Page 27 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Unit 2: Graphics Systems

to the central processing unit. The display buffer memory stores the computer produced display Notes
list or display program. The program holds point and line plotting commands with (x, y) or
(x, y, z) end point co-ordinates, as well as character plotting commands. The display controller
interprets commands for plotting points, lines and characters and sends digital and point
coordinates to a vector generator. The vector generator then changes the digital coordinate
values to analogue voltages for beam-deflection circuits that relocate an electron beam writing
on CRT’s phosphor coating.

Figure 2.4: Vector Display Architecture

.

Move
100 CUP
300
LINE
400 I/O Port
300
CHAR
(Data
CAR
(interaction data) commands)
MOVE
100
100
LINE Display controller CRT
.
.
JMP
Keyboard Mouse
Display buffer memory

In vector display beam is deflected from end point to end point, therefore this technique is also
called random scan. We know as beam, strikes phosphor it produces light. But a phosphor light
decay after few milliseconds and therefore it is necessary to repeat through the display list to
refresh the phosphor at least 30 times per second to avoid flicker. As display buffer is used to store
display list and it is used for refreshing, the display buffer memory is also called refresh buffer.
In Random Scan System, an electron beam is directed to only those parts of the screen where a
picture is to be drawn. The picture is drawn one line at a time, so also called vector displays or
stroke writing displays. After drawing the picture the system cycles back to the first line and
design all the lines of the picture 30 to 60 time each second.

Figure 2.5: Random Scan System

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