Computer Graphics Hardware

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Computer Graphics Hardware

• Keywords Logical Input devices, Display devices,
  Vector and Raster Displays, Video Cards

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Logical Input Devices

• Classify objects in terms of what it does
• Provides level of abstraction
• Enhances portability (device independent design
  of interface)
• Shield application from physical details

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Classes of Logical Input Devices

• Locator - to indicate a position or orientation
  (subclasses)
• Pick - to select a displayed entity
• Valuator - to input a single real number
• String - to input a character string
• Choice - to select from a set of possible actions
  or choices

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Physical Input Devices (1)

• Keyboard string/choice input
• Mouse pick/locator device with relative
  positioning and indirect input.
• Tablet pick/locator device with absolute
  positioning and indirect input.
• Joystick/Trackball locator/valuator
• Knobs (e.g. Volume control) valuator devices

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Physical Input Devices (2)

• 3D laser scanner/digitizer
• Data Gloves
• Motion Capture Devices
• Eye trackers

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Display Hardware

• Hard Copy devices
• Printers, plotters
• Transient displays
• LCD, CRT, projectors

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Cathode Ray Tubes

• Most common is Cathode Ray Tube (CRT) monitor
• Horizontal and vertical deflectors focus an
  electron beam on any spot on a phosphor coated
  screen
• Colour systems have groups of 3 different
  phosphors, for red, green and blue
• Electrons hit the screen phosphor molecules and
  excite them

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Cathode Ray Tubes

• Most phosphors relax back to the ground state by
  emitting a photon of light which is called
  flourescence, which decays in under a millisecond
• Some molecules may be further excited, and emit a
  light call phosphorescence, which decays slower,
  but still rapidly (15-20 milliseconds)
• Therefore, the screen must be refreshed by
  redrawing the image

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Phosphors

• Phosphors have a colour (usually red, green or
  blue
• They also are characterized by their persistence
  (time to decay of emitted light)
• High persistence cheap and good for text, bad for
  animation
• Low persistence, good for animation, but need
  high refresh rate (50-60 Hz usually sufficient to
  avoid flicker)
• The maximum number of points, or pixels, that can
  be displayed without overlap is called the
  resolution, eg. 1024 X 768

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Vector Display Devices

• Also Vector Scan Displays, Random Scan Devices,
  Line Plotters
• The electron beam directly draws the picture
• e.g

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Vector Scan Displays

• Advantages are high resolution, easy animation,
  and requires little memory (just display program)
• Disadvantages are limited colour capability and
  flicker occurs as complexity of image increases.

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Raster Scan Devices

• Scans the screen from top to bottom in a regular
  pattern (common TV technology)
• A Raster is a matrix of pixels (picture elements)
  covering the screen
• The electron beam is turned on/off so the image
  is a collection of dots painted on screen one row
  (scan line) at a time.

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Frame Buffer

• An image is stored in a special graphics memory
  area called a frame buffer (or bit map)
• Each memory location corresponds to a pixel
• A display processor scans this memory controls
  the electron beam at each pixel accordingly
• For a monochrome system, each pixel is either on
  or off, so only one bit per pixel is required,
  and the electron beam is either on or off
• For gray scale images, 8 bits per pixel gives 256
  different intensities of gray

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Colour Raster Scan Systems
Phosphor dot triangle

• 3 Electron guns used, for R G and B
• Each pixel consists of 3 dots of phosphor,
  arranged as triangle
• Combining different intensities of phosphors can
  generate different colours

Shadow mask holes are arranged so that each beam
can only excite its own color phosphor
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Colour Lookup table (LUT)

• In many colour raster systems, the display
  controller includes a colour lookup table.
• The value of a pixel in the frame buffer is not
  used to directly control the beam, but is an
  index into the LUT
• The entry in the LUT is used to directly control
  the colour of the pixel
• Eg. 1 byte per pixel in frame buffer, 6 bits for
  each of R,G and B in LUT, then an application can
  choose 256 (28) colours out of 262,144 (26x26x26)
  available colours.

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Raster Scan Systems Conclusion

• Advantages of Raster Scan systems
• Low cost
• Refresh rate independent of image complexity
• Can handle colour and filled areas
• Disadvantages
• Mathematical models of objects must be scan
  converted by the host processor.
• Aliasing