Page 30 - DCAP313_LAB_ON_COMPUTER_GRAPHICS

Lab on Computer Graphics

Notes
Figure 2.8: Graph Representation

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vertical retrace
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horizontal retrace
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Advantages
• Produce realistic images
• Also produced different colours
• And shadows scenes
Disadvantages
• Low resolution
• Expensive
• Electron beam directed to whole screen
2.4.1 Difference between Raster Scan and Random Scan Technique

Raster-scan System
• Raster displays have less resolution.
• The lines produced are ziz-zag as the plotted values are discrete.
• High degree realism is achieved in picture with the aid of advanced shading and hidden
surface technique.
• Decreasing memory costs have made raster systems popular.
Random Scan System
• Random displays have high resolutions since the picture definition is stored as a set of
line drawing commands and not as a set of intensity values.
• Smooth lines are produced as the electron beam directly follows the line path.
• Realism is difficult to get.
• Random-scan system’s are generally costlier
2.4.2 Important Characteristics of Video Display Devices

Following are the characteristics of video display devices:
Persistence: The major difference between phosphors is their persistence. It decides how long
they continue to emit light later than the electron beam is removed. Persistence is defined as the
time it takes the emitted light from the screen to decay to one-tenth of its original intensity. Lower
persistence phosphors require higher refreshing rates to maintain a picture on the screen without
flicker. However it is useful for displaying animations. On the other hand higher persistence
phosphors are valuable for displaying static and highly complex pictures.
Resolution: Resolution indicates the maximum number of points that can be displayed without
overlap on the CRT. It is defined as the number of points per centimetre that can be plotted
horizontally and vertically. Resolution depends on the type of phosphor, the intensity to be
displayed and the focusing and deflection systems used in the CRT.
Aspect Ratio: It is the ratio of vertical points to horizontal points to produce equal length lines
in both directions on the screen. An aspect ratio of 4/5 means that a vertical line plotted with
four points has the same length as a horizontal line plotted with five points.

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