Computer Graphics

Vector Scan and Raster Scan Display

Vector Scan Display 

In Random-Scan Display electron beam is directed only to the areas of screen where a picture has to be drawn. It is also called vector display, as it draws picture one line at time. It can draw and refresh component lines of a picture in any specified sequence. A Pen plotter is an example of random-scan device. The number of lines regulates refresh rate on random-scan displays. An area of memory called refresh display files stores picture definition as a set of line drawing commands. The system returns back to first-line command in the list, after all the drawing commands have been processed.

ADVANTAGES:

Higher resolution as compared to raster scan display.

Produces smooth line drawing.

Less Memory required.

DISADVANTAGES:

Realistic images with different shades cannot be drawn.

Color limitations.

Raster Scan Display

Raster Scan Displays are most common type of graphics monitor which employs CRT. It is based on television technology. In raster scan system electron beam sweeps across the screen, from top to bottom covering one row at a time. A pattern of illuminated pattern of spots is created by turning beam intensity on and off as it moves across each row. A memory area called refresh buffer or frame buffer stores picture definition. This memory area holds intensity values for all screen points. Stored intensity values are restored from frame buffer and painted on screen taking one row at a time. Each screen point is referred to as pixels. 

In raster scan systems refreshing is done at a rate of 60-80 frames per second. Refresh rates are also sometimes described in units of cycles per second / Hertz (Hz). At the end of each scan line, electron beam begins to display next scan line after returning to left side of screen. The return to the left of screen after refresh of each scan line is known as horizontal retrace of electron beam. At the end of each frame electron beam returns to top left corner and begins the next frame.

ADVANTAGES:

Real life images with different shades can be displayed.

Color range available is bigger than random scan display.

DISADVANTAGES:

Resolution is lower than random scan display.

More memory is required.

Data about the intensities of all pixel has to be stored.

Difference between Raster and vector Scan Display: