</strong> melting ink on to paper the print head vaporises the ink which then<br />
condenses on to special paper very close to the ribbon. By this method the size <strong>of</strong> the<br />
dots can be controlled and the primary colours can be blended together, doing away<br />
with the need for dithering. This is a qualitative improvement over other printing<br />
technologies and produces very high quality continuous tone (not composed <strong>of</strong> dots)<br />
output, its main disadvantage being the high cost <strong>of</strong> consumables (ink and paper).<br />
Colour laser printing works by using a laser and a photosensitive drum to place<br />
electrostatic charges on the paper corresponding to the printing positions: when the<br />
paper is taken through a colour toner reservoir the magnetically-charged toner is<br />
attracted to the paper as dust and is later heat-bonded to fix it. The results, on the very<br />
best printers, are indistinguishable from photographs, and unsurprisingly such<br />
machines are very expensive; however, because they can print to ordinary <strong>of</strong>fice<br />
paper the cost per print is low.<br />
Less common devices are colour plotters - both pen and electrostatic - which<br />
are extensively used in Computer-Aided Design (CAD) and scientific disciplines such<br />
as meteorology where a relatively small colour range is sufficient and large printouts<br />
are required; the average <strong>of</strong>fice or home user is unlikely to come across these devices.<br />
3.5 Limitations <strong>of</strong> Colour Output<br />
Computer graphics technology is still far from the point where the full range <strong>of</strong><br />
visible colours can be rendered either on screen or on paper. The main reason for this<br />
is that neither monitors or printers are yet capable <strong>of</strong> producing totally pure primary<br />
colours. Figure 3.4 shows the approximate colour range <strong>of</strong> current monitors within<br />
the CIE chromaticity diagram (see the following Chapter on Colour).<br />
Figure 3.4<br />
Realisable colours on a typical colour monitor, in relation to the full range <strong>of</strong> visible colours.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 22 University <strong>of</strong> Hull
<strong>Graphics</strong> Hardware<br />
_________________________________________________________________________________<br />
3.6 Colour Scanners<br />
Scanners are used to digitise an image on paper, photographic negative, or slide<br />
into a bitmap. This is accomplished by passing light containing a known set <strong>of</strong><br />
wavelengths - usually similar to daylight - over the image and receiving that light in<br />
photosensitive semiconductors known as Charged Coupled Devices (CCDs) which<br />
emit voltages proportional to the intensity <strong>of</strong> the light falling upon them. The image is<br />
scanned using red, green and blue filters either using three CCDs - one per primary<br />
colour - or one, in which case each scan requires three passes to view the image under<br />
the three different filters. Naturally, scanners with three CCDs are more expensive<br />
than those with only one, but as they only require one pass to scan an image are<br />
obviously faster.<br />
There are two types <strong>of</strong> hard copy scanner: hand-held and flatbed. Hand-held<br />
scanners contain a light source and CCDs and pick up the image by being swept down<br />
the page; if the image to be scanned is wider than the scanner head the process is<br />
repeated across the page and the resulting strips are 'stitched together' by s<strong>of</strong>tware<br />
inside the computer. With flatbed scanners the paper is placed on a glass surface and<br />
the light is moved over it in a similar way to a photocopier. Hand-held scanners are<br />
cheaper than flatbeds but the latter come into their own when most <strong>of</strong> the images to<br />
be scanned are page-sized. Scanning resolution and colour depth varies from 256<br />
colours (8-bit) at 300 dpi to 16.7 million colours (24-bit) at 1200 dpi for high-end<br />
products. The crucial factor, as ever, is cost.<br />
Flatbed scanners can be used to scan photographic slides and negatives, but this<br />
is usually performed with specialised scanners known as Film Recorders which scan<br />
at higher resolutions than hard copy scanners, typically up to 3048 x 2072 pixels in<br />
24-bit colour (≈ 3000 dpi).<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 23 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
4.1 What is Colour?<br />
Chapter Four<br />
Colour<br />
Colour is the human perception <strong>of</strong> the visible portion <strong>of</strong> the electromagnetic<br />
spectrum, the full range <strong>of</strong> which stretches from low frequency radio waves to gamma<br />
rays (Figure 4.1). This visible portion <strong>of</strong> the spectrum comprises radiation <strong>of</strong><br />
wavelengths from roughly 380 to 700 nanometres (1 nm = 10 -9 m, or 1 billionth <strong>of</strong> a<br />
metre), which we see as a range <strong>of</strong> colours from violet (360 nm) through blue (480<br />
nm) and yellow (580 nm) to red (700 nm).<br />
Wavelength<br />
3km<br />
3cm<br />
0.3 mm<br />
3000 nm<br />
300 nm<br />
30 nm<br />
0.003 nm<br />
0.0003 nm<br />
0.00003 nm<br />
Radio waves<br />
Microwave<br />
Infrared<br />
Ultraviolet<br />
Xrays<br />
Gamma rays<br />
Cosmic rays<br />
Figure 4.1<br />
The Electromagnetic spectrum.<br />
Frequency (Hz)<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
10<br />
5<br />
10<br />
12<br />
14<br />
15<br />
16<br />
20<br />
21<br />
22<br />
Red<br />
Violet<br />
380 nm<br />
Visible Spectrum<br />
700 nm<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 24 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
4.2 The Human Visual System<br />
The human eye is an extraordinarily sensitive and sophisticated visual system<br />
capable <strong>of</strong> distinguishing very subtle differences in shade and hue. In essence it's a<br />
simple mechanism (Figure 4.2).<br />
Muscle<br />
Iris<br />
Cornea<br />
Lens<br />
Figure 4.2<br />
Simplified cross-section <strong>of</strong> the human eye<br />
Vitreous Humor<br />
Retina<br />
Fovea<br />
(focal point)<br />
Light is focussed on to the sensitive retina at the back <strong>of</strong> the eye by the flexible<br />
lens, the shape <strong>of</strong> which is controlled by muscles. The retina is composed <strong>of</strong> two<br />
types <strong>of</strong> light-sensitive cells, rods and cones. Rods function in dim light and are<br />
concentrated in the peripheries <strong>of</strong> the retina: they are responsible for our night sight<br />
and are not colour sensitive. Cones contain photopigments rendering them sensitive to<br />
colour, but only operate in good light. Rods vastly outnumber the cones over most <strong>of</strong><br />
the retina with the exception <strong>of</strong> the focal point <strong>of</strong> the eye, the fovea (Figure 4.3).<br />
Figure 4.3<br />
Schematic cross-section <strong>of</strong> retina near fovea, showing distribution <strong>of</strong> rods and cones.<br />
Blind spot<br />
Optic nerve<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 25 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
4.3 The Perception <strong>of</strong> Colour and Brightness<br />
Within the visible spectrum our eyes are more sensitive to some wavelengths<br />
than others. Broadly speaking we can see yellows and reds rather better than blues<br />
and violets, longer wavelengths better than shorter. There are a number <strong>of</strong> reasons for<br />
this:<br />
• the lens and vitreous humor absorb more long wavelengths than short<br />
• there are many more red-sensitive cones than green and blue (roughly 65% red,<br />
32% green, 3% blue)<br />
• the lens cannot adjust sufficiently to properly focus the shorter wavelength blues<br />
on to the fovea (this is why blues can <strong>of</strong>ten appear blurred)<br />
For these physical reasons our perception <strong>of</strong> colour differences varies according<br />
to the position <strong>of</strong> the colours in the visible spectrum. We find it easier to perceive<br />
changes in colour at the red end than at the blue. Moreover, we see reds and yellows<br />
as inherently brighter than blues and violets, so that to give the appearance <strong>of</strong> equal<br />
brightness a blue area has to have a greater intensity than a red.<br />
Our perception <strong>of</strong> a colour's brightness is similarly non-linear. Brightness is our<br />
perception <strong>of</strong> intensity, which represents the energy in the light (the peak, or<br />
amplitude <strong>of</strong> its waveform). Although we can distinguish in the order <strong>of</strong> 10 billion<br />
intensity levels from near-darkness to unbearable glare changes in intensity at low<br />
levels result in much greater increases in brightness than the same changes at higher<br />
levels. Replacing a 50W bulb with a 100W will create a greater brightness increase<br />
than changing the 100W for a 200W even though each increase represents a doubling<br />
<strong>of</strong> intensity.<br />
4.4 Colour Models<br />
A colour classification system based on the physical properties <strong>of</strong> light is not<br />
really suitable to measure the human perception <strong>of</strong> colour. A colour described as 'EM<br />
radiation <strong>of</strong> wavelength 700nm' would make little sense to most <strong>of</strong> us, to whom the<br />
word 'red' is rather easier to envisage. Moreover, colour mixtures with no one<br />
wavelength would be very difficult to describe. Instead, colour models (sometimes<br />
known as colour spaces) have been devised based on the concept <strong>of</strong> colour mixing,<br />
whereby 3 primary colours can combine to produce the full colour range. Not only<br />
are these more intuitive than purely physical descriptions but they also provide a<br />
numerical description <strong>of</strong> colour that can be used for displays and printing.<br />
4.4.1 Additive and Subtractive Colours<br />
We perceive the colours <strong>of</strong> objects either by the light they emit or the light they<br />
reflect. A tomato on television appears red because its image emits red light, whereas<br />
a real tomato gets its colour by absorbing all other colours in white light apart from<br />
red, which it reflects.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 26 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
Computer monitors and televisions shine by their own light and produce colours<br />
by the addition <strong>of</strong> differing amounts <strong>of</strong> primary colours - this is known as additive<br />
mixing. On the other hand colour printing uses the concept <strong>of</strong> subtractive mixing<br />
whereby primary coloured inks subtract colour from the incident light. Thus, in<br />
additive schemes the combination <strong>of</strong> the three primaries at full intensity produces<br />
white, whereas in subtractive schemes it produces black.<br />
4.4.2 The CIE Diagram<br />
In 1931 the Commission Internationale de L'Eclairage developed the CIE<br />
chromaticity diagram (Figure 4.4) based on empirical data gathered on human colour<br />
perception, and this diagram has been the baseline for colour modelling since. All <strong>of</strong><br />
the observable colours are placed in a horseshoe shape between x-y axes so that each<br />
colour has unique coordinates. All the visible pure colours (hues, or spectral colours<br />
because they occur in the visible spectrum) are to be found on the edge <strong>of</strong> the curve.<br />
The colours occurring inside the curve and on the straight 'purple line' which<br />
completes the curve are mixtures <strong>of</strong> hues. Being 2-dimensional the CIE Diagram<br />
cannot show intensity/luminance, but this factor can be derived using simple<br />
mathematics.<br />
Figure 4.4<br />
The CIE chromaticity diagram.<br />
The diagram has a number <strong>of</strong> useful properties. For example, a line drawn<br />
between any two points on the curve defines all the colours that can be derived from<br />
mixing those two colours; similarly, finding all the possible mixes <strong>of</strong> three colours is<br />
done by drawing a triangle connecting the three points.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 27 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
Whilst the CIE Diagram is used extensively by television engineers, more<br />
suitable colour models - some <strong>of</strong> which have been derived from it - have been<br />
invented for use in computer graphics.<br />
4.4.3 Red, Green and Blue (RGB)<br />
As monitors and televisions use red, green and blue for primary colours, it is<br />
unsurprising that the RGB model is widely used for visual display. This is <strong>of</strong>ten<br />
visualised as a cube (Figure 4.5).<br />
Cyan<br />
(0,G,B)<br />
Blue<br />
(0,B,0)<br />
Green<br />
(0,G,0)<br />
Black<br />
(0,0,0)<br />
White<br />
(R,G,B)<br />
Magenta<br />
(R,0,B)<br />
Figure 4.5<br />
The RGB cube. (Adapted from Burger and Gillies [1989].)<br />
Yellow<br />
(G,R,0)<br />
Red<br />
(R,0,0)<br />
From a hardware point <strong>of</strong> view, the RGB model is easy to work with as the<br />
different percentages <strong>of</strong> red, green and blue in a particular colour can be directly<br />
mapped to electron gun intensities. It is, though, unintuitive to use: given a particular<br />
colour it is not always easy to decide how to adjust the RGB balance in the colour to<br />
make it darker or lighter or to add tints <strong>of</strong> a non-primary colour.<br />
4.4.4 Hue, Light and Saturation (HLS)<br />
HLS uses the more friendly concepts <strong>of</strong> hue and saturation. Hue is the pure<br />
colour as people see it - red, yellow, green - and saturation expresses the relative<br />
amounts <strong>of</strong> pure hue and white in a colour, ranging from 0% (no hue at all, ie grey) to<br />
100% (pure hue). The third factor - light - can be thought <strong>of</strong> as the brightness <strong>of</strong> the<br />
colour. HLS is easiest to visualise as a diagram (Figure 4.6).<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 28 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
Yellow<br />
(180º)<br />
Green<br />
(120º)<br />
Red<br />
(240º)<br />
L=1<br />
(white)<br />
L=0<br />
(black)<br />
Cyan<br />
(60º)<br />
Magenta<br />
(300º)<br />
Blue<br />
(0º)<br />
Figure 4.6<br />
The HLS cone. Saturation increases away from the central axis. (Adapted from Burger and Gillies [1989].)<br />
The hues are defined by their counter-clockwise angle from the benchmark blue<br />
line, saturation is the distance from the central axis with 100% saturation (pure colour<br />
or hue) at the circle edge, and light is the distance <strong>of</strong> the circle along the vertical axis.<br />
The central axis also represents the greyscale, being the line <strong>of</strong> 0% saturation.<br />
4.4.5 Hue, Saturation and Value (HSV)<br />
The visualisation <strong>of</strong> HSV is similar to HLS ins<strong>of</strong>ar as hue is measured as an<br />
angle and Saturation and Value conceptually correspond with Saturation and Light in<br />
the HLS model. However, HSV differs from HLS in both the visual analogy - HSV is<br />
conceived as a single cone (Figure 4.7) the top <strong>of</strong> which marks V = 1 - and in the<br />
method <strong>of</strong> calculating Saturation and Value 12 .<br />
Green Yellow<br />
Cyan Red<br />
Blue<br />
V<br />
Magenta<br />
V = 0<br />
Figure 4.7<br />
The HSV hexagonal cone. As with HLS, saturation increases away from the central axis. (Adapted from Burger and<br />
Gillies [1989].)<br />
HSV and HLS can be simply related arithmetically to RGB values so it is not<br />
difficult to translate a shade represented in either <strong>of</strong> these systems into electron gun<br />
intensities for screen display. At least two - RGB and HLS - and <strong>of</strong>ten all three can be<br />
found in modern graphics packages so that the user has a choice <strong>of</strong> colour models.<br />
12 See Burger & Gillies [1989], pp 333-338, for details <strong>of</strong> the calculation method.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 29 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
4.4.6 Cyan, Magenta, Yellow and Black (CMYK)<br />
For colour hard copy the subtractive colour model in use is the CMYK system<br />
(Figure 4.8). This uses Cyan, Magenta and Yellow as the three primary mixing<br />
colours. Whilst theoretically these colours can create black, in practice a pure black<br />
pigment is usually included in the scheme in order to print text and to produce deep<br />
black fills.<br />
Green<br />
Yellow<br />
Figure 4.8<br />
The CMY cube.<br />
Cyan<br />
White<br />
Black<br />
Red<br />
4.4.7 Other Colour Models<br />
Blue<br />
Magenta<br />
There are a number <strong>of</strong> variations on the HLS theme, including HSI (Hue,<br />
Saturation, Intensity) and HVC (Hue, Value, Chroma). HSI and HVC are based upon<br />
the non-linear human perception <strong>of</strong> colour in contrast to HLS and HSV which are<br />
based on the linear manner in which the computer produces colour. Whilst they are<br />
more complex mathematically than linear models and therefore make the computer<br />
work harder such perception-based models are easier to use from a human standpoint<br />
and are likely to supersede the machine-based models over time.<br />
Although not a colour model as such it is worth mentioning the Pantone<br />
Matching System used in colour printing. This is a set <strong>of</strong> standard shades, each shade<br />
having a unique shade number and consisting <strong>of</strong> specified percentages <strong>of</strong> each <strong>of</strong> the<br />
CMY primaries. This allows colours to be printed consistently by disparate hardware.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 30 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
4.5 The Use <strong>of</strong> Colour<br />
4.5.1 Lighting and Backgrounds<br />
Different coloured lighting changes the colours <strong>of</strong> objects which we see by<br />
reflected light - this can easily be verified by standing under a sodium street lamp and<br />
looking at a colour magazine. This is not a problem in the field <strong>of</strong> computer graphics<br />
as virtually all colour printouts are intended to be viewed in white light, and <strong>of</strong> course<br />
colour monitors luminesce so the display is unaffected by any incident light.<br />
However, the colour <strong>of</strong> backgrounds affects the appearance <strong>of</strong> foreground<br />
objects. Obviously blue text will have much less contrast against a violet background<br />
than against a red (and none at all against a blue!). Less obvious, however, is that a<br />
colour on a light background appears to be more saturated than if it were on a dark<br />
background. It's also interesting to note that a large area <strong>of</strong> a colour appears more<br />
saturated than a small area <strong>of</strong> the same colour.<br />
4.5.2 Warm and Cool Colours<br />
As a result both <strong>of</strong> our non-linear colour perception and <strong>of</strong> colour associations<br />
found in nature it is possible to develop rules for the use <strong>of</strong> colour. Blue appears<br />
distant to our perceptions for physiological reasons (see The Perception <strong>of</strong> Colour and<br />
Brightness above) and is also the colour <strong>of</strong> the sky and sea. To our eyes, then, it has<br />
properties <strong>of</strong> distance and tranquillity. On the other hand, we perceive red very<br />
strongly in comparison to blue and it is also the colour <strong>of</strong> blood and fire. Not<br />
unnaturally we associate it with passion, heat and activity.<br />
A rough division <strong>of</strong> the visible spectrum can be made into 'warm' and 'cool'<br />
colours, warm colours corresponding to long wavelengths (reds, yellows) and cool to<br />
short wavelengths (blues, violets). This scheme is usually conceived as a colour<br />
wheel (Plate 3).<br />
Colours opposite each other on the wheel (180° apart) are known as direct<br />
complements and provide the most vivid and vibrant contrasts. The two colours 30°<br />
either side <strong>of</strong> a direct complement are known as the split complements; using split<br />
complements <strong>of</strong> a colour results in greater harmony than the use <strong>of</strong> the direct<br />
complement, at the loss <strong>of</strong> some vibrancy and contrast.<br />
The use <strong>of</strong> colour is a large and complex topic which is dealt with ably in a<br />
wide range <strong>of</strong> works (see the Bibliography). There are, however, a number <strong>of</strong> simple<br />
guidelines that should be borne in mind when using colour, whatever the purpose:<br />
• cool colours are distancing and should generally be used in backgrounds,<br />
whereas warm colours grab the attention and are better placed in the<br />
foreground<br />
• the eye finds it difficult to focus on short wavelengths, so avoid the use <strong>of</strong><br />
blues and violets for text and where edges are important<br />
• be sparing in the use <strong>of</strong> saturated colours - the eye tires quickly when faced<br />
with vibrant shades<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 31 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
• bear in mind that colours <strong>of</strong>ten have psychological and symbolic associations;<br />
this can be particularly important when addressing an international audience<br />
as these associations are <strong>of</strong>ten very culture-specific.<br />
4.5.3 Colour Deficiency<br />
Sometimes incorrectly termed 'colour blindness', colour deficiency is the result<br />
<strong>of</strong> a minor genetic error which mainly affects men. Roughly 8% <strong>of</strong> men, and 9% <strong>of</strong><br />
the population as a whole, exhibit some form <strong>of</strong> colour deficiency. This ranges from<br />
complete colour blindness, which is quite rare, to mild colour insensitivity. The most<br />
common deficiency is dichromatism in which the person's retina lacks the green or<br />
red photopigment so that they are unable to distinguish red or orange from green or<br />
yellow.<br />
Colour deficiency is not usually a problem for the people affected, and indeed<br />
some are completely unaware that their colour vision is impaired. They still see a<br />
tomato as a shade which they call 'red' even though they may be lacking a red<br />
photopigment; problems only arise when they are required to distinguish between<br />
colours to which they are insensitive, usually red and green. It is therefore important<br />
for designers who use colour to be aware <strong>of</strong> this and to avoid, for example, using red<br />
text on a green background.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 32 University <strong>of</strong> Hull
Colour<br />
_________________________________________________________________________________<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 33 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
Chapter Five<br />
<strong>Graphics</strong> Packages<br />
This Chapter looks at the graphics programs available to the 'ordinary' user -<br />
that is, the sort <strong>of</strong> package that can be bought from the dealers that advertise in the<br />
mainstream computer press. It lists some <strong>of</strong> the leading product in each application<br />
category and explains how graphics are incorporated into useful applications, either<br />
by using programming and/or authoring or by employing the inherent features <strong>of</strong> the<br />
Windows and Macintosh Graphical User Interfaces (GUIs). However, the relative<br />
merits <strong>of</strong> packages are not discussed as this is a survey <strong>of</strong> the market rather than a<br />
review <strong>of</strong> individual products.<br />
5.1 Popular Microcomputer <strong>Graphics</strong> Packages<br />
5.1.1 Painting and Drawing<br />
The most popular types <strong>of</strong> graphics application are undoubtedly general purpose<br />
painting and drawing packages. Such a package allows the user to create original<br />
artwork, or to edit and manipulate existing computer graphic images. These graphics<br />
can then be used in a number <strong>of</strong> ways, including:<br />
• desktop publishing<br />
• enhancing documents with diagrams and pictures<br />
• business presentations<br />
• training and education<br />
• databases<br />
and so on - the list is potentially endless. Although <strong>of</strong>ten used interchangeably,<br />
the terms painting package and drawing package refer to bitmapped and vector-based<br />
(sometimes called object-oriented) applications respectively; a generic term that<br />
includes both might be 'art package'. Tables 5.1 and 5.2 list some <strong>of</strong> the best-known<br />
computer art packages available at the time <strong>of</strong> writing 13 for both Macintosh and PC.<br />
13 It must be emphasised that these tables are neither exhaustive nor necessarily indicate the best packages on<br />
the market: new packages emerge seemingly daily and any list is no better than a snapshot <strong>of</strong> a dynamic<br />
market.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 34 University <strong>of</strong> Hull
<strong>Graphics</strong> Packages<br />
_________________________________________________________________________________<br />
Package PC Mac Notes<br />
Claris MacPaint √ Monochrome only.<br />
Electronic Arts Studio/8 √ Also available as Studio/32, for 32-bit colour<br />
Aldus SuperPaint √<br />
Windows Paintbrush √ Comes free with Micros<strong>of</strong>t Windows.<br />
Table 5.1<br />
Popular bitmapped computer art packages<br />
Package PC Mac<br />
Adobe Illustrator √ √<br />
Aldus Freehand √ √<br />
CA CricketDraw √ √<br />
Claris MacDraw √<br />
Corel Draw! √<br />
Harvard <strong>Graphics</strong> √<br />
Micrografx Designer √<br />
Table 5.2<br />
Popular object-oriented (vector) computer art packages<br />
Most modern commercial applications now include both bitmapped and objectoriented<br />
editors in the full package to enable the user to handle any graphic they may<br />
come across. For example, the Corel Draw! suite <strong>of</strong> applications includes the objectoriented<br />
Draw! itself and the bitmap editor PhotoPaint!<br />
It is quite common for large Clip Art libraries to be distributed with graphics<br />
applications. Clip Art is the generic term applied to computer graphics images that are<br />
in the public domain and can be used freely, without copyright restrictions. Both<br />
vector and bitmapped Clip Art is available in a wide variety <strong>of</strong> file formats and in an<br />
astonishing range <strong>of</strong> subjects. Given the size <strong>of</strong> modern programs and the amount <strong>of</strong><br />
Clip Art bundled with them the preferred distribution medium for commercial<br />
applications is rapidly becoming the CDROM.<br />
5.1.2 Presentation<br />
The use <strong>of</strong> slide shows to present, promote and sell ideas, services and products<br />
has a long history predating the advent <strong>of</strong> computers. Data is <strong>of</strong>ten much more<br />
comprehensible when presented visually as an image (such as a pie chart or bar<br />
graph), and across all fields people whose job it is to make presentations to audiences<br />
know that slides can complement and enhance their performance. Modern micros<br />
with their rapidly improving graphics capabilities are ideal vehicles for creating and<br />
displaying 'slide shows'. Each 'slide' is simply a computer graphic - usually vectorbased<br />
- created using standard techniques which can then be printed to film, paper or<br />
acetate, or displayed direct from the computer.<br />
Computer-generated slide shows have a number <strong>of</strong> significant advantages over<br />
traditional methods:<br />
• distribution: a large presentation can be saved to disk and copied or downloaded<br />
cheaply and quickly<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 35 University <strong>of</strong> Hull
<strong>Graphics</strong> Packages<br />
_________________________________________________________________________________<br />
• ease and cost <strong>of</strong> production: anyone with a micro and presentation s<strong>of</strong>tware can<br />
create a presentation, eliminating the cost and delay <strong>of</strong> using pr<strong>of</strong>essional bureaux<br />
• ease <strong>of</strong> editing: changes can be made to slides with little effort or waste<br />
• special effects: using the computer for the presentation means that sophisticated<br />
visual effects can be included in a 'show', including sound, video and animation<br />
As a consequence, a large and lucrative market in presentation s<strong>of</strong>tware has<br />
exploded into being in recent years. In addition, it is now quite common for large<br />
drawing packages to include slide show 'modules' within the package. Table 5.3 is a<br />
sample <strong>of</strong> the specialist presentation packages currently on the market.<br />
Package PC Mac<br />
Aldus Persuasion √ √<br />
CA Cricket Presents √ √<br />
MacroMedia Action √ √<br />
Micrografx Charisma √<br />
Micros<strong>of</strong>t Powerpoint √<br />
Symantec More √<br />
Table 5.3<br />
Presentation Packages<br />
5.1.3 Photography<br />
As hardware becomes more and more powerful so the field <strong>of</strong> photography has<br />
been brought into computing. Photographs - whether negative, print or slide - can<br />
now be digitised by scanners and saved as bitmaps, despite the enormous memory<br />
overheads <strong>of</strong> high-resolution true-colour photos. This service, which was once the<br />
exclusive province <strong>of</strong> pr<strong>of</strong>essionals, has been brought on to the high street by Kodak<br />
with their proprietary PhotoCD technology: ordinary members <strong>of</strong> the public can walk<br />
into a retail photographic shop and have their negatives digitised on to a CDROM.<br />
In recent years a number <strong>of</strong> packages have come on the market that are capable<br />
<strong>of</strong> manipulating digitised photographs. These programs are essentially sophisticated<br />
bitmap editors tailored to the requirements <strong>of</strong> photographic editing which allow an<br />
impressive array <strong>of</strong> image processing effects to be applied to the photographic image,<br />
such as changing its colour balance or smoothing or enhancing certain features, as<br />
well as detailed editing down to the level <strong>of</strong> individual pixels. Indeed, in skilful hands<br />
the image can be retouched drastically without any sign <strong>of</strong> these changes appearing in<br />
the final print; the old saying that 'the camera never lies' has never been less true than<br />
today. Because <strong>of</strong> the high memory requirements <strong>of</strong> photographic bitmaps, and the<br />
amount <strong>of</strong> processing involved in applying image processing techniques to them,<br />
photographic applications require high-end machines to run efficiently.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 36 University <strong>of</strong> Hull
<strong>Graphics</strong> Packages<br />
_________________________________________________________________________________<br />
Package PC Mac Notes<br />
Adobe Photoshop √ √<br />
Aldus PhotoStyler √<br />
Corel PHOTO-PAINT √ Supplied with Corel Draw!<br />
Kodak PhotoCD Access √ √ Just used for decoding PhotoCD files, and only<br />
supports simple operations such as clipping and<br />
scaling.<br />
Micrografx Photo Magic √ Supplied with Micrografx <strong>Graphics</strong> Works<br />
Table 5.4<br />
Photographic editing applications<br />
5.1.4 <strong>Graphics</strong> Utilities<br />
Traditionally the field <strong>of</strong> computer graphics has been beset by a pr<strong>of</strong>usion <strong>of</strong><br />
incompatible file formats and this has been a fertile ground for the growth <strong>of</strong> utility<br />
programs which convert images from one format to another. Increasing user demand<br />
for sophisticated bitmap editing facilities, including the application <strong>of</strong> special effects<br />
to the image, also spawned a large number <strong>of</strong> utilities, as until the recent past both<br />
painting and drawing packages were relatively basic. Modern packages, however,<br />
incorporate graphics file conversion filters for a wide range <strong>of</strong> file formats, and an<br />
<strong>of</strong>ten bewildering armoury <strong>of</strong> image processing tools to create special effects, so the<br />
niches occupied by graphics utilities are gradually shrinking.<br />
The vast majority <strong>of</strong> these utilities are in the public domain, either as shareware<br />
- s<strong>of</strong>tware which can be copied freely and used on a 'try before you buy' basis - or<br />
freeware. There are so many utilities, and they vary so much in quality (although all<br />
are cheap), that it would be a fruitless and inaccurate exercise to compile a list <strong>of</strong> the<br />
'best-known' or 'most common'. The best advice for users who feel they may need one<br />
or more utilities - possibly because their art package has only rudimentary facilities -<br />
is to consult sources <strong>of</strong> shareware, usually either a dealer or a s<strong>of</strong>tware archive.<br />
5.1.5 Animation<br />
Animation is a very recent arrival on the microcomputer scene primarily<br />
because it requires a fast processor and a large amount <strong>of</strong> memory. Essentially,<br />
animation is no more than a series <strong>of</strong> frames flashed before our eyes so quickly that<br />
they give the appearance <strong>of</strong> movement. In this respect, computer animation is no<br />
more sophisticated than 'what-the-butler-saw' machines. However, computers do<br />
vastly ease the animator's job by being able to extrapolate frames from a start point<br />
and an end point. If, for example, the start scene is a car at the left edge <strong>of</strong> the screen<br />
and the end scene the same car at the right edge the computer can use in-betweening<br />
to create the intermediate frames. When the scene is played from start to end it<br />
appears that the car is moving across the screen. In traditional animation these<br />
intermediate scenes would have to be drawn by hand.<br />
Animation packages are vector-based, object-oriented applications. Each figure<br />
in an animation is an object (an actor, in animation jargon) the properties and<br />
movements <strong>of</strong> which are defined by the user. This means that all the information<br />
about a particular animation - objects, backgrounds, movement - can be stored quite<br />
compactly, in the same way that vector graphics produce compact files.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 37 University <strong>of</strong> Hull
<strong>Graphics</strong> Packages<br />
_________________________________________________________________________________<br />
Because animation is such a new field in the microcomputer world, and because<br />
its market isn't as well defined as that <strong>of</strong> art or presentation packages, there are<br />
relatively few packages on the market. Table 5.5 shows the most common.<br />
Package PC Mac Notes<br />
Autodesk Animator √<br />
Corel Move! √ Supplied with Corel Draw!<br />
Gold Disk Animation Works √<br />
Interactive<br />
√<br />
MacroMind Director √<br />
Table 5.5<br />
Animation packages<br />
5.2 Incorporating <strong>Graphics</strong> into Applications and<br />
Documents<br />
<strong>Graphics</strong> <strong>of</strong>ten need to be used within programs to have the most effect. For<br />
example, a series <strong>of</strong> photographs <strong>of</strong> archeological sites may be quite nice to look at in<br />
a drawing package, but their impact and educational content is exponentially<br />
increased by placing them within a program which contains text about the sites, uses<br />
image processing techniques to bring out hidden features (old field boundaries, for<br />
instance), and cross-references sites according to age and period to show germane<br />
features <strong>of</strong> the civilisations that created them.<br />
We have already looked at one method <strong>of</strong> using graphics in a program, in the<br />
previous section on Presentation Packages, and in the following sections we explore<br />
other methods <strong>of</strong> using the informational power <strong>of</strong> computer graphics within<br />
applications.<br />
5.2.1 Programming Languages and Authoring Tools<br />
These are the 'traditional' methods <strong>of</strong> creating applications. Programming<br />
languages allow the programmer to have close control over the computer, but <strong>of</strong><br />
course require programming skills to use and have a steep learning curve . Their use<br />
is thus restricted to IT pr<strong>of</strong>essionals and enthusiastic users with a lot <strong>of</strong> time. These<br />
days programming languages incorporate such a vast array <strong>of</strong> graphics-handling<br />
functions that it's <strong>of</strong>ten easier to list what programmers can't do with graphics than<br />
what they can.<br />
Authoring tools allow authors without programming skills to produce<br />
applications, usually in particular subject areas. Using simple actions such as<br />
selecting text and applying menu commands complex structures can be created, the<br />
'pre-programming' <strong>of</strong> these structures already having been performed by the<br />
developer. Usually a small amount <strong>of</strong> simple programming in a language specific to<br />
the package is required to get the most out <strong>of</strong> the system but full-blown programming<br />
skills are never called for. All authoring packages have graphics-handling capabilities<br />
which can range from the rudimentary to the sophisticated <strong>of</strong>ten depending,<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 38 University <strong>of</strong> Hull
<strong>Graphics</strong> Packages<br />
_________________________________________________________________________________<br />
unsurprisingly, on the cost <strong>of</strong> the package. The main drawbacks <strong>of</strong> authoring tools<br />
are:<br />
• Proprietary s<strong>of</strong>tware. An authoring package is linked inextricably with the<br />
company that produced it, unlike general-purpose programming languages<br />
which are in the public domain. This usually results in a severe lack <strong>of</strong><br />
portability because an application produced in a particular package can only be<br />
run in that one package (or a cut-down runtime version <strong>of</strong> it used for<br />
distributing the application).<br />
• Efficiency. Applications produced by an authoring system are usually slower<br />
and less efficient than if they had been written in a programming language.<br />
• Specialisation. Although most tools attempt to be general-purpose application<br />
generators, in practice they tend to be very good in certain areas and poor in<br />
others.<br />
On the other hand ordinary users - even computer novices - can produce usable<br />
applications with authoring tools after only a short learning period, and it is this<br />
productivity together with ease <strong>of</strong> use that are the main selling points <strong>of</strong> authoring<br />
packages and the reasons for their enduring popularity. Table 5.6 lists some <strong>of</strong> the<br />
most common.<br />
Package Company PC Mac Notes<br />
Authorware<br />
Pr<strong>of</strong>essional<br />
Authorware Inc √ √<br />
Guide Info-Access √ Primarily for hypertext applications.<br />
Hypercard Claris √ The most popular application generator in the<br />
Mac world.<br />
IconAuthor AimTech Corp. √<br />
Toolbook Asymetrix √ Sometimes called 'Hypercard for the PC'<br />
because <strong>of</strong> its similarities to the Mac program.<br />
Table 5.6<br />
Authoring packages.<br />
5.2.2 Desktop Publishing (DTP)<br />
Desktop Publishing is the name given to the process <strong>of</strong> producing documents<br />
containing both text and graphics - newspapers, magazines, etc - using a computer.<br />
The phenomenal growth in the power and affordability <strong>of</strong> DTP packages for micros<br />
has put publishing into the hands <strong>of</strong> anyone with a micro and a laser printer. DTP<br />
applications allow very sophisticated text formatting and the placing <strong>of</strong> a wide variety<br />
<strong>of</strong> graphics within the publication. Their graphics-handling capabilities are usually<br />
restricted to cropping and resizing, so the editing <strong>of</strong> a graphic has to be performed in<br />
a specialist graphics package.<br />
DTP owes its very existence to the rapid advances in the graphics capabilities <strong>of</strong><br />
computers, particularly micros. Not only is its raison d'etre the incorporation <strong>of</strong><br />
graphics into text, but the WYSIWYG (What You See Is What You Get) display <strong>of</strong><br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 39 University <strong>of</strong> Hull
<strong>Graphics</strong> Packages<br />
_________________________________________________________________________________<br />
DTP programs, whereby the document appears on the screen exactly as it will print, is<br />
extremely graphically intensive.<br />
Despite being a relatively mature field - in microcomputing terms - DTP is<br />
dominated by just a few packages (Table 5.7), although given the abilities <strong>of</strong> modern<br />
Word Processing programs the line between WP and DTP is becoming ever more<br />
tenuous.<br />
Package PC Mac<br />
Adobe Illustrator √<br />
Aldus PageMaker √ √<br />
Quark Xpress √<br />
Ventura Publisher √<br />
Table 5.7<br />
Leading DTP packages<br />
5.2.3 Placing <strong>Graphics</strong> into Non-<strong>Graphics</strong> Files<br />
<strong>Graphics</strong> can also be placed into files generated by non-graphics packages by<br />
cut and paste techniques. If both packages work within the same operating<br />
environment, such as the Macintosh System 7 which uses a common data format<br />
regardless <strong>of</strong> the type <strong>of</strong> file, then data can simply be copied from the source to the<br />
destination via the clipboard (an area <strong>of</strong> memory set aside for temporary storage). For<br />
example, you could create a drawing in MacDraw, select that drawing, copy it to the<br />
clipboard, and paste it into a MacWrite document.<br />
A qualitative improvement on simple cut and paste is Object Linking and<br />
Embedding (OLE) introduced by Micros<strong>of</strong>t in Windows version 3.1. Central to OLE<br />
is the object, defined as data created by a Windows application that can be placed -<br />
either by linking or embedding - into another Windows application.<br />
The capacity to use OLE has to be specifically built into a Windows application<br />
by its programmers, although this is now the case with the vast majority <strong>of</strong><br />
commercial Windows programs. Programs that support OLE can be either, or both:<br />
a server, which can supply objects to other applications<br />
a client, which can accept objects from server applications<br />
Each object is created in its own specific server application - say, Windows<br />
Paintbrush - and is then placed as an object in the client file. This object can be either<br />
a link to the server file or a copy <strong>of</strong> the server file embedded into the client.<br />
The main qualitative difference between OLE and simple cut and paste is that<br />
the object can be edited from within the destination file. To edit a graphic placed into<br />
a database record, say, the user simply double-clicks the mouse over the graphic and<br />
the originating graphics application is opened.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 40 University <strong>of</strong> Hull
<strong>Graphics</strong> Packages<br />
_________________________________________________________________________________<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 41 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
Chapter Six<br />
Computer <strong>Graphics</strong> in Higher Education<br />
If a picture is worth a thousand words, there can be few more obvious uses for it<br />
than in the area <strong>of</strong> imparting knowledge to others. Text is not a very natural teaching<br />
medium for us. Not only does it have a low information density but it also results in<br />
processing overheads for the brain, ins<strong>of</strong>ar as it is a symbolic representation <strong>of</strong><br />
information that has to be decoded. Still and moving images, on the other hand,<br />
usually require no decoding (although they need a lot <strong>of</strong> interpretation) and have high<br />
information densities. Moreover, the human visual system has evolved to be highly<br />
efficient at information gathering and processing, so the presentation <strong>of</strong> information<br />
as graphics takes advantage <strong>of</strong> this natural ability. (This is by no means to deride text<br />
- plainly it would not be possible to put across the information in this book purely<br />
graphically - but simply to recognise its limitations.)<br />
It would not be accurate to say as yet that computer graphics has qualitatively<br />
changed the delivery <strong>of</strong> education and training, although it has certainly significantly<br />
enhanced the quality <strong>of</strong> teaching. In contrast to science, where new technologies<br />
open up new avenues <strong>of</strong> research and change the ways that science is carried out, the<br />
practice <strong>of</strong> teaching changes slowly and is driven by theories <strong>of</strong> learning as well as<br />
more contingent socio-economic factors. Primary <strong>of</strong> these factors, <strong>of</strong> course, is the<br />
availability <strong>of</strong> funding, and this has meant that the use <strong>of</strong> computer graphics in public<br />
education has lagged behind private training. Corporations are willing to spend large<br />
sums on high-end training facilities because <strong>of</strong> the real productivity gains that can be<br />
realised and measured in cash terms. This contrasts with public education where<br />
'productivity' is a more slippery concept and where the information being conveyed is<br />
<strong>of</strong> a different nature from training: broadly speaking education is about concepts and<br />
techniques - the Why and How - whereas training concentrates narrowly on the How.<br />
This concentration upon technique makes training much more amenable to the<br />
use <strong>of</strong> computer graphics than education. It's a fairly simple matter to use graphics to<br />
illustrate the assembly <strong>of</strong> an electronic circuit, but rather harder to explain the<br />
quantum mechanics that enable the semiconductors in the circuit to work.<br />
However, there are areas <strong>of</strong> education where graphics can, and are, being put to<br />
good use. In the sciences visualisation - the graphical representation <strong>of</strong> data - is used<br />
extensively for both data analysis and teaching, and in many fields images are<br />
essential to grasp fundamental concepts - the structure <strong>of</strong> a molecule in chemistry, or<br />
the process <strong>of</strong> cell division in biology, for example. The teaching <strong>of</strong> history,<br />
particularly <strong>of</strong> the 20th century, could benefit from the use <strong>of</strong> the vast amount <strong>of</strong> still<br />
and moving images in the archives, as could the teaching <strong>of</strong> Art by the use <strong>of</strong> image<br />
databases.<br />
There are a few subjects which would not benefit substantially from the use <strong>of</strong><br />
computer graphics in their teaching: it's difficult to see how courses in such areas as<br />
English Literature, Philosophy, Music, or Theology, could be much enhanced by<br />
images whether moving or still. These, though, are the exceptions rather than the rule,<br />
and in most subjects the judicious use <strong>of</strong> graphics can only improve the teaching<br />
material. If nothing else, the inclusion <strong>of</strong> interesting images can make learning fun,<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 42 University <strong>of</strong> Hull
Computer <strong>Graphics</strong> in Higher Education<br />
_________________________________________________________________________________<br />
which <strong>of</strong> course has a positive effect on the student's attitude and thus on knowledge<br />
uptake.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 43 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
Glossary<br />
Actor An active object in an animation.<br />
ADC Analogue to Digital Converter. A specialised chip which converts an analogue<br />
waveform into digital data. The reverse process is carried out by a Digital to<br />
Analogue Converter (DAC).<br />
Additive Colours Colours produced by the addition <strong>of</strong> light from luminescent<br />
primary sources, which in the case <strong>of</strong> computer monitors and televisions are red,<br />
green and blue. See also RGB.<br />
Aliasing Image imperfections whereby jagged edges and staircasing appear on lines<br />
and edges due to the limitations <strong>of</strong> raster systems in the representation <strong>of</strong> lines and<br />
curves. These imperfections may be removed by anti-aliasing, a technique which<br />
varies the intensities <strong>of</strong> pixels along the line.<br />
Authoring Tool A program which enables the user (author) to create applications<br />
without writing computer progrms.<br />
Bandwidth The range <strong>of</strong> frequencies in which a signal is transmitted. The greater the<br />
bandwidth the more information the signal carries.<br />
Bitmap A computer graphic composed <strong>of</strong> individual dots known as pixels.<br />
CCD Charged Coupled Device. A photosensitive semiconductor which emits a<br />
voltage proportional to the intensity <strong>of</strong> the light falling upon it. Used in scanners.<br />
CDROM An acronym for Compact Disk Read Only Memory. A CDROM is<br />
physically identical to an audio CD, and is used to store large amounts <strong>of</strong> data (up to<br />
600MB). This makes it particularly useful for distributing large graphical or video<br />
files, and for reference works like dictionaries. A CDROM is read by a CDROM<br />
drive which can be either inside or outside the computer.<br />
CGA Colour <strong>Graphics</strong> Adaptor. An obsolete PC monitor standard, capable <strong>of</strong><br />
displaying low-resolution graphics (640x200 pixels) in 16 colours.<br />
Character Generator A device implemented in display system hardware which<br />
creates standard text characters on screen. A PC uses its character generator when<br />
operating in text mode.<br />
CIE Diagram A conceptual colour space empirically derived from data on human<br />
colour perception by the Commission Internationale de L'Eclairage in 1931.<br />
Clip Art Digital images in the public domain which can be used without copyright<br />
restrictions.<br />
CMYK Standing for Cyan, Magenta, Yellow and blacK, the colour scheme used for<br />
printing. Although in theory cyan, magenta and yellow inks can combine to form<br />
black, in practice the mixture is <strong>of</strong>ten not sufficiently dark so additional black ink is<br />
used.<br />
Colorimetry The science <strong>of</strong> colours, or more explicitly - given that colour is a purely<br />
human perception <strong>of</strong> the visible part <strong>of</strong> the electromagnetic spectrum - the science <strong>of</strong><br />
the human perception <strong>of</strong> colour.<br />
Colour Model Empirical colour description scheme based on the concept <strong>of</strong> colour<br />
mixing, whereby each shade is specified by a unique combination <strong>of</strong> primary factors<br />
such as hue, saturation and brightness.<br />
Colour Wheel Spectral hues arranged in a circle so that opposite colours (180º apart)<br />
are complementary.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 44 University <strong>of</strong> Hull
Glossary<br />
_________________________________________________________________________________<br />
Compression The process <strong>of</strong> removing redundant data from a file to reduce its size.<br />
CRT Cathode Ray Tube. A CRT produces an image by firing electrons at<br />
phosphorescent dots on the inside <strong>of</strong> the screen. In a colour CRT three electron guns<br />
and three colours <strong>of</strong> phosphor dot - red, green and blue - are used.<br />
DAC Digital to Analogue Converter. A specialised chip which converts digital data<br />
into an analogue waveform. The reverse process is carried out by an Analogue to<br />
Digital Converter (ADC).<br />
Desktop Publishing (DTP) The use <strong>of</strong> computer s<strong>of</strong>tware to produce printed<br />
publications which can incorporate both text and graphics.<br />
Digitise To convert a real-world image or sound clip into binary data so that it can be<br />
read by computers.<br />
Direct Complement The direct complement <strong>of</strong> a colour is the colour which lies<br />
opposite it on the colour wheel; it can also be thought <strong>of</strong> as its 'negative'.<br />
Dithering On systems with a limited number <strong>of</strong> colours dithering is a method <strong>of</strong><br />
simulating out-<strong>of</strong>-range colours by mixing pixels/dots <strong>of</strong> existing colours. For<br />
example, a 16-colour system can display 256-colour images using dithering. If there<br />
are only two colours available, such as is the case with monochrome printers, the<br />
process is known as half-toning; this is how black-and-white newspaper pictures are<br />
produced.<br />
DPI Dots per inch, a measure <strong>of</strong> image resolution on output devices such as monitors<br />
and printers.<br />
EGA Enhanced <strong>Graphics</strong> Adaptor. PC monitor mode capable <strong>of</strong> displaying 16<br />
colours at 640x350 resolution.<br />
Electron Gun Used in CRTs (such as televisions and desktop computer monitors).<br />
An electron gun fires a beam <strong>of</strong> electrons at phosphorescent dots on the inside <strong>of</strong> the<br />
screen. See CRT.<br />
False Colour Artificial colour applied to a greyscale image in image processing in<br />
order to enhance salient features <strong>of</strong> the image.<br />
Film Recorder A hardware device which prints a computer graphic to a<br />
photographic medium, usually 35mm slides. Film recorders are used by slide bureaux<br />
to convert computer-based presentations into slides.<br />
Framestore See Video RAM.<br />
Graphical User Interface (GUI) An interface between an application or operating<br />
system and the user which communicates with the user by employing graphical<br />
elements, such as windows and icons. Sometimes known as a WIMP Interface<br />
(Windows, Icons, Menus, Pointer). Examples include the Macintosh System 7,<br />
Micros<strong>of</strong>t Windows, and Sun OpenWindows.<br />
Greyscale The range <strong>of</strong> grey shades available on a system.<br />
Greyscale Image A monochrome bitmap where pixel values are interpreted as<br />
different shades <strong>of</strong> grey on a scale from pure white to pure black.<br />
HLS Hue, Light and Saturation. An additive colour model appropriate for visual<br />
displays, developed by Tektronix.<br />
HSV Hue, Saturation and Value. An additive colour scheme for visual displays.<br />
Hue A pure - 100% saturated - colour. A hue is a spectral colour, one that occurs in<br />
the visible spectrum.<br />
Hue, Light and Saturation See HLS.<br />
Hue, Saturation and Value See HSV.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 45 University <strong>of</strong> Hull
Glossary<br />
_________________________________________________________________________________<br />
Interlacing A method <strong>of</strong> producing high-resolution displays by scanning every<br />
alternate line in a frame, thus taking two frames to display the whole picture.<br />
Normally found in less expensive systems.<br />
Interpolation A method <strong>of</strong> enlarging bitmaps by adding extra pixels, substituting<br />
each pixel in the original image by a n x n square <strong>of</strong> pixels where n is the degree <strong>of</strong><br />
enlargement.<br />
Liquid Crystal Display A visual display which exploits the light polarisation<br />
properties <strong>of</strong> special molecules (liquid crystals) when small voltages are applied to<br />
them. LCD displays are used in portable computers because <strong>of</strong> their lightness and low<br />
power consumption.<br />
Lookup Table (LUT) A subset <strong>of</strong> the full palette available on a system. The LUT is<br />
a set <strong>of</strong> index values which point to colour values in the full palette.<br />
Megaflop A million floating-point operations per second. Often used as a measure <strong>of</strong><br />
processor speed.<br />
Object A constituent part <strong>of</strong> a vector graphic composed <strong>of</strong> other objects and/or<br />
primitives.<br />
Object Linking and Embedding (OLE) A feature introduced by Micros<strong>of</strong>t with<br />
Windows 3.1 allowing data from any OLE-aware Windows application file to be<br />
placed into another similar file, even if they are <strong>of</strong> different application types (eg a<br />
sound file being placed into a word processing document).<br />
Object-Oriented In graphics terms, referring to a vector-based drawing package.<br />
Operand See Parameter.<br />
Optical Media Storage media in which laser light is used to read and write disk data,<br />
such as CDROMs and Laserdisks. Optical media are generally read-only, as the laser<br />
used in the writing process burns pits into the disk surface, but some expensive<br />
optical disks are read/write.<br />
Page Description Language Commonly used in Desktop Publishing, PDLs are<br />
effectively programming languages which instruct output devices how to output<br />
documents. One <strong>of</strong> the most common PDLs is Adobe Postscript.<br />
PAL The main analogue video standard used in the world outside North America<br />
and Japan.<br />
Palette The full range <strong>of</strong> colours available on a system.<br />
Pantone A standard colour matching scheme for printing which specifies the<br />
percentage <strong>of</strong> each primary colour used to produce a particular shade. Each shade has<br />
a unique shade number. The scheme was created to achieve uniformity and<br />
consistency in colour printing.<br />
Parameter Data supplied to a command, usually in parentheses. For example, the<br />
common graphical primitive rectangle(x1,y1,x2,y2) takes the four parameters x1, y1,<br />
x2, y2.<br />
Phosphor Dot A dot coloured red, green or blue on the screen surface, which<br />
phosphoresces when struck by electrons. An RGB triplet <strong>of</strong> phosphors comprises a<br />
pixel.<br />
Pixel Picture Element. A discrete dot on a television or monitor, or the smallest<br />
element in a bitmap.<br />
Primary Colours Pure hues which when mixed can produce the full range <strong>of</strong> visible<br />
shades.<br />
Primitive The lowest level vector graphics object, such as a line or a rectangle. All<br />
objects are ultimately composed <strong>of</strong> primitives.<br />
Raster Scan Device See Cathode Ray Tube.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 46 University <strong>of</strong> Hull
Glossary<br />
_________________________________________________________________________________<br />
Ray Tracing An important form <strong>of</strong> rendering which produces more 'realistic' scenes<br />
by taking into account factors such as the lighting <strong>of</strong> the scene and the reflectivity or<br />
opacity <strong>of</strong> the scene objects.<br />
Refresh Rate The number <strong>of</strong> times a second a display is redrawn. To give a flickerfree<br />
display this needs to be at least 25 times a second.<br />
Rendering The generation <strong>of</strong> artificial 3-D scenes using geometrical and lighting<br />
data.<br />
RGB Standing for Red, Green and Blue, the colour scheme used by colour monitors<br />
and televisions. Different intensities <strong>of</strong> each colour combine to produce the full<br />
colour range.<br />
Saturation The amount <strong>of</strong> pure hue in a colour, on a linear scale from 0% saturation<br />
- no hue at all, resulting in a white or grey - to 100% saturation which represents the<br />
pure hue itself. Pastel colours lie within this range.<br />
Scanner A hardware device for digitising (converting to binary data) printed<br />
material, such as pictures on paper or 35 mm slide.<br />
Shadow Mask In a CRT, a sheet <strong>of</strong> metal with regular apertures which focusses the<br />
beams from the electron guns onto their corresponding phosphors.<br />
Spectral Colour A colour <strong>of</strong> the visible spectrum, a pure fully-saturated hue.<br />
Split Complement The split complements <strong>of</strong> a colour are those which lie either side<br />
<strong>of</strong> its complement on the colour wheel.<br />
Subtractive Colours Colours produced by the subtraction <strong>of</strong> colours from incident<br />
light. A tomato appears red in daylight because it absorbs all other colours in the<br />
visible spectrum other than red, which it reflects. See also CMYK.<br />
Super VGA A wide term, covering a number <strong>of</strong> screen devices with superior<br />
resolution and/or colour capability to VGA.<br />
VGA Video <strong>Graphics</strong> Adaptor 14 . A PC monitor standard <strong>of</strong> at least 640 x 480 x 16<br />
colours.<br />
Video Card A slot-in expansion card containing video RAM and - usually - a<br />
processor, which improves the resolution and colour depth <strong>of</strong> the display. Some video<br />
cards, known as accelerator cards, also speed up the display by reducing the screen<br />
redraw time.<br />
Video RAM (VRAM) The amount <strong>of</strong> dynamic memory devoted to screen display,<br />
usually resident on the video card. Also known as the framestore.<br />
Window A rectangular viewing area which is processed separately from the rest <strong>of</strong><br />
the screen.<br />
WORM Write Once Read Many times. A type <strong>of</strong> read-only optical disk usually<br />
employed for archiving.<br />
WYSIWYG "What You See Is What You Get", pronounced "wizziwig". Normally<br />
applied to Word Processing and Desk Top Publishing packages which operate in a<br />
graphical mode. What appears on the screen is what will appear on hard copy.<br />
14 Some textbooks translate the acronym as 'Video Gate Array'.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 47 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
Author Title<br />
Annotated Bibliography<br />
Ammeraal, Leendert <strong>Graphics</strong> Programming in Turbo C. 1989. Wiley &<br />
Sons, Chichester.<br />
Guide to graphics programming using Borland Turbo C<br />
Arnold, D B<br />
Duce, D A<br />
Baker, M P<br />
Hearn, D<br />
Barlow, Horace<br />
Blakemore, Colin<br />
Weston-Smith, Miranda<br />
version 2.0<br />
ISO Standards for Computer <strong>Graphics</strong>: The First<br />
Generation. 1990. Butterworths, London.<br />
A detailed treatment <strong>of</strong> the ISO-approved graphics<br />
standards - PHIGS, GKS, CGM, etc - and the<br />
standardisation and approval procedures.<br />
Computer <strong>Graphics</strong>. 1986. Prentice-Hall, London.<br />
Theoretical and practical study <strong>of</strong> underlying concepts <strong>of</strong><br />
computer graphics, including maths (algebra, matrices)<br />
and Pascal code, well-diagrammed throughout.<br />
Images and Understanding. 1990. Cambridge<br />
University Press, Cambridge.<br />
Collection <strong>of</strong> papers from an international conference on<br />
the title subject, held in 1986.<br />
Beard, Nick Visualisation - a series <strong>of</strong> 4 articles in Personal<br />
Computer World, Vol 15 (11 & 12) & Vol 16 (1&2) (Nov<br />
1992 - Feb 1993). A non-technical look at data<br />
Browne, Jimmie<br />
McMahon, Chris<br />
Burger, Peter<br />
Gillies, Duncan<br />
visualisation.<br />
CADCAM: From Principles to Practice. 1993.<br />
Addison-Wesley, Wokingham.<br />
A standard introductory technical texbook on the subject.<br />
Interactive Computer <strong>Graphics</strong>. 1989. Addison-<br />
Wesley, Wokingham.<br />
Comprehensive and advanced treatment <strong>of</strong> computer<br />
graphics with mathematical concepts and algorithms.<br />
Carlson, W E A Survey <strong>of</strong> Computer <strong>Graphics</strong> Encoding and<br />
Storage Formats. Computer <strong>Graphics</strong>, April 1991, Vol<br />
25(2), pp 67-75.<br />
A survey <strong>of</strong> several graphics file formats, including<br />
descriptions <strong>of</strong> common compression methods.<br />
Durrett, H J (ed) Color and the Computer. 1987. Academic Press Inc,<br />
Orlando, Florida.<br />
A collection <strong>of</strong> papers on the title subject, discussing<br />
topics such as Colour Science, colour and humancomputer<br />
interaction, and colour hardware.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 48 University <strong>of</strong> Hull
Annotated Bibliography<br />
_________________________________________________________________________________<br />
Foley, J<br />
van Damm, A<br />
Feiner, S<br />
Hughes, J<br />
Gonzalez, Rafael C<br />
Woods, Richard E<br />
Computer <strong>Graphics</strong> Principles & Practice. 1990.<br />
Addison-Wesley, Reading, Mass.<br />
Very large and all-embracing text on the theory and<br />
practice <strong>of</strong> computer graphics, particularly mathematical<br />
concepts and algorithms.<br />
Digital Image Processing. 1992. Addison-Wesley,<br />
Reading, Mass.<br />
Comprehensive introductory text to the field <strong>of</strong> Image<br />
Processing.<br />
Hopgood, F R A Using Colour in Computer <strong>Graphics</strong>. 1991. Advisory<br />
Group on Computer <strong>Graphics</strong>, Loughborough University.<br />
A short and concise paper from ACOCG's Technical<br />
Hopgood, F R A<br />
Duce, D A<br />
Johnston, D J<br />
Report Series.<br />
A Primer for PHIGS: C Programmer's Edition. 1992.<br />
Wiley & Sons, Chichester.<br />
Technical desciption <strong>of</strong> the PHIGS standard from a C<br />
programmer's perspective, with full details <strong>of</strong> the C<br />
language binding and example source code.<br />
Jute, André Colour for Pr<strong>of</strong>essional Communicators. 1993.<br />
Batsford, London.<br />
A friendly and colourful 'how to' guide to the use <strong>of</strong><br />
Kay, David C<br />
Levine, John R<br />
colour for communication.<br />
<strong>Graphics</strong> File Formats. 1992. Windcrest/McGraw-Hill,<br />
NY.<br />
Excellent reference on all the graphics file formats in<br />
current use, giving detailed technical information about<br />
each format.<br />
Latham, Roy The Dictionary <strong>of</strong> Computer <strong>Graphics</strong> Technology<br />
and Applications. 1991. Springer-Verlag, NY.<br />
A comprehensive dictionary <strong>of</strong> computer graphics<br />
terminology.<br />
Low, Adrian Introductory Computer Vision and Image<br />
Processing. 1991. McGraw-Hill, Maidenhead.<br />
Mealing, Stuart The Art and Science <strong>of</strong> Computer Animation. 1992.<br />
Intellect Books, Oxford.<br />
Comprehensive and accessible textbook covering all<br />
aspects <strong>of</strong> the field <strong>of</strong> animation.<br />
Peterson, Ivars The Mathematical Tourist. 1988. W H Freeman, NY.<br />
A non-mathematical look at some aspects <strong>of</strong> modern<br />
mathematics, particularly mathematics which can be<br />
expressed graphically, such as fractals.<br />
Pickover, Clifford Computers, Pattern, Chaos and Beauty.1990. St<br />
Martin's Press, New York.<br />
An eclectic work on the images that can be generated on<br />
computers using non-linear mathematical functions .<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 49 University <strong>of</strong> Hull
Annotated Bibliography<br />
_________________________________________________________________________________<br />
Rimmer, Steve Bit-Mapped <strong>Graphics</strong>. 1990. Windcrest Books, Blue<br />
Ridge, PA.<br />
A highly technical book for C and Assembler<br />
programmers on common bitmap formats and how to<br />
display and manipulate them. Contains plenty <strong>of</strong> source<br />
Rogers, D F<br />
Adams, J A<br />
code.<br />
Mathematical Elements for Computer <strong>Graphics</strong>.<br />
1990. McGraw-Hill, NY.<br />
Heavily theoretical mathematical treatment <strong>of</strong> computer<br />
graphics concepts, from simple 2D and 3D<br />
transformations to surfaces and splines.<br />
Rowell, Jan Picture Perfect: Color Output for Computer<br />
<strong>Graphics</strong>. 1991. Tektronix Inc, Beaverton, Oregon.<br />
A glossy, friendly booklet on colour and colour printing<br />
with an unsurprising bias towards Tektronix products.<br />
Well written and informative, though.<br />
Smith, W J<br />
Using Computer Color Effectively: An Illustrated<br />
Thorell, L G<br />
Reference. 1990. Prentice-Hall, New Jersey.<br />
Stonier, Tom Information and the Internal Structure <strong>of</strong> the<br />
Universe. 1990. Springer-Verlag, London.<br />
Subtitled "An Exploration into Information Physics" the<br />
book outlines the author's view that information is an<br />
integral part <strong>of</strong> the natural world.<br />
Warren, Lorraine Understanding IT: Computer-based Presentations.<br />
1994. University <strong>of</strong> Hull.<br />
A highly readable guide to producing presentations with<br />
presentation packages.<br />
Watt, Alan Fundamentals <strong>of</strong> Three-Dimensional Computer<br />
<strong>Graphics</strong>. 1989. Addison-Wesley, Reading, Mass.<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 50 University <strong>of</strong> Hull
Annotated Bibliography<br />
_________________________________________________________________________________<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 51 University <strong>of</strong> Hull
_________________________________________________________________________________<br />
animation, 36-37<br />
authoring packages, 37-38<br />
bitmapped graphics, 5-7<br />
advantages, 6<br />
colour depth, 5<br />
disadvantages, 6<br />
memory, 5, 6, 13-14<br />
resizing, 6<br />
resolution, 5<br />
brightness, 25<br />
cathode ray tube (CRT), 16<br />
charged coupled device (CCD), 22<br />
Clip Art, 34<br />
colour<br />
additive, 19, 26<br />
definition, 23<br />
depth, 5<br />
dithering, 19<br />
greyscale, 7, 16<br />
perception, 24-25<br />
spectral, 26<br />
subtractive, 19, 26<br />
warm and cool, 30-31<br />
colour models<br />
CIE diagram, 26-27<br />
CMYK, 19, 29<br />
HLS, 27-28<br />
HSI, 29<br />
HSV, 28-29<br />
HVC, 29<br />
RGB, 27<br />
colour printers, 18-21<br />
dot matrix, 20<br />
dye sublimation, 21<br />
inkjet, 20<br />
laser, 21<br />
limitations, 21<br />
plotters, 21<br />
resolution, 19<br />
thermal wax, 21<br />
colour wheel, 30<br />
computer memory (RAM), 15<br />
desktop publishing (DTP), 38-39<br />
dichromatism, 31<br />
direct complements, 30<br />
disk storage, 14-15<br />
Index<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 52 University <strong>of</strong> Hull
Index<br />
_________________________________________________________________________________<br />
optical media, 14<br />
drawing package, 4<br />
electron guns, 6, 15<br />
eye<br />
colour perception, 24-25<br />
diagram, 24<br />
retina, 24<br />
rods and cones, 24<br />
false colour, 7<br />
file compression, 8-10, 15<br />
fractal, 9<br />
Huffman coding, 9<br />
lossless and lossy, 9<br />
run length encoding (RLE), 8<br />
file formats, 10-13<br />
conversion between, 36<br />
graphics packages<br />
animation, 36-37<br />
desktop publishing, 38-39<br />
photo editing, 35-36<br />
presentation, 34-35<br />
utilities, 36<br />
hue, 27<br />
image processing, 6<br />
image processing, 37<br />
intensity, 25<br />
interlacing, 17<br />
interpolation, 19<br />
liquid crystal display (LCD), 16-17<br />
lookup table, 6<br />
matrix addressing, 16<br />
Micros<strong>of</strong>t Windows, 39<br />
monitors, 15-18<br />
CRT, 16<br />
LCD, 17<br />
object, 3, 13<br />
object linking and embedding (OLE), 39<br />
Pantone, 29<br />
phosphor dot, 15<br />
photographs, digitised, 35<br />
pixel, 5, 14<br />
primitive, 3, 14<br />
saturation, 27<br />
shadow mask, 15<br />
split complements, 30<br />
SVGA, 16<br />
System 7, 39<br />
vector graphics, 3-5<br />
vector graphics<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 53 University <strong>of</strong> Hull
Index<br />
_________________________________________________________________________________<br />
advantages, 4<br />
disadvantages, 4<br />
memory, 13<br />
uses, 5<br />
video card, 15<br />
video cards, 18<br />
video ram (VRAM), 5, 15, 18<br />
visual system, human, 23-24<br />
__________________________________________________________________________________________________________<br />
Understanding IT: Computer <strong>Graphics</strong> 54 University <strong>of</strong> Hull