Computer graphics involves drawing pictures on screens through programming and manipulating data. The primary output device is a video monitor, which uses a cathode ray tube. In a raster scan display, an electron beam sweeps across the screen line by line to display pixels stored in a refresh buffer. This avoids screen flicker. A random scan display directs the electron beam only where needed for high resolution line drawings, but is limited to simple images. Raster graphics systems use a video controller to access a frame buffer in memory and refresh the screen.

1. COMPUTER GRAPHICS
NEHRUREVATHY
DEPARTMENT OF BCA

2. Computer Graphics-Definition:
• It is an art of drawing pictures on computer screens with the help of
programming.
• It involves computations, creation, and manipulation of data.
• It is also a rendering tool for the generation and manipulation of
images.

3. Cathode Ray
Tube
• The primary output device in a
graphical system is the video monitor.
• The main element of a video monitor
is the Cathode Ray Tube CRT.
operation of CRT :
The electron gun emits a beam of
electrons Called cathode
The electron beam passes through
focusing and deflection systems that direct
it towards specified positions on the
phosphor-coated screen.
When the beam hits the screen, the
phosphor emits a small spot of light at
each position contacted by the electron
beam.
It redraws the picture by directing the
electron beam back over the same screen
points quickly –REFRESH CRT.

5. RASTER SCAN DISPLAY
• In a raster scan system, 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.
• 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.

7. REFRESH BUFFER
• 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.
• Each screen point is referred to as a pixel .
• To avoid flickers,we use interlaced scan line.
INTERLACED SCAN LINE
HORIZONTAL RETRACE
VERTICAL RETRACE

8. • HORIZONTAL RETRACE:
Returning to the left of the screen after refreshing each scan line.
• VERTICAL RETRACE:
At the end of each frame,the electron beam returns to the top left corner
of the screen to begin the next frame.

9. RASTER
SCAN
DISPLAY

10. • Advantages:
1. avoid flickers
2.realistic images
• Disadvantage:
1.low resolution
• Example:
Home television printer

11. RANDOM SCAN DISPLAY
• The electron beam is directed only to the part of the screen where
the picture is to be drawn .
• It is also called vector display, stroke-writing display, or calligraphic
display.
• Picture definition is stored as a set of line-drawing commands in an
area of memory referred to as the refresh display file.
• Random-scan displays are designed to draw all the component lines
of a picture 30 to 60 times each second.

12. RANDOM SCAN DISPLAY

13. • Advantages:
1.high resolution
2.require little memory
3.produce smooth line drawings
4.require intelligent beam
• Disadvantages:
1.cannot draw a complex images
2. limited color capabilities:very expensive.
• Note:
1. used for line drawing applications
2.cannot be used to display shaded images.

14. RASTER SCAN SYSTEMS
• Simple Raster Graphics system

15. RASTER SCAN SYSTEM
• Video controller-Special purpose processor:
Also called as display controller.
Control the operation of the display
It access the frame buffer to refresh the screen
A fixed area of the system memory is reserved for the frame
buffer and the video controller can directly access that frame
buffer.
Screen surface are represented as xypixels ---x increases as
left to right and y increases bottom to top.

16. Raster graphics system with affixed portion of the
system memory reserved for the frame buffer

17. Basic refresh operation of video controller

18. Basic refresh operation of video controller

19. • A cycle is slow(To speed up the processing,video controller can
retrieve multiple pixel values from the refresh buffer on each pass)

21. DISPLAY PROCESSOR

23. RASTER SCAN DISPLAY PROCESSOR

24. • Display processors can perform various other tasks like:
1. creating different line styles,
2. displaying color areas, etc
• . Typically display processors are utilized to interface input devices,
such as mouse, joysticks.

26. RANDOM SCAN SYSTEM

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