Page 35 - DCAP504_Computer Graphics

Computer Graphics

The raster-scan display is based on a television technology. It is the most common type of graphics
monitor which employs a CRT. An electron beam is swept across the screen in a raster-scan system, one
row at a time from top to bottom. The beam intensity is turned on and off to create a pattern of
illuminated spots as the electron beam moves across each row. The picture definition is stored in a
memory called the refresh buffer or frame buffer. A set of intensity values hold this memory area for all
the screen points. Then, the stored intensity values are retrieved from the refresh buffer and painted on
the screen one row at a time. The figure 3.5 displays a raster-scan system that displays an object as a set
of discrete points across each scan line.
Figure 3.5: A Raster-scan System

Source: Computer Graphics, Donald Hearn, M. Pauline Baker, C version, Second Edition., Chapter 2-Overview of
Graphics Systems, Page 61.
A raster-scan system can store the intensity information for each screen point that makes it well suited
for the realistic display of scenes containing subtle shading and color patterns. A screen point is referred
to as a pixel or pel. Other examples of systems using raster-scan methods are TVs and printers.

The capability of the raster system depends on the intensity range for pixel positions. In a black-and-
white system, every screen point is either on or off. Therefore, only one bit per pixel is required to
control the intensity of screen positions. For a bi-level system, a bit value of 1 indicates that the electron
beam is to be turned on at that position, and a value of 0 indicates that the beam intensity is to be turned
off. In addition, bits are required when color and intensity variations can be displayed. Up to 24 bits per
pixel are included in high-quality systems, which can require several megabytes of storage for the frame
buffer, depending on the resolution of the system. One 3 MB (megabytes) of storage for the frame buffer
is required for a system with 24 bits per pixel and a screen resolution of 1024 x 1024. The frame buffer is
commonly called a bitmap for a black-and-white system with 1 bit per pixel, and the frame buffer is
often referred to as a pixmap for systems with multiple bits per pixel.
While some systems are designed for higher refresh rates, the refreshing on raster-scan displays are
carried out at the rate of 60 to 80 frames per second. The refresh rates are described in units of cycles per

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