Image processing and page representation: part 4
| Published date | 01 April 1989 |
| Pages | 248-251 |
| DOI | https://doi.org/10.1108/eb044901 |
| Date | 01 April 1989 |
| Author | Tony Cawkell |
Technology Tutorial
Image processing and page
representation: part 4
Tony Cawkell
Citech
Ltd,
PO Box
565,
Iver,
Bucks SL0
0QZ,
England
Abstract:
The penultimate article in this series commences
with a few remarks about the size and ebullience of the
market for
'output'
devices, followed by a consideration of
display techniques. The functioning of a Cathode Ray Tube
display is
explained,
moving on to colour tubes and the
steadily improving methods for handling colour and
graphics. The article gives a brief description of alternative
displays including plasma, electroluminescent, liquid
crystal,
and light emitting diode panels.
Outputs
'Outputs' covers the displayed or printed reproduction of im-
ages,
a subject which is inextricably mixed with other aspects
of
the
subject and so has been partially covered already.
In common with other branches of the image processing
market, 'outputs' is ebullient. A whole page colour
advertisement in The Daily Telegraph costs £35,000, and the
price for The Guardian and The Times is similar. An updated
model of an existing laser printer was advertised in July,
probably a dozen times, the space costs being up to £400,000.
This printer costs £256 more than the existing model for
which you will get more memory, more fonts, and faster and
more elaborate graphics, 'to give your documents that
sparkling, professional look'.
The image processing market
is
characterised by jumps —
the first inexpensive scanner, the first laser printer, and so on,
— followed by consolidation, price reductions, supposed
value for money, 'improvements' (not necessarily price re-
ductions), and the addition of bells and whistles before the
next jump occurs. 'New Features' are essential; a model with
25 features sells better than one with five. Complication
usually increases and planned obsolescence may accompany
it. For instance, older, simpler, adequate versions of word
processing software are unavailable, having being replaced
by more expensive, elaborate, cleverer versions, with extras
which may be seen as improvements by some and bells and
whistles by others.
The market is booming. This means extra benefits for the
highly discriminating user. Those unable to resist the hype
may be saddled with expensive new models/features which
they will never use.
In part
1
of
this
series, mention was made of
CRT
graphics
and other aspects such as communications, resolution,
halftones, colour, and data volumes. Part 2 included 'Print
versus CRT presentation' and halftone reproduction was also
discussed. This enables me to continue without repeating
basic image processing topics.
Displays 1. Cathode Ray Tubes
In a CRT display an electron beam is deflected across the
screen horizontally, rapidly flies back, sweeps again slightly
lower down, and so on, eventually covering the whole screen
with a series of scanning lines called a
raster.
The beam must
be switched on for the raster
to
be
visible.
To
display
a
pattern
representing an image,
the
beam
is
switched on and off in such
a way that a dot pattern appears on a series of adjacent scan-
ning lines.
The complete dot pattern required to represent the
characters or graphics over the whole screen area (field) is
held in a 'frame buffer' or 'video buffer' RAM store. The
contents of the buffer are updated
as
necessary under program
control and repeatedly copied
onto the
screen in
its
entirety by
the action of successively reading the contents of each storage
cell and displaying it on the screen at the right moment as a
change in spot brightness. Provided the same field is repeated
40 or more times per second, a flicker-free apparently
stationary display is produced.
A Vector display is used instead of raster for specialised
graphic applications such as Computer Aided Design (CAD).
Images are described in terms of their component line
structure — straight lines, circles, polygons, and so on. The
data for
a
screen display takes the form of a set of instructions
for co-ordinate positioning and spot movements, for instance
'draw a circle centred at co-ordinates 230,385, with a radius
of two inches'.
In colour tubes the beams from three electron guns pass
through holes in a mask aligned with three-dot 'triads' on the
screen which glow with different
colours.
The beam from the
'red' gun can only strike a red dot, the 'blue' gun only a blue,
and the 'green' gun only a green. The eye perceives the
particular colour produced by the combined effect of the
primary-colour dots in a triad each glowing with a particular
intensity, as controlled by signal levels supplied to the three
guns.
The size of the fluorescing spot is the single most
important factor determining
CRT
resolution.
A size of about
0.13 mm diameter is claimed for the best monochrome tubes;
the minimum spot size in the best colour tubes (limited by the
dot pitch required in a triad dot structure) is just over 0.30 mm
[1].
A tube with a 0.13 mm spot can display 300 dots per inch
on a '19 inch' screen (measured diagonally) which usually
248 The Electronic Library, Vol. 7, No. 4, August 1989
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