Computer graphics
Introduction
Since the early 1980s, affordable personal computers have come equipped with the capability to
display graphics. Increasingly, museums are using these capabilities for collections management,
education and publishing.
How computers display images
Modern computer graphics displays are built up using rectangular grids of thousands of dots called pixels
(the name is derived from picture elements), much like the tiles in a mosaic or the stitches in a tapestry.
Although the display on the monitor or screen is always composed of a rectangular grid of pixels,
there are two fundamentally different ways the computer can actually store and manipulate
images. The two categories are often referred to as paint-type and draw-type images.
Paint-type images
Paint-type images are actually stored and manipulated as a rectangular grid of pixels - there are no
objects with attributes, just a series of tiny squares with colour values. Paint-type images can either
be created entirely by computer, or they can be sampled initially from other sources, for example,
scanning a photograph or capturing a frame of video.
The paint-type image of a detail from an 18th century woman's portrait shown here simulates the
appearance of an area of the image under magnification. Because the image (like the monitor's
display) comprises of a number of pixels, the best quality occurs when the image is displayed at
l00% magnification.
The image can be displayed enlarged or reduced, but the number of pixels in the image file remains
constant (unless. the image is actually being edited). This means that enlarging a display of an image
simply involves using more than one screen pixel to display each image pixel, and therefore does not
increase the actual information being displayed. Conversely, displaying the image at less than full
size results in some of the image information being discarded from the display.
When magnifying or reducing paint-type images, it is best to stick to factors of two (x2, x4, or x1/2,
x1/4) to ensure that the pixels are added or removed evenly. Arbitrary scaling factors can
sometimes result in interference patterns known as the Moiré Effect.
The three most important measurements for paint-type images are width, height and colour depth.
The width and height are simply the dimensions of the image measured as a number of pixels.
Colour depth is the number of bits (binary digits), used to describe the colour of each pixel. A bit
can either be a 0 or a 1, and the more bits there are the more unique combinations of 0s and 1s
can be stored. For example, if an image has a colour depth of 4 bits, then there are 16 unique
combinations of 0s and 1s:
0000; 0001; 0010; 0011; 0100; 0101; 0110; 0111;
1000; 1001; 1010; 1011; 1100; 1101; 1110; 1111.
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Because this allows software to differentiate between 16 different values, 16 different colours can
be stored. The table below shows some common values for colour depth:
Colour Depth
(bits per pixel)
Number of Colours
1
4
8
16
24
2
16
256
65,536
16,777,216
These three 'dimensions' can be used to imagine a three-dimensional solid, a metaphor that is
useful because its volume is proportional to the size of the actual image file in the computer's
memory, and the size of the file on disk.
Paint-type image files can be very large. For example an image 640 pixels wide by 480 pixels high (a
VGA screen), with 8-bit colour depth (256 colours) would be 2,457,600 bits (300 Kb) in size. Higher
monitor resolutions and increased colour depths multiply the problem. A 24-bit image (16.8 million
colours) filling a 1024 by 768 pixel display would occupy 18,874,368 bits (2.25 Mb) of memory.
There is some disagreement about the optimum colour depth for images. Some believe that the
best solution, as the cost of storage and computing power continue to drop, is to use 24-bit images
with millions of colours. Others believe that it is wasteful, and that 256 colours can produce
excellent results as long as the colours in the image's palette is picked very carefully by the
software (an adaptive palette).
To make the issue of image size even more complex, there are a variety of compression
techniques that can be applied to make the image file even smaller.
When to use
Where photo-realistic images are required.
Software
Popular paint-type software includes: Microsoft Paint (supplied with Windows); Adobe Photoshop;
ZSoft PC Paintbrush; Corel PhotoPaint; Aldus PhotoStyler; and JASC Paint Shop Pro.
How to acquire images
The most common way is to scan a photograph, slide or negative using a flatbed scanner. The
original can be scanned too, if it is flat (e.g. a map or diagram). Other techniques include: grabbing
frames from a video signal using a video capture device; or using a digitizing camera to record the
information digitally direct from the object being photographed.
Another method is PhotoCD. This was developed by Kodak and is available through high street
photo-processors. Here a CD-ROM of approximately 100 paint-type images is produced direct
from 35mm film, taken using an ordinary camera.
File formats
BMP
Bitmap, also known as Device Independent Bitmap (DIB), is the primary paint-type image format
for Microsoft Windows. They can be saved using no compression, or with Run Length Encoded
(RLE) compression. Maximum of 24-bit (16.8 million colours).
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EPS
Encapsulated PostScript. Adobe PostScript is the industry standard page description language.
EPS files can contain both paint and draw type information, and can often become extremely large.
FIF
Fractal Image Format. Iterative Systems' proprietary image file format that uses fractal
compression technology to reduce file sizes.
GIF
CompuServe Graphics Interlaced Format. Designed for transmission over modem links, GIF files
are compact images that can be stored in an interlaced format. This means that one line of pixels
in every four will be decoded first, allowing the user to see what the image will look like before the
whole file is downloaded. It has a maximum of 8-bit (256 colours). This type of image is used on
the World Wide Web.
JPG
Joint Photographic Experts Group format. This format began as a compression algorithm designed
by a group of graphics experts. Later, a standard file format (the JPEG File Interchange Format)
was added, and now all files are supposed to use the extension JIF, although JPG is still used.
JPG/JIF files offer extremely high compression ratios, for 24-bit (16.8 million) colour or 8 bit (256)
grayscale images only. The compression is lossy - image detail is actually discarded. Compression
ratios from 1 (no compression) to 100 (very high compression) can be selected, and compression
ratios of 25:1 produce little noticeable degradation of quality. This type of image is used on the
World Wide Web.
PCD
PhotoCD. It offers five different versions of each image, at five different image sizes: 192 by 128
pixels, 384 by 256 pixels, 768 by 512 pixels, 1536 by 1024 pixels, and 3072 by 2048 pixels, all at
24-bit colour. The smallest image is useful as a thumbnail - a small, memory efficient image used
for rapid searching. The medium resolutions are well suited for display on computer monitors,
whereas the larger images are designed for printing hard copies on paper. A Professional PhotoCD
format is now available that provides an even larger version of the image, at the expense of
capacity - approximately 33 images can be stored on a single Professional PhotoCD disk.
PCX
Zsoft Paintbrush - a proprietary format that became a de facto standard simply by being one of the
earliest in the industry. The latest versions of the format support up to 24-bit colour images.
PICT
The native format of the Apple Macintosh graphics engine. (Can also be used for draw-type images).
RLE
Run Length Encoded bitmap - a version of the Windows bitmap format that uses compression. See BMP.
TGA
Truevision Targa format, a format designed for Truevision's Targa and Vista hardware products.
Colour depths can be up to 32-bit (16.8 million colours plus additional information).
TIF
Tagged Image File Format - designed to become a standard, but with so many features that there
are many variations. There are six different ways to encode TIFF files: No compression, Packbits
compression, and LZW compression in either 1, 4, 8 and 24-bit colour, or Huffman compression,
Fax Group 3 compression and Fax Group 4 compression for 1-bit images (the technical details of
the various types of compression are beyond the scope of this fact sheet). TIFF files are well suited
for transferring images between Macintosh and PC.
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Draw-type images
A draw-type image, often referred to as a vector or scalable image, contains a set of objects whose
characteristics are stored mathematically. Each individual object within the image retains its own
characteristics, such as the co-ordinates of its vertices (corners), the thickness and colour of its
outline, the colour of its interior etc. Because each object retains its individual 'identity' editing
actions can be targeted at specific elements of an image.
The following is a draw-type image that contains a dark ellipse with no border, a lighter rectangle with
a thick black border, and a lighter polygon with a light border. The illustration simulates the vertices of
the polygon about to be edited by 'dragging' the small square 'handle' with the mouse pointer.
The image information is stored mathematically, so the image is independent of any display
device's capabilities. It can be scaled (enlarged or reduced), or rotated indefinitely without any loss
of image information, and will always display at the highest resolution of the output device (see
Hardware, below), whether it is a monitor, a printer, a plotter or a typesetter. Certain variants of
draw-type images, such as those used by Computer-Aided Design/Manufacturing (CAD/CAM)
systems or Geographic Information Systems (GIS) can store measurements to a very high degree
of accuracy. The image files tend to be quite small.
When to use
Draw-type images are ideally suited to tasks that involve mathematical rather than visual accuracy,
such as CAD, GIS, and cartography. They are also ideal for producing diagrams and charts.
Software
Examples of commonly used software for producing draw-type images include: CorelDRAW!,
Adobe Illustrator, Aldus Freehand, Microsoft Draw, and AutoDesk AutoCAD).
File formats
There is a bewildering array of different file formats available for graphics files, each with
advantages and disadvantages. This short glossary explains some of the differences between
them. Files are generally referred to by their three-letter file extensions.
AI
Adobe Illustrator format, based on a subset of the Encapsulated PostScript format.
CGM
Computer Graphics Metafile. Used by a number of software packages.
DXF
AutoCAD format. Can be used for 2-D and 3-D drawings.
EPS
Encapsulated PostScript. Adobe PostScript is the industry standard page description language.
EPS files can contain both paint and draw type information, and can often become extremely large.
PICT
The native format of the Apple Macintosh. (Can also be used paint-type images).
WMF
Windows Metafile - the native format for Microsoft Windows.
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Hardware
The quality and speed of the graphics displayed by a computer are dependant on two hardware
components; the monitor (or screen), and a collection of electronic components referred to as the
graphics adaptor. The graphics adapter is usually, but not always, mounted on a separate
removable circuit board and is often referred to as the video card.
Because there are so many different combinations of graphics adapter and monitor, special
software is often needed to translate the output of graphical software to a format the hardware can
understand. This type of software is known as a video driver. One of the benefits of using a
graphical interface such as Microsoft Windows is that only Windows needs a specific video driver all the application programs can share the same driver. The earliest resolution for Microsoft
Windows was called VGA, or Video Graphics Array. This had a display resolution of 640 pixels
wide by 480 pixels high, with 16 colours (4-bit). Extensions to this standard are referred to as
SVGA, or Super VGA. Here screens to can be 800 by 600 pixels, 1024 by 768 pixels or 1280 by
1024 pixels, with 256, 32,000, 64,0000, 16.8 million colours, or more. The maximum resolution is
dependant on the amount of memory on the video card being used.
There are also a number of techniques used to improve the speed of graphics adapters. The term
Windows Accelerator refers simply to a graphics adapter that has been specially designed to
speed up the graphics performance of Microsoft Windows.
Sources of help and advice
Besser and Trant, Introduction to Imaging, The Getty Art History Information Program
Foley, van Dam et al. Computer Graphics, Principles & Practice, Addison Wesley
Murray and Van Ryper, Encyclopedia of Graphic File Formats, O'Reilly & Associates Inc.
This work is licensed under the Creative Commons Attribution-NonCommercial-Share Alike 2.0 UK: England & Wales License. To view a copy of
this license, visit http://creativecommons.org/licenses/by-nc-sa/2.0/uk/
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