Lecture-03: Random Scan / Vector Scan Display (CGA)

 Vector Scan Display/ Random Scan Display/ Calligraphic Display/ Stroke- Writing Display

A random scan display (or vector/calligraphic display) draws images by directing an electron beam directly along the paths of lines and shapes, rather than scanning the entire screen. This technique, which produces high-resolution, smooth line drawings, is ideal for drafting and CAD applications but unsuitable for complex, filled-in, or shaded images.

                                                 

                                                               Figure: Vector/Random Scan Display

 Components of Random (Vector) Scan Display

Based on the diagram, the main components are:

1.    CPU (Central Processing Unit)

o   Sends display commands and data to the display system.

2.    I/O Port

o   Transfers information and display commands from CPU to display controller.

3.    Display Controller

o   Interprets display commands.

o   Controls the CRT beam movement.

o   Generates signals to draw lines and characters.

4.    Display Buffer Memory (Display List Memory)

o   Stores display list (list of drawing commands).

o   Contains:

§  Point plotting commands

§  Line drawing commands

§  Character plotting commands

5.    Display List

o   Sequence of vector instructions (start point, end point, intensity).

6.    CRT (Cathode Ray Tube)

o   Electron beam draws lines directly on the screen.

7.    Input Devices (Keyboard, Mouse)

o   Used for user interaction and updating display data.

 

Working Procedure of Random (Vector) Scan Display

1.    Command Generation

o   CPU generates display commands (point, line, character).

2.    Storing in Display List

o   Commands are stored in display buffer memory as a display list.

3.    Display Controller Processing

o   Display controller reads one command at a time from display list.

o   It interprets the command and generates X and Y deflection signals.

4.    Beam Movement

o   CRT electron beam moves directly from one coordinate to another.

o   It draws only required lines (not entire screen).

5.    Refreshing

o   After completing all vectors, the controller repeats the process continuously to refresh the image.

👉 In random scan, the beam follows the path of the image (vector by vector), not row by row.

Advantages of Random (Vector) Scan Display

1.    High resolution for line drawings.

2.    Smooth and sharp lines (no jagged edges).

3.    Memory requirement is less (stores only line commands).

4.    Suitable for CAD and engineering drawings.

5.    No pixel-based distortion for straight lines.

 

Disadvantages of Random (Vector) Scan Display

1.    Not suitable for filled images or realistic pictures.

2.    Complex hardware design.

3.    Expensive compared to raster scan.

4.    Flickering occurs if too many lines are drawn.

5.    Limited color capability.

 

Advantage of Random Scan Display over Raster (CRT) Display

(Note: Random scan also uses CRT, but compared to Raster scan CRT display)

1.    Produces smoother lines.

2.    Better for line-based graphics (e.g., engineering drawings).

3.    Requires less memory (stores vectors instead of pixels).

4.    No need to refresh the entire screen pixel by pixel.