Interactive graphics systems typically employ several
processing units. In addition to CPU, a special purpose display processor is
used to interact with the CPU and to control the operation of the display
device as shown in figure below:
The display processor is used to convert digital information
from the CPU into analog value needed by the display device. This
digital-analog conversion depends on the type of display devices used and the
particular graphics functions that are to be hardware implemented. For many
graphics monitors, the coordinate origin is defined at the lower left screen
corner as shoen in figure below:
In the figure, the X and Y are used to store the coordinates
of the screen pixels. First, set X to 0 and Y to Ymax. This value is used to
set intensity of the CRT beam. Then X is incremented by 1 and process is
repeated for next pixel along the scan line.When the last pixel has been
processed, then X is reset to 0 and Y is decremented by 1. The screen must be
refeshed at rhe rate of sixty frames per second.
Following figure shows one way to set up the organization of
a raster scan system containing a separate display processor sometimes called a
display processor. The purpose of display processor is to free the CPU from the
graphics chores. In this, a separate display processor memory area can also be
available.
A major task of display processor is to perform a procss
called scan conversion. It is the process of separating contigous graphics
objects as a collection of ellipcs, rectangles and polygons.
Example: Characters can be defined with rectangle grids or
with curved outlines as shown in figure below:
The display processor also designed to perform a number of
additionaloperations. These functions include various line style(dashed,
dotted, solid), displayinf color area and so on. Also designed to interface
with interactive input devices such as mouse. Sometimes the display processor
in a random graphics system is referred to as a display processing unit or a
graphic controller. The following figure shows the set up of organization of
random graphics systems.
Display Systems-Refresh CRTs !
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