Raster Scan Displays

• A raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television.

• It is a systematic process of covering the area progressively, one line at a time.

• Although often a great deal faster, it is similar in the most-general sense to how one's gaze travels when one reads lines of text.

What is Raster Scan?

Principle:

In a raster scan, an image is subdivided into a sequence of (usually horizontal) strips known as "scan lines".

Each scan line can be transmitted in the form of an analog signal as it is read from the video source, as in television systems, or 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.

Raster: A rectangular array of points or dots

Pixel: One dot or picture element of the raster

Scan Line: A row of pixels

Raster Scan Displays

In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom.

Raster Scan Displays

Picture definition is stored in a memory area called the refresh buffer or frame buffer.

Raster Scan Displays

Stored intensity values then retrieved from refresh buffer and “painted” on the screen one row (scan line) at a time.

Raster Scan Displays

Raster Scan Displays

Horizontal retrace: The return to the left of the screen, after refreshing each scan line.

Raster Scan Displays

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.

Raster graphics cannot be scaled to a higher resolution without loss of apparent quality.

Raster Image

Applications:

Interlaced Scanning:

Raster Scan System Random Scan SystemResolution

It has poor or less Resolution It has High Resolution

Electron-Beam Electron Beam is directed from top to

bottom and one row at a time on screen, but electron beam is directed to whole screen.

Electron Beam is directed to only that part of screen where picture is required to be drawn, one line at a time so also called Vector Display.

CostIt is less expensive It is Costlier

Refresh RateRefresh rate is 60 to 80 frame per second. Refresh Rate is 30 to 60 times per second.

Picture Definition It Stores picture definition in Refresh Buffer

also called Frame Buffer.It Stores picture definition as a set of line commands called Refresh Display File.

Line DrawingZig – Zag line Smooth line

Realism in display

It contains shadow, advance shading and hidden surface technique so gives the realistic display of scenes.

It does not contain shadow and hidden surface technique so it can not give realistic display of scenes.

Image Drawing It uses Pixels along scan lines for drawing an

image.

It is designed for line drawing applications and uses various mathematical function to draw.

Advantages• produce realistic images• also produced different colors• and shadows scenes.

Disadvantages• low resolution• expensive• electron beam directed to whole screen

Raster Scan Displays

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