Raster Scan Displays

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Raster: A raster scan, or raster scanning, is the rectangular pattern of image capture in television.

• It’s a rectangular array of points or dot.• The word raster comes from the Latin word 

rastrum (a rake).

• An image is subdivided into a sequence of strips known as “scan lines” which can be further divided into discrete pixels for processing in a computer system.

• This ordering of pixels by rows is known as raster order, or raster scan order.

• In raster scanning, the beam sweeps horizontally left-to-right at a steady rate, then blanks and rapidly moves back to the left, where it turns back on and sweeps out the next line.

The use of raster scanning in television was proposed in 1880 by French engineer Maurice Leblanc.

The term raster was used for a halftone printing screen pattern as early as 1894.

The first use of raster specifically for a television scanning pattern is often credited to Baron Manfred von Ardenne who wrote in 1933.

 in January 1930 it was proven by demonstrations that the Braun tube was prototyped in the laboratory with point sharpness and point brightness for the production of a precise, bright raster.

In analog TV, originally it was too costly to create a simple sequential raster scan of the type just described with a fast-enough refresh rate and sufficient horizontal resolution, although the French 819-line system had better definition than other standards of its time.

HISTORY:

The image is stored in a frame buffer containing the total screen area and each memory locations corresponds to pixel.

Frame buffer also contains the colour for each pixel.

Examples- Television panels ,printer (99% of raster scan )

FRAME BUFFER

VIDEO CONTROLLER

• It is used to scan each and every line of the refresh or buffer memory• The lines are scanned from left to right and when

line is finishes ,it get to the next line and so on ..• It has the direct access to refresh buffer to

retrieve all the coordinates corresponding to each pixels.• Two registers are used to represent the

coordinates .

Interlacing On some raster systems(TV), each frame is displays in

two parsing using an interlaced refresh procedure.

Interlacing is primarily used for slower refresh rates.

An effective technique to avoid Flicker. (Flicker occurs on CRTs when they are driven at a low refresh rate, allowing the brightness to drop for time intervals sufficiently long to be noticed by a human eye).

Interlacing

Raster Image

The quality of raster image is determined by total number pixels (resolution), and the amount of information in each pixel (color depth).

Raster Image

ADVANTAGES

• Raster scan is a technology which is very much efficient.

• It requires little memory.• This technology is less costlier.

Disadvantage To increase size of a raster image the pixels defining the image are be increased

in either number or size spreading the pixels over the large area causes the image to lose detail and clarity.

Produces jagged line that are plotted as discrete points

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