Page 29 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Unit 2: Graphics Systems

Raster scan is the most ordinary method of displaying images on the CRT screen. In this method, Notes
the horizontal and vertical deflection signals are generated to shift the beam all over the screen
in a pattern. Here, the beam is swept back and forth from the left to the right across the screen.
When the beam is shifted from the left to the right, it is ON. The beam is OFF, when it is shifted
from the right to the left. When the beam reaches the bottom of the screen, it is made OFF and
rapidly retraced back to the top left to start once more. A display produced in this way is called
raster scan display. In raster scan displays a special area of memory is dedicated to graphics only.
This memory area is called frame buffer. It holds the set of intensity values for all the screen
points. The stored intensity values are retrieved from frame buffer and displayed on the screen
one row (scan line) at a time. Each screen point is referred to as a pixel. Each pixel on the screen
can be specified by its row and column number. Thus by specifying row and column number
we can specify the pixel position on the screen. Intensity range for pixel positions depends on
the capability of the raster system. It can be a simple black and white system or colour system.
In a simple black and white system, each pixel position is either on or off, so only one bit per
pixel is needed to control the intensity of the pixel positions. Additional bits are required when
colour and intensity variations can be displayed. Up to 24 bits per pixel are included in high
quality display systems, which can require several megabytes of storage space for the frame
buffer. On a black and white system with one bit per pixel, the frame buffer is called a bitmap.
F or systems with multiple bits per pixel, the frame buffer is often referred to as a pixmap.

It is the most common type of graphics monitor based on television technology. In a raster scan
system, the electron beam is swept across the screen, one row at a time from top to bottom. When
electron beam moves across each row the beam intensity is turned ON and OFF to generate a
pattern of illuminated spots. Picture meaning is stored in a memory called frame buffer which
holds the set of intensity values, which are then retrieved from the frame buffer and indicated
on the screen one row at a time as shown.

Figure 2.7: Raster Scan System

(a) (b)

(c) (d)

At the end of each line the beam should be turned off and redirect to the left hand side of the
CRT, this is called Horizontal Retrace. At the end of each frame, the electron beam return to top
left corner of the screen to begin the next frame called Vertical Retrace.

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