Computer Graphics: Question Set – 05

State advantages of Random Scan system.

Following are the advantages of random scan display:

  • Good choice for a device with very high resolution
  • Requires less memory.
  • Produces smooth lines.
  • Useful in displaying static drawing.

Enlist disadvantages of Random Scan system

  • Cannot handle the complex or natural scene.
  • Supports limited color only.
  • Such devices are costly.
  • Not useful to show animation.

Differentiate Raster scan vs. Random scan display

Sr. No.Raster Scan SystemRandom Scan System
1.The electron beam scans the entire screen to draw a picture.The electron beam scans only the part of the screen where picture information is present.
2.Due to the discrete nature of the system, it has low resolution.Electron beam directly follows the path of lines, it draws smooth lines, which gives better resolution.
3.Picture definition is stored as a set of discrete intensity values in the frame buffer.Picture definition is stored as a line drawing commands in the display list.
4.The intensity value is stored for each pixel, it is well suited to display realistic scene.The system is designed to display lines, so it cannot be opted to display a realistic scene.
5.Pixel / spatial location of the screen is used to draw an image.Mathematical functions are used to draw an image.
6.The refresh rate is independent of a number of objects in the scene.When a number of primitives are too large, random scan device flickers.
7.Scan conversion is required.Scan conversion is not required.
8.Scan conversion hardware is required as it displays real-time dynamic scene with extensive computation.Scan conversion hardware is not required.
9.Raster scan system displays are economical.Random scan system are more costly.
10.Used to display a dynamic scene.Used to display static picture.
11.The video controller is required.The video controller is not required.
raster scan system
Raster scan display
random scan display
Random scan display

What do you mean by ‘field’ in interlaced display system?

On some raster-scan systems and in TV sets, each frame is displayed in two passes using an interlaced refresh procedure. The scene is displayed using half-frames, called fields.

What is meant by interlacing

Interlacing is the technique of displaying all of the picture’s odd scan lines first, followed by all of the picture’s even scan lines on the monitor screen. It is also referred to as interleaving.

Interlacing is useful for devices that have a reduced refresh rate. By partially painting the landscape, it nearly doubles the refresh rate.

Progressive and interlaced displayed are depicted in following figure.


What do you mean by scan line ?

Scan line is a row in raster scanning. Video controller paints the scene scan line by scan line starting from top of the screen. Scan line is plotted progressively from left to right. After scanning one scan line, CRT beam is moved to first column of next scan line.

What is the role of video controller in raster scan systems?

Video controller is responsible for following:

  • Refresh the screen
  • Retrieve intensity values for each pixel from frame buffer
  • Generate animation
  • Contains look up table for fast rendering of the screen

Out of raster scan system and random scan systems, which type of system is beneficial for rendering architectural drawings? Why?

Random scan systems are useful for displaying static scenes, whereas raster scan systems are useful for displaying animation. Random scan methods are appropriate for displaying scenes with straight lines.

Architectural drawings are composed of straight lines and are static in character. As a result, random scan systems are useful for rendering architectural drawings.

Why is the electron beam allowed to over scan?

When an electron beam is moved away from a screen pixel, the intensity of that pixel steadily decreases. After a while, the scene will be removed from the screen. So, in order to keep the scene on screen, we must repeatedly scan the full screen.

State advantages and disadvantages of beam penetration method.


  • Inexpensive way to produce colors.


  • Suitable to random scan display only.
  • Can produce only four colors.

Enlist advantages and disadvantages of Direct-View Storage Tubes (DVST)


  • No refreshing is required
  • Complex pictures can be displayed at very high resolutions without flicker.


  • Do not display color.
  • Selected parts of picture cannot be erased.
  • To manipulate a small part, entire scene needs to be erased.
  • Not useful to display animation.

What do you mean by Emissive display devices?

LED (Light Emitting Diodes), plasma panels, electroluminescent displays, and other devices transform electrical energy into light to produce a display.

Flat CRT is a better variant of CRT in which electron beams are propelled parallel to the screen rather than perpendicular to the screen. They are then deflected by 90 degrees. Although such flat CRTs save space, they are not as popular as other emissive display systems.

What do you mean by Non-emissive display devices?

They employ the optical effect to transform natural or synthetic light into a graphic scene on the screen.

LCD is an example of such a device (Liquid Crystal Display).

Scholarly Reading: Non emissive display

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