Raster Graphics Hardware

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Raster Graphics Hardware
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EXAMPLE RASTER GRAPHICSARCHITECTURE
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BASIC DEFINITIONS

• RASTER A rectangular array of points or dots.
• PIXEL (Pel) One dot or picture element of the
  raster
• SCAN LINE A row of pixels

Video raster devices display an image by
sequentially drawing out the pixels of the scan
lines that form the raster.
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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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CRT Monitor
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Electron Gun

• Contains a filament that, when heated, emits a
  stream of electrons.
• Electrons are focused with an electromagnet into
  a sharp beam and directed to a specific point of
  the face of the picture tube.
• The front surface of the picture tube is coated
  with small phosphor dots.
• When the beam hits a phosphor dot it glows with
  a brightness proportional to the strength of the
  beam and how often it is excited by the beam.

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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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Shadow Mask

• Shadow mask has one small hole for each phosphor
  triad.
• Holes are precisely aligned with respect to both
  the triads and the electron guns, so that each
  dot is exposed to electrons from only one gun.
• The number of electrons in each beam controls
  the amount of red, blue and green light generated
  by the triad.

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Scanning An Image

• Frame The image to be scanned out on the CRT.
• Some minimum number of frames must be displayed
  each second to eliminate flicker in the image.

CRITICAL FUSION FREQUENCY Typically 60 times
per second for raster displays. Varies with
intensity, individuals, phosphor persistence,
room lighting.
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Interlaced Scanning

• Scan frame 30 times per second
• To reduce flicker, divide frame into two
  fieldsone consisting of the even scan lines and
  the other of the odd scan lines.
• Even and odd fields are scanned out alternately
  to produce an interlaced image.

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Scanning

• VERTICAL SYNC PULSE Signals the start of the
  next field.
• VERTICAL RETRACE Time needed to get from the
  bottom of the current field to the top of the
  next field.
• HORIZONTAL SYNC PULSE Signals the start of the
  new scan line.
• HORIZONTAL RETRACE Time needed to get from the
  end of the current scan line to the start of the
  next scan line.

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Video Formats

• NTSC - 525x480, 30f/s, interlaced
• PAL - 625x480, 25f/s, interlaced
• VGA - 640x480, 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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CRTs (cont.)

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

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Frame Buffers

• A frame buffer may be thought of as computer
  memory organized as a two-dimensional array with
  each (x,y) addressable location corresponding to
  one pixel.
• Bit Planes or Bit Depth is the number of bits
  corresponding to each pixel.
• A typical frame buffer resolution might be
• 640 x 480 x 8
• 1280 x 1024 x 8
• 1280 x 1024 x 24

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1-Bit Memory. Monochrome Display(Bit-map
Display)
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3-Bit Color Display
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True Color Display24 bitplanes, 8 bits per color
gun. 224 16,777,216
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Color Map Look-Up Tables

• Extends the number of colors that can be
  displayed by a given number of bit-planes.

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Pseudo Color 28 x 24 Color Map LUT
Could be used to define 256 shades of green or 64
shades each of red, blue, green and white, etc.
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Display Processor
Also called either a Graphics Controller or
Display CoProcessor or Graphics Accelerator or
Video Card

• Specialized hardware to assist in scan converting
  output primitives into the frame buffer.
• Fundamental difference among display systems is
  how much the display processor does versus how
  much must be done by the graphics subroutine
  package executing on the general-purpose CPU.

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Video Controller

• Cycles through the frame buffer, one scan line at
  a time. Contents of the memory are used the
  control the CRT's beam intensity or color.

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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.
• Full color obtained by having separate
  monochromatic projector for each of the R,G, B
  color channels

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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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Input Devices

• Logical Devices
• Locator, to indicate a position and/or
  orientation
• Pick, to select a displayed entity
• Valuator, to input a single value in the space of
  real numbers
• Keyboard, to input a character string
• Choice, to select from a set of possible actions
  or choices
• Locator Devices Tablet, Mouse, Trackball,
  Joystick, Touch Panel, Light Pen
• Keyboard devices Alphanumeric keyboard (coded -
  get single ASCII character, unencoded - get
  state of all keys - more flexible)
• Valuator Devices Rotary dials (Bounded or
  Unbounded), Linear sliders
• Choice Devices Function keys