Visual Displays

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Visual Displays
Bowman, et al., pp. 29-59

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Outline

• Image Quality Issues
• Pixels
• Color
• Video Formats
• Liquid Crystal Displays
• CRT Displays
• Projection Displays

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Image Quality Issues

• Screen resolution
• Color
• Blank space between the pixels
• Intentional image degradation

• Brightness
• Contrast
• Refresh rate
• Sensitivity of display to viewing angle

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Pixels

• Pixel - The most basic addressable image element
  in a screen
• CRT - Color triad (RGB phosphor dots)
• LCD - Single color element
• Screen Resolution - measure of number of pixels
  on a screen (m by n)
• m - Horizontal screen resolution
• n - Vertical screen resolution

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Other meanings of resolution

• Pitch - Size of a pixel, distance from center to
  center of individual pixels.
• Cycles per degree - Addressable elements (pixels)
  divided by twice the FOV measured in degrees.
• The human eye can resolve 30 cycles per degree
  (20/20 Snellen acuity).

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Color

• There are no commercially available small pixel
  technologies that can individually change color.
• Color is encoded by placing different-colored
  pixels adjacent to each other.
• Field sequential color uses red, blue and green
  liquid crystal shutters to change color in front
  of a monochrome screen.

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Video Formats

• NTSC - 525x480, 30f/s, interlaced
• PAL - 625x480, 25f/s, interlaced
• VGA - 640x480, 60f/s, noninterlaced
• SVGA 800x600, 60f/s noninterlaced
• RGB - 3 independent video signals and
  synchronization signal, vary in resolution and
  refresh rate
• Time-multiplexed color - R,G,B one after another
  on a single signal, vary in resolution and
  refresh rate

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Liquid Crystal Displays

• Liquid crystal displays use small flat chips
  which change their transparency properties when a
  voltage is applied.
• LCD elements are arranged in an n x m array call
  the LCD matrix
• Level of voltage controls gray levels.
• LCDs elements do not emit light, use backlights
  behind the LCD matrix

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LCDs (cont.)
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LCDs (cont.)

• Color is obtained by placing filters in front of
  each LCD element
• Usually black space between pixels to separate
  the filters.
• Because of the physical nature of the LCD matrix,
  it is difficult to make the individual LCD pixels
  very small.
• Image quality dependent on viewing angle.

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LCDs (cont.)

• LCD resolution is often quoted as number of color
  elements not number of RGB triads.

Example 320 horizontal by 240 vertical elements
76,800 elements Equivalent to 76,800/3 25,500
RGB pixels "Pixel Resolution" is 185 by 139
(320/1.73, 240/1.73)
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LCDs (cont.)

• Passive LCD screens
• Cycle through each element of the LCD matrix
  applying the voltage required for that element.
• Once aligned with the electric field the
  molecules in the LCD will hold their alignment
  for a short time

• Active LCD screens
• Each element contains a small transistor that
  maintains the voltage until the next refresh
  cycle.
• Higher contrast and much faster response than
  passive LCD

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Advantages of LCDs

• Flat
• Lightweight
• Low power consumption

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Cathode Ray Tubes (CRTs)
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Color CRT
Red, Green and Blue electron guns. Screen
coated with phosphor triads. Each triad is
composed of a red, blue and green phosphor
dot. Typically 2.3 to 2.5 triads per pixel.

• FLUORESCENCE - Light emitted while the phosphor
  is being struck by electrons.
• PHOSPHORESCENCE - Light given off once the
  electron beam is removed.
• PERSISTENCE - Is the time from the removal of
  excitation to the moment when phosphorescence has
  decayed to 10 of the initial light output.

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CRTs (cont.)

• Strong electrical fields and high voltage
• Very good resolution
• Heavy, not flat

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Projection Displays

• Use bright CRT or LCD screens to generate an
  image which is sent through an optical system to
  focus on a (usually) large screen.

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Projector Technologysee http//electronics.howstu
ffworks.com/projection-tv.htm

• Two Basic Designs
• Transmittive projectors - Shine light through the
  image-forming element (CRT tube, LCD panel)
• Reflective projectors - Bounce light off the
  image-forming element (DLP)
• In both types of projectors, a lens collects the
  image from the image-forming element, magnifies
  the image and focuses it onto a screen

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CRT Projectors

• CRT Based
• One color CRT tube (red, blue, green phosphors)
  displays an image with one projection lens.
• One black-and-white CRT with a rapidly rotating
  color filter wheel (red, green, blue filters) is
  placed between the CRT tube and the projection
  lens.
• Three CRT tubes (red, green, blue) with three
  lenses project the images. The lenses are aligned
  so that a single color image appears on the
  screen.

CRT-based projectors are usually heavy and large
compared to other technologies Usually better
range of color and brightness
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LCD Projectors

• Use a bright light to illuminate an LCD panel,
  and a lens projects the image formed by the LCD
  onto a screen
• Compact since LCD Chip small compared to CRTs
• Less Heat, Less Power
• Screen Door effect
• One pixel can burn out
• Different colors are polarized but not in the
  same orientation. Passive stereo polarization
  will not work.

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DLP (Digital Light Processing) Projectors

• The Chip in a DLP Projector is a Digital
  Micromirror Device.
• Essentially every pixel on a DMD Chip is a
  reflective mirror.
• Higher resolution is possible than with LCD
  technology
• No screen door effect.
• Consistent Polarization
• More expensive

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Advantages/Disadvantagesof Projection Display

• Very large screens can provide large FoV and can
  be seen by several people simultaneously.
• Image quality can be fuzzy and somewhat dimmer
  than conventional displays.
• Sensitivity to ambient light.
• Delicate optical alignment.

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Displays in Virtual Reality

• Head-Mounted Displays (HMDs)
• The display and a position tracker are attached
  to the users head
• Head-Tracked Displays (HTDs)
• Display is stationary, tracker tracks the users
  head relative to the display.
• Example CAVE, Workbench, Stereo monitor

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Visually Coupled Systems

• A system that integrates the natural visual and
  motor skills of an operator into the system he is
  controlling.
• Basic Components
• An immersive visual display (HMD, large screen
  projection (CAVE), dome projection)
• A means of tracking head and/or eye motion
• A source of visual information that is dependent
  on the user's head/eye motion.

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Differences HMD/HTD

• HMD
• Eyes are fixed distance and location from the
  display screen(s)
• Line-of-sight of the user is perpendicular to the
  display screen(s) or at a fixed, known angle to
  the display screen(s).
• Only virtual images in world

• HTD
• Distance to display screen(s) varies
• Line-of-sight to display screen(s) almost never
  is perpendicular
• Usually much wider FoV than HMD
• Combines virtual and real imagery