Computer Graphics

1
Computer Graphics

• Chapter 2
• Overview of Graphics Systems

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1. Video Display Devices (1/15)

• Typical Output Device Video Monitor
• Refresh CRT(Cathode-Ray Tubes)
• A beam of electrons (cathode ray)
• Electron gun gt ??? ??
• Focusing systems gt ??? ? ??? ??
• Deflection systems gt ??? ??? ??
• Phosphor-coated screen gt ??? ??
• The light emitted by the phosphor fades very
  rapidly
• ? To redraw the picture repeatedly ? Refresh CRT

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1. Video Display Devices (2/15)

• Refresh Cathode-Ray Tubes (cont.)
• Electron Gun
• Heated metal cathode To boil off electrons
• Control Grid To control the intensity of the
  beam
• Intensity of the electron beam
• Controlled by voltage levels on the control grid

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1. Video Display Devices (3/15)

• Refresh Cathode-Ray Tubes (cont.)
• Focusing Systems
• To force electron beam to converge to a small
  cross section as it strikes the phosphor
• Electrostatic focusing or Magnetic focusing

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1. Video Display Devices (4/15)

• Refresh Cathode-Ray Tubes (cont.)
• Deflection Systems
• Can be controlled by either with electric fields
  or with magnetic fields
• Two pairs of deflection plates to control
• Vertical deflection
• Horizontal deflection

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1. Video Display Devices (5/15)

• Refresh Cathode-Ray Tubes (cont.)
• Phosphor-coated screen
• The transfer of the CRT beam energy to the
  phosphor
• The electrons in the beam collide with the
  phosphor coating
• Their kinetic energy is absorbed by the phosphor
• Converted by friction into heat energy, causes
  excited phosphor electrons ? spot of light

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1. Video Display Devices (6/15)

• Refresh Cathode-Ray Tubes (cont.)
• Persistence
• How long phosphors continue to emit light after
  the CRT beam is removed?
• Defined as the time it takes the emitted light
  from the screen to decay to one-tenth of its
  original intensity
• Usually in the range from 10 to 60 microseconds
• Lower-persistence phosphor
• ? requires higher refresh rates
• ? useful for animation
• High-persistence phosphor
• ? useful for static picture

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1. Video Display Devices (7/15)

• Refresh Cathode-Ray Tubes (cont.)
• Resolution
• The maximum number of points that can be
  displayed without overlap on a CRT (Ex 1280 x
  1024)
• Spot intensity has a Gaussian distribution
• Spot size depends on intensity
• Aspect ratio
• The ratio of vertical points to horizontal points

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1. Video Display Devices (8/15)

• Raster-Scan Displays

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1. Video Display Devices (9/15)

• Raster-Scan Displays (Cont.)
• Raster-Scan display the most common type of CRT
• Refresh Buffer (Frame Buffer)
• Picture definition is stored in memory
• Pixel or Pel (Picture Element)
• Each screen point
• Intensity range for each pixel position
• Bitmap black-and white system with one bit per
  pixel
• Pixmap multiple bit per pixel (Grayscale
  display)

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1. Video Display Devices (10/15)

• Raster-Scan Displays (cont.)
• Refreshing on raster-scan displays
• Refresh rates
• In units of cycles per second, or Hertz(Hz)
• Horizontal retrace
• The return to the left of the screen
• Vertical retrace
• At the end of each frame, the return to the top
  left corner of the screen to begin the next frame
• Interlacing Each frame is displayed in two
  passes (fields)
• Even-line field and odd-line field
• To avoid flicker (Sacrifice in resolution)

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1. Video Display Devices (11/15)

• Random-Scan Displays

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1. Video Display Devices (12/15)

• Random-Scan Displays (cont.) gt Skip
• CRT has the electron beam directed only to the
  parts of the screen where a picture is to be
  drawn
• Vector display (stroke-writing or calligraphic
  display)
• Refresh rate depends on the number of lines to be
  displayed
• Picture definition is stored as a set of
  line-drawing commands
• Refresh display file, display list, vector file,
  display program
• Designed to draw all the component lines of a
  picture 30 to 60 times each second
• Designed for line-drawing applications
• Can not display realistic shaded scenes

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1. Video Display Devices (13/15)

• Color CRT Monitors
• Combination of phosphors (R, G, B)
• The two basic techniques
• Beam-penetration method
• Shadow-mask method
• Beam-penetration Method gt Skip
• Random-scan monitor
• Two layers of phosphor (red, green)
• A beam of slow electrons ? outer red layer
• A beam of fast electrons ? inner green layer
• Screen color controlled by the speed of electrons
• Only four colors (red, green, orange, and yellow)
  are possible

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1. Video Display Devices (14/15)

• Shadow-mask method (Color Television)
• Has three phosphor color dots (R, G, B) at each
  pixel position
• The three electron beams are deflected and
  focused as a group onto the shadow mask.
• Various colors can be obtained by varying the
  intensity levels of three electron beams.

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1. Video Display Devices (15/15)

• Other display devices
• Flat-panel display
• Plasma Display Panel (PDP) gt ?? ?? ??
• Electroluminescent Display (EL)
• Liquid Crystal Display (LCD) gt ???? ?? (??)
• Organic Light Emitting Diodes (OLED) gt ??, ?? ?
• Stereoscopic display

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2. Raster-Scan Systems (1/3)

• Video Controller

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2. Raster-Scan Systems (2/3)

• Video Controller

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2. Raster-Scan Systems (3/3)

• Raster-Scan Display Processor
• Display processor, graphics controller, display
  coprocessor
• Major task Scan conversion
• Graphics commands (lines, circles, ) are scan
  converted into a set of discrete intensity points
  in the frame buffer

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3. Input devices

• Keyboard
• Mouse
• Trackball
• Joystick
• Data Glove
• Digitizer
• Image Scanner
• Touch Panel
• Light Pen
• Voice System

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4. Graphics Software Standards

• GKS (Graphics Kernel System)
• PHIGS (Programmers Hierarchical Interactive
  Graphics Standard)
• OpenGL
• VRML (Virtual-Reality Modeling Language)