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Saturday, 15 February 2014

Computer Graphics Notes lab 5 Video Display Devices


The most commonly used output device in a graphics system is a video monitor. The operation of most video monitors is based on the cathode-ray-tube (CRT) design.

A CRT is an evacuated glass tube. An electron gun at the rear of the tube produces a beam of electrons which is directed towards the front of the tube (screen). The inner side of the screen is coated with phosphor substance which gives off light when it is stroked by electrons. It is possible to control, the point at which the electron beam strikes the screen, and therefore the position of the dot upon the screen by deflecting the electronic beam.

The deflection system of the cathode-ray-tube consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. When electron beam passes to the horizontal and vertical deflection plates, it is bent or deflected by the electric fields between the plates. The horizontal plates control the beam to scan from left to right and retrace from right to left. The vertical plates control the beam to go from the first scan line at the top to the last scan line at the bottom and retrace from the bottom back to top. There are two techniques used for producing images on the CRT screen
   Vector scan or Random scan
    Raster scan

Vector scan CRT display/directly traces out only the desired lines on CRT i.e. if we want a line connecting point A with point B on the vector graphics display, we simply drive the beam deflection to go directly from point A to point B. If we want to move the beam from point A to point B without showing line between points, we can blank the beam as we move it. To move the beam across CRT, the information about both, magnitude and direction is required.
It consists of display controller, CPU, display buffer memory and a CRT. A display controller is connected as an I/O peripheral to the CPU. The display buffer memory stores the computer produced display list or display program. The program contains point and line plotting commands as well as character plotting commands. The display controller interprets commands for plotting points, lines and characters and sends digital and point coordinates to a vector generator. The Vector generator then converts the digital coordinate values to analog voltages for beam deflection circuits that displace an electron beam writing on
The CRT’s phosphor coating.

In vector displays, beam is deflected from end point to end point, hence this technique is also called random scan. As beam, strikes phosphor it emits light. But phosphor light decays after few milliseconds and therefore it are necessary to repeat the display list to refresh the phosphor at least 30 times per second to avoid flicker. As display list and it is used for refreshing, the display buffer memory is also called refresh buffer.

The architecture of a raster display consists of disjplay controller, CPU, video controller, refresh buffer, keyboard, mouse and the CRT.

The display image is stored in the form of 1s and Os in the refresh buffer. The video controller reads this refresh buffer and produces the actual image on the screen. It does this by scanning one scan line at a time, from top to bottom and then back to top.

Raster scan CRT
Raster scan is the common method of displaying images on the CRT screen. In this method, the horizontal and vertical deflection signals are generated to move the beam all over the screen in a pattern. Here, the beam is swept back and forth from the left to the right across the screen. When the beam is moved from tire left to the right, it is ON. The beam is OFF, when it is moved from the right to the left. When the beam reaches the bottom of the screen, it is made OFF and rapidly retraced back to the top left to start again. A display produced in this way is called raster scan display. Raster scanning process is similar to reading different lines on the page of a book. After completion of scanning of one line, the electron beam flies back to the start of next line and process repeats. In the raster scan display, the screen image is maintained by repeatedly scanning the same image. This process is known as refreshing of screen.


Random Scan Display
Raster Scan Display
In vector scan display the beam is moved between the end points of the graphics primitives.

In raster scan display the beam is moved all over the screen one scan line at a time, from top to bottom and then back to top.

Vector display flickers when the number of primitives in the buffer becomes too large.

In raster display, the refresh process is independent of the complexity of the image.

Vector display draws a continuous and smooth lines

Raster display can display mathematically smooth lines, polygons and boundaries of curved primitives only by approximating them with pixels on the raster grid.

Vector Scan Display
Raster Scan Display

Cost is more.
Cost is low.

Vector display only draws lines and characters.

Raster display has ability to display areas filled with solid colors or patterns.

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