Raster Scan Display and Random Scan
Display in CG
Raster Scan Display
The
most common types of graphics monitor employing a CRT is the raster-scan
display, based on television technology.
In a raster
scan display, the electron beam is swept across the screen, one row at a
time from top to bottom. As the electron beam moves across each row, the beam
intensity is turned on and off to create a pattern of illuminated spots.
Picture
definition is stored in memory area called the refresh buffer or frame
buffer. This memory area holds the set of intensity values for all the
screen points. Stored intensity values are then retrieved from the refresh
buffer and “painted” on the screen one row (scan line) at a time (Figure
1.5). Each screen point is referred to as a pixel or pel
(shortened forms of picture element).
Refreshing
on raster-scan displays is carried out at the rate of 60 to 80 frames
per second, although some systems are designed for higher refresh rates.
Sometimes, refresh rates are described in units of cycles per second, or
Hertz (Hz), where a cycle corresponds to one frame.
At the
end of each scan line, the electron beam returns to the left side of the screen
to begin displaying the next scan line. The return to the left of the screen,
after refreshing each scan line, is called the horizontal retrace of the
electron beam. And at the end of each frame (displayed in 1/80th to 1/60th of a
second), the electron beam returns (vertical retrace) to the top left corner of
the screen to begin the next frame.
On some
raster-scan systems (and in TV sets), each frame is displayed in two
passes using an interlaced refresh procedure. In the first pass, the beam
sweeps across every other scan line from top to bottom. Then after the vertical
retrace, the beam sweeps out the remaining scan lines (figure 1.5). Interlacing
of the scan lines in this way allows us to see the entire screen displayed in
one-half the time it would have taken to sweep across all the lines at once from
top to bottom
Random Scan Display
Random
scan monitors draw a picture one line at a time and for this reason, are also
referred to as vector displays (or stroke-writing or calligraphic
displays). The component lines of a picture can be drawn (Figure 1.6) and
refreshed by a random-scan system in any specified order.
The refresh rate on a random-scan system depends on the
number of lines to be displayed. Picture definition is now stored as a set of
line-drawing commands in an area of memory referred to as the refresh display
file.
Sometimes
the refresh display file is called the display list, display program, or simply
the refresh buffer. To display a specified picture, the system cycles through
the set of commands in the display file, drawing each component line in turn.
After
all line-drawing commands have been processed, the system cycles back to the
first line command in the list. Random-scan displays are designed to draw al
the component lines of picture 30 to 60times each second.
