2. Video Display Devices

The primary output device in graphics system is a video monitor. The operation of most video monitors is based on the standard Cathode-Ray Tube (CRT) design.

Refresh Cathode-Ray Tube

The following figure illustrates the basic operation of a CRT. A beam of electrons (cathode rays) emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions 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, the picture is redrawn repeatedly and quickly.

  • The primary components of an electron gun are the heated metal cathodes and a control grid as shown in the figure.
  • Heat is supplied to the cathode by current through filament, which produces electrons.
  • And the electrons are accelerated toward the phosphor coated screen.
  • Intensity of the electron beam is controlled by setting voltage levels on the control grid. A high negative voltage shut off the beam, and a smaller negative voltage simply decreases the number of electrons in the beam.
  • The focusing system is needed to force the electron beam to converge into a small spot as it strikes the phosphor.
  • Focusing is accomplished with either electric or magnetic filed.
  • Additional focusing hardware is used in high precision systems to keep the beam in focus at all screen positions.
  • A phosphor with low persistence is useful for animation; a high-persistence phosphor is useful for displaying computer pictures. (Persistence means, how long the phosphor can continuously emit the light.)
  • The maximum number of points that can be displayed without overlap on a CRT is referred as the resolution.
  • Or the resolution is referred as the number of points per centimeter that can be plotted horizontally and vertically. Eg. 1280 ?? 1024
  • Aspect ratio: Ratio of vertical points to horizontal points necessary to produce equal-length lines in both directions on the screen.

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. 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 a memory area called the refresh buffer or frame buffer used for redrawn. Each screen point is referred to as a pixel or pel (picture element)

Intensity range for pixel positions depends on the capability of the raster system. In a B&W system, each screen point is either on or off. So only one bit is needed. The frame buffer in B&W system is called as bitmap. For multi-color systems the frame buffer is called as pixmap.

Refreshing on raster-scan displays is carried out at the rate of 60 to 80 frames per second. The unit for refreshing rate is Hertz (Hz).

Random-Scan Displays

Here, the CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random-scan monitors draw a picture one line at a time, called as vector display.

Refresh rates on a ransom-scan system depends on the number of lines to be displayed. Picture definition is stored as a set of line-drawing commands in the refresh display file or refresh buffer. To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line.

These systems are designed for the line-drawing applications and can???t display realistic shaded scenes.

Color CRT Monitors

A CRT monitor displays color pictures by using a combination of phosphors that emit different colored light. Two basic methods are used for producing color displays (i) Beam penetration and (ii) shadow-mask method.

Beam-Penetration Method

            Two layers of phosphor, usually red and green are coated onto the screen. A beam of slow electrons excites only the outer red layer, fast electrons penetrates through inner red layer and excites the inner green layer. The electrons with intermediate speed produce the combinations of red and green color. The speed of electrons is controlled by the beam-acceleration voltage. But the quality of the pictures is not good here.

Shadow-Mask Method

            It can produce a much wider range of color than the beam-penetration method. A shadow-mask CRT has three phosphor color dots at each pixel position with red, green and blue. This type of CRT has three electron guns, one for each color. The three electron beams are deflected and focused as a group onto the shadow mask, which contains corresponding holes for each pixel position.

            The color variation is obtained by varying the intensity levels of the three electron beams. For example, turning off the green and red beam, we will get only blue color.

Direct-View Storage Tubes

            An alternative method for maintaining a screen image is to store the picture information inside the CRT instead of refreshing the screen.

            A direct-view storage device stores the picture information as a charge distribution just behind the phosphor coated screen. Two electron guns are used, the primary gun is used to store the picture pattern, the second, the floor gun, maintains the picture display.

Advantage:

Because of no refreshing is needed very complex pictures can be displayed at very high resolutions without flicker.

Disadvantage:

it can not display colors, and selected parts of a picture can???t be erased. To eliminate the selected part of a picture, the entire screen must be erased and redrawn. It is a time consuming process.

Flat-Panel Displays

            It refers to a class or video devices that have reduced volume, weight and power requirements composed to CRT. A main advantage of flat-panel displays is that they are thinner than CRTs and we can hang them on walls or wear on our wrists. The two categories of flat-panel displays are

  • Emissive displays: the device which convert electrical energy into light. (Plasma panel)
  • Non-Emissive displays: used optical effects to convert sunlight or light from some other source into graphic patterns. (LCD)

Examples:

  1. Plasma Panel - are constructed by filling the region between two glasses with a mixture of gases that usually included neon.
  2. Thin-Film Electroluminescent Display - similar to plasma panel, but the region between two plates is filled with phosphor.
  3. LED - A matrix of diodes is arranged to form the pixel positions in the display, and the picture definition is stored in a refresh buffer.
  4. LCD - commonly used in small systems, such as calculators, and portable, laptop computers. These non-emissive devices produce a picture by passing polarized light from the surroundings or from an internal light source through a liquid-crystal material that can be aligned either block or transmit the light.

Three Dimensional Viewing Devices

            Graphical monitors for the display of 3D scenes have been devised using a technique that reflects a CRT image from a vibrating, flexible mirror. The operation is illustrated in the figure. As the vertical mirror vibrates, it changes focal length. These vibrations are synchronized with the display of an object on a CRT so that each point on the object is reflected from the mirror into a spatial position corresponding to the distance of that point from a specified viewing position. This allows us to walk around an object or scene and view it from different sides.

Stereoscopic and Virtual-Reality Systems

To obtain a stereoscopic projection, first to obtain two views of a scene generated from a viewing direction corresponding to each eye. When we simultaneously look at the left view with the left eye and the right view with the right eye, the two views merge into a single image and we perceive a scene with depth.

Stereoscopic viewing is also a component in virtual-reality systems, where users can step into a scene and interact with the environment.