Series: Computer Graphics 1.2

Raster Scan Systems!

        In a raster- scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in memory area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and “ painted” on the screen one row (scan line) at a time (fig.below). Each screen point is referred to as a pixel.

       Refreshing on raster-scan displays is carried out at the rate of 60 to 80 frames per second, although some systems are designed for higher refresh rates. Sometimes, refresh rates are described in units of cycles per second, or Hertz (Hz), where a cycle corresponds to one frame. At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line. The return to the left of the screen, after refreshing each scan line, is called the horizontal retrace of the electron beam. And at the end of each frame (displayed in 1/80th to 1/60th of a second), the electron beam returns (vertical retrace)to the top left corner of the screen to begin the next frame.

       On some raster-scan systems (and in TV sets), each frame is displayed in two passes using an interlaced refresh procedure. In the first pass, the beam sweeps across every other scan line from top to bottom. Then after the vertical retrace, the beam sweeps out the remaining scan lines(fig.below). Interlacing of the scan lines in this way allows us to see the entire screen displayed in one-half the time it would have taken to sweep across all the lines at once from top to bottom.

Raster Display System

Advantages

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

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Disadvantages

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

Series: Computer Graphics 1.1

Display Systems and CRTs !

1) Cathode Ray Tube

       A cathode ray tube (CRT) is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. Most desktop computer displays make use of CRTs. The CRT in a computer display is similar to the "picture tube" in a television receiver.

      All CRT's have three main elements: an electron gun, a deflection system, and a screen. The electron gun provides an electron beam, which is a highly concentrated stream of electrons. The deflection system positions the electron beam on the screen, and the screen displays a small spot of light at the point where the electron beam strikes it.

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2)  CRT with Refresh Buffers (Refresh CRT)

      A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam towards specified position on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT.

The basic components of CRT is shown in figure below:

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Electron Gun

The primary components of an electron gun in a CRT are the heated metal cathode and a control grid. The cathode is heated by an electric current passed through a coil of wire called the filament. This causes electrons to be boiled off the hot cathode surface. In the vacuum inside the CRT envelope, negatively charged electrons are then accelerated toward the phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positively charged metal coating on the in side of the CRT envelope near the phosphor screen, or an accelerating anode can be used. Sometimes the electron gun is built to contain the accelerating anode and focusing system within the same unit.

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Focusing System

The focusing system is used to create a clear picture by focusing the electrons into a narrow beam. Otherwise, electrons would repel each other and beam would spread out as it reaches the screen. Focusing is accomplished with either electric or magnetic fields.

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Deflection System

Deflection of the electron beam can be controlled by either electric fields or magnetic fields. In case of magnetic field, two pairs of coils are used, one for horizontal deflection and other for vertical deflection. In case of electric field, two pairs of parallel plates are used, one for horizontal deflection and second for vertical deflection as shown in figure above.

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CRT Screen

The inside of the large end of a CRT is coated with a fluorescent material that gives off light when struck by electrons. When the electrons in the beam is collides with phosphor coating screen, they stopped and their kinetic energy is absorbed by the phosphor. Then a part of beam energy is converted into heat energy and the remainder part causes the electrons in the phosphor atom to move up to higher energy levels. After a short time the excited electrons come back to their ground state. During this period, we see a glowing spot that quickly fades after all excited electrons are returned to their ground state.

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Persistence

It is defined as the time they continue to emit light after the CRT beam is removed. Persistence is defined as the time it take the emitted light from the screen to decay to one-tenth of its original intensity. Lower-persistence phosphors require higher refresh rates to maintain a picture on the screen without flicker. A phosphor with low persistence is useful for animation ; a high-persistence phosphor is useful for displaying highly complex, static pictures. Although some phosphor have a persistence greater than 1 second, graphics monitor are usually constructed with a persistence in the range from 10 to 60 microseconds.

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Resolution

The number of points per centimeter that can be used be plotted horizontally and vertically. Or Total number of points in each direction.

The resolution of a CRT is depend on

1. type of phosphor
2. intensity to be displayed
3. focusing and deflection system

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Aspect Ratio

It is ratio of horizontal to vertical points.

Example: An aspect ratio of 3/4 means that a vertical line plotted with three points has same length as horizontal line plotted with four points.

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HelloWorld

Hello Everyone!
Myself Abhishek Chourasiya and i'm currently a student pursuing my Master's degree in computer science from Birla Institute of Technology , Mesra. I'm also a Microsoft Student Partner and love to develop software and games for windows , windows phone and Android platforms.

 Today i'm just diving into the world of blogging for the first time , and as i have a deep interest in computer science field i've decided to start blogging and sharing some knowledge and learning with everyone.


So , let's jumpstart to a whole new learning experience on some most popular computer science topics.

Cheers!