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SOME ASPECTS OF CARTOGRAPHIC VISUALISATION OF THE SCREEN
- MUTUAL RELATION OF SCAN PIXELS ANS SCREEN PIXELS
Dr.sc. Brankica Malic
Civil engineering Faculty of University J.J. Strossmayer
Osijek, Croatia
KEY WORDS: scan pixel, screen pixel, hardware zoom, transformation matrix
ABSTRACT:
This paper analyses the raster graphics on two types of Cathode Ray Tube (CRT) screens with shadow mask and with
tension mask, regarding hardware zoom (pixel replication zoom).
The results can be generalized through mathematical description of the transformation of scan pixel to screen pixel
matrix.
KURZFASSUNG:
Dieser Artikel analysiert Rastergraphiken an zwei Typen des Kathodenstrahl-Bildschirms, am Streifenmasken- und
Lochmasken-Bildschirm, durch die Anwendung eines reinen Hardware-Zooms, sog. „pixel replication zoom“.
Diese Erkenntnisse werden durch die mathematische Beschreibung der Abbildung einer Bildmatrix in die
Bildschirmpixelmatrix verallgemeinert.
1. INTRODUKTION
Computer graphics are classified as two types, as raster
graphics and as vector graphics. Raster graphics, which
are, for example, acquired through the scanning of the
maps, are made up of individual dots of the image, the so-
called pixels arranged in the raster form. Vector graphics
are made up of dots, i.e., of a sequence of coordinates
which are then, through mathematical formulae,
connected to form lines, curves or surfaces. This paper
will discuss the relation between scan pixels and screen
pixels within raster graphics.
2. THE FORM AND THE SIZE OF SCREEN PIXELS
IN CRT SCREENS
The construction of CRT (Cathode Ray Tube) computer
screens, both with shadow mask (with “delta” electron gun
arrangement) and with tension mask (with “in-line”
electron gun arrangement) depends on the selected
screen resolution. Screen resolution is the most important
feature of image quality: the lower the resolution, the
coarser the image. Since the screen image is made up of
many individual screen dots, the so-called screen pixels,
which are arranged in columns and in rows, the resolution
is declared as columns x rows. Common resolutions are
640 x 480; 800 x 600; ...; 1600 x 1200.
Each screen pixel on a color screen is made up of three
phosphorus dots glowing in red, green and blue (RGB)
colors when they are hit with an electron beam. Details on
the image structure on CRT screens can be found in
professional literature (Foley, 1994, Lang, 1995, Moers,
1996). The form and the size of a screen pixel depend on
the type of the screen (CRT with shadow mask or CRT
with tension mask), on screen size and on selected
resolution. Rectangular screen pixels are found in screens
with tension masks, whereas the screens with shadow
mask have triangular screen pixels, which always have
the same, constant dimensions, regardless of the selected
resolution (see Figure 1).
7
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Figure 1: Formation of screen pixels in screens with a
tension mask and in 17”- screens with a shadow mask.
With the tension mask screens the dimension of the
electron beam changes depending on the selected
resolution. Here the width of the electron beam is constant
due to the construction of the mask (so-called aperture
grill pitch) and the phosphorus stripes, and only the
vertical stretching of the electron beam changes. Thus the
height of the screen pixel is changed as well (marked with
b in the table 2), whereas the width of the screen pixel
remains constant (marked with a in the table 2). The
measured sizes of screen pixels in 15” and 20” screens
with tension masks are contained in table 2 (Malic, 1998).
Due to the change in the vertical stretching of the electron
beam in screens with tension masks, there is a change in
the length of the way that the electron beam must pass,
depending on the resolution selected. At the resolution of
640 x 480, the electron beam, with maximum height, must
pass only through 480 rows to fully form the screen
image. To achieve the same effect with the resolution of
1280 x 1024, the electron beam must pass through 1024
rows.
