Computer Graphics: Types and Applications

Computer Graphics and its Applications

• Computer graphics: art of drawing pictures using computers.
• Deals with the creation, manipulation, and storage of images and objects.

Applications of Computer Graphics

• Computer-Aided Design (CAD)
• Presentation
• Computer graphics
• Art and entertainment
• Education and training
• Visualization
• Graphical user interface
• Image Processing

Types of Computer Graphics

• Non-Interactive Computer Graphics:
  • Passive, lacking user control.
  • Example: screen savers.
• Interactive Computer Graphics:
  • Features two-way communication.
  • User has some degree of control.
  • Examples: Ping-pong games, animation in movies.

Refresh Cathode Ray Tubes (CRTs)

• The primary output device in a graphical system is the video monitor.
• CRT: specialized vacuum tube for producing images.
• A beam of electrons, emitted by an electron gun, passes through focusing and deflection systems to directed toward phosphor coated screen.
• The phosphor emits a small spot of light when contacted by the electron beam.
• The picture is redrawn repeatedly by quickly directing the electron beam back over the same points to keep the phosphor glowing.
• The primary components of an electron gun in a CRT are the heated metal, cathode and a control grid.
• Heat is supplied to the cathode by directing a current through the filament.
• Intensity of the electron beam is controlled by setting voltage levels on the control grid.
• The amount of light emitted by the phosphor coating depends on the number of electrons striking the screen.
• The intensity of the electron beam is controlled by setting voltage levels on the control grid, which fits over the cathode.
• A smaller negative voltage on the control grid decreases the number of electrons passing through, while a high negative voltage stops electrons completely.
• The focusing system converges the electron beam into a small spot as it strikes the phosphor, prevents the electrons from repelling each other
• The electron beam’s deflection is controlled by electric or magnetic fields.
• Horizontal deflection uses one pair of coils, vertical deflection uses another pair.
• With electrostatic deflection, two pairs of parallel plates inside the CRT envelope are used.
  • One pair of plates is mounted horizontally to control vertical deflection.
  • The other pair is mounted vertically to control horizontal deflection.
• Negatively charged electrons are accelerated by high positive voltage towards the phosphor coating.
• The accelerating voltage can be generated with an accelerating anode or a positively charged metal coating near the phosphor screen.
• Electrons stop upon collision with the phosphor coating, where their kinetic energy is absorbed.
• Part of the beam energy converts to heat, and the remainder elevates electrons in the phosphor atoms to higher quantum energy levels.
• After a short time, excited phosphor electrons return to their ground state, releasing extra energy as light.
• Persistence:
  • The duration it takes for a CRT's emitted light to decay to one tenth of its original intensity.
  • Lower persistence phosphors need high refresh rates.
  • Lower-persistence phosphors require higher refresh rates to maintain a picture on the screen without flicker.
  • Low-persistence phosphors are better for animation, high-persistence phosphors for complex, static images.
• Resolution:
  • the maximum number of points displayable on a CRT without overlap.
  • The number of points per cm that can be plotted horizontally and vertically.
  • Resolution on high quality systems is commonly 1280 by 1024.
  • Higher resolution systems are often referred to as high-definition systems.
• Intensity distribution:
  • Intensity is highest at the pixel center and decreases with a Gaussian distribution towards the edges.
• Aspect Ratio: The ratio of vertical to horizontal points necessary to produce equal length lines on the screen.

Color Cathode Ray Tubes

• A shadow-mask CRT has three phosphor color dots at each pixel position.
• RGB monitors' color pictures are created by using phosphors emitting different-colored light in combination.
• Combining emitted light from different phosphors generates a color range.
• Two ways for producing color displays: the beam penetration method and the shadow mask method.

Color CRT Tubes - Beam Penetration Method

• Two layers of phosphor, usually red and green, are coated onto the inside of the CRT screen.
• The displayed color depends on how far the electron beam penetrates.
  • A beam of slow electrons excites just the outer red layer.
  • A very fast electron beam penetrates to excite the inner green layer.
  • Intermediate speeds create combinations of red and green (orange, yellow).
• Advantage: inexpensive for random-scan monitors.
• Disadvantage: only four colors possible, lower picture quality.

Color CRT Tubes - Shadow Mask Method

• This CRT type uses three electron guns (one per color), a shadow-mask grid behind the phosphor-coated screen, and three phosphor color dots at each pixel position.

• Each phosphor dot emits a light (red, green, or blue).

• There are two possible arrangements: Delta delta method and In-line Method

• The Delta-Delta method is commonly used in color CRT systems.

• Shadow masks ensure the electron beams hit the correct phosphors.

• The three electron beams are deflected and focused as a group onto the shadow mask.

• Passing through a hole in the shadow mask means light activates a dot triangle which appears as one colored pixel.

• Phosphor dots in the triangles allow each beam to activate only its corresponding color dot.

• Color variations in shadow-mask CRTs are obtained by varying intensity levels of the electron beams.

• Delta-Delta method is commonly used in color CRT systems.

Color CRT Tubes - Inline Method

• In the inline method, three electron guns and the corresponding red-green-blue dots are aligned along one scan line rather than in a triangular pattern.
• This setup allows better electron gun alignment and is commonly used in high-resolution CRTs.
• In some low-cost systems, the electron beam can only be set to on or off, the color is limited, such setups only generate eight colors.
• The more expensive system can set intermediate intensity levels, allowing several million different colors to be generated
• RGB colour CRT's are designed to be RGB monitors.
• These use shadow masks, and the level of intensity is set for each gun.
• Resolution on high quality systems is commonly 1280 by 1024. A RGB color system with 24 bits of storage per pixel is known as full color system, or true color system.

Color CRT Tubes - Advantages

• Produce realistic images.
• Generate millions of colors.
• Enable shadow scenes.

Color CRT Tubes - Disadvantages

• Low resolution.
• Expensive.