Page 34 - DCAP504_Computer Graphics

Unit 3: Overview of Graphics I/O Devices

Direct View Storage Tube Display
Of all the CRT (Cathode Ray Tube) display technologies in use today, the Direct View Storage Tube
(DVST) display is the simplest. The DVST is a CRT with a long persistence phosphor. The duration of
visibility of the line of character is nearly up to an hour or till the time it is erased. The intensity of the
CRT’s electron beam is sufficiently increased in order to draw a line on the display. This affects the
phosphor to get its bright state. To enable phosphor to get into the dark state, the entire tube has to be
flooded with a specific voltage for half a second.
When the entire tube is flooded, all the lines and characters are also erased. This is one of the limitations
for animation using this display. This method is not useful for interactive drawings. However, this
display has many advantages which include a flicker free display, capability of displaying any number
of lines and is easier to program.
Calligraphic Refresh Display
Calligraphic refresh display refers to a random scan display or a line drawing display. It uses very short
persistence phosphor. Due to the short persistence, the picture has to be redrawn on the CRT a number
of times per second. This redrawing on the CRT is called refreshing the CRT.

The rate of refresh should not be less than 30 times per second. Otherwise a flickering
image will be seen, which is quite irritating to the viewer.
In addition to the CRT, this display requires two more elements called the display buffer and the
display controller. The display buffer is contiguous memory containing all the information required to
draw the picture on the CRT. The display controller takes this information and gives it to the CRT to
display at the refresh rate. The number of lines that are displayed depends on the size of the display
buffer and the speed of the display controller.
The advantages of DVST over CRT are:
1. Refreshing is not required.
2. Displaying very complex pictures at very high resolution without flicker.

The disadvantages of DVST are:
1. Generally, it does not display color.
2. It is not possible to erase a selected part of the image.
3. It consumes several seconds to erase and redraw process for complex images.
3.2.3 Raster Scan Display Systems

A raster scan display system is the rectangular pattern of image capture and reconstruction in
televisions. Most raster displays have some specialized hardware to assist in scan converting output
primitives into the pixmap, and to perform the raster operations, such as moving, copying, and
modifying pixels or blocks of pixels. This hardware is called as a graphics display processor. The
fundamental difference among display systems is how much the display processor does against how
much must be done by the graphics subroutine package executing on the general-purpose CPU that
drives the raster display.

A graphics controller refers to the graphics display processor (emphasizing its
similarity to the control units for other peripheral devices).

The relationship between the pixmap and the address space of the general-purpose computer’s memory
is another major differentiator in raster systems, whether the pixmap is part of the general-purpose
computer’s memory or separate.

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