4ICT10 Computer Graphics and Virtual Reality

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4ICT10 Computer Graphics and Virtual Reality

• 3. Graphics Hardware
• Dr Ann McNamara

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Summary so Far

• Introduction Applications
• Diverse spectrum of applications
• Model/Render/Animate

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Objectives

• Elements of Computer Graphics Pictures
• Overview of some of the Hardware used to display
  images

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Elements

• Output Primitives
• Polylines
• text
• filled regions
• raster images
• Attributes

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Polylines
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Polygons
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Attributes of lines polygons

• Manner in which lines are dashed
• Manner in which edges are joined

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Text
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Filled-Regions
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Raster Images
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Raster Images
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Pixel Map
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Creating Raster Images

• Hand Designed
• paint process can automate
• Computed Images
• algorithm used to render a scene
• Scanned Images
• digitized photograph

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Raster Images

• Easily manipulated

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Gray-Scale Raster Images

• Bi-level
• only 2 pixel values
• figure 1.26
• Pixel Depth
• 2 bits per pixel produces 4 gray levels
• 4 bits per pixel produces 16 gray levels
• 8 bit per pixel produces 256 gray levels

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Pixel Depth
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Colour Raster Images

• Each pixel value represents a colour
• Ordered Triple

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Graphics Hardware
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Basic Definitions
Video raster devices display an image by
sequentially drawing out the pixels of the scan
lines that form the raster.
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Graphics DisplayTechnology

• Cathode Ray Tube CRT
• Most common device
• originally used for RAM
• Manchester University 1950s
• Ivan Sutherland 1963 Sketchpad
• first interactive system
• Xerox Parc (late 1970s)
• high resolution windowing systems
• Xerox Star

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Cathode Ray Tubes

• vector
• very high resolution line drawings
• random access beam pattern, variable refresh
• low storage cost
• raster
• discrete image composed of pixels (picture
  elements)
• fixed beam pattern and refresh time
• high storage cost

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Cathode Ray Tube
Early Example (1890)
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Cathode Ray Tube

• Phosphors emit light when excited by electrons
• Electron Gun
• Beam current controlled by monitor
• Deflection Yoke steers electron stream

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Cathode Ray Tube

• Raster Scan
• top left to top right
• shuts off while redirected, begins below
• Image formed by modulating beam current
• Why dont we just see a dot flying around?
• Flicker
• 60-80 times per second (hertz)

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CRT Raster Scan Pattern
Field 1
Field 2
Interlaced
Non Interlaced
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CRT Raster Scan Pattern

• Frame Rate number of complete screen updates
  per second
• Field Rate number of vertical re-traces per
  second

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CRT Signal Characteristics
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Colour Displays

• But, electrons dont come in different colours?

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Colour Production
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Electromagnetic Spectrum
Low
High
380nm
780nm
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Colours

• CRT systems employ additive colour mixing
• Red Green Yellow
• Red Blue Magenta
• Green Blue Cyan
• Red Green Blue White
• We get intermediate colour mixes by varying the
  intensity of each of the 3 primaries.
• Light Orange 100 R 50 G 20 B

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Colour Primaries
Subtractive Colour Mixing
Additive Colour Mixing
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Colour Mixing

• Additive spectrum of light is the result of
  addition of individual spectra
• CRT colour mixing
• LCD projectors
• Subtractive colour resulting from the selective
  absorption of light wavelengths
• paints
• dyes

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RGB Colour Space

• Each CRT is capable of a finite but continuous
  range of intensities.
• Assume that
• 0 ? no intensity (i.e. no phosphor emission)
• 1 ? full intensity
• This defines a colour space with vectors defining
  colours

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RGB Colour Space
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Device Dependency

• This is a vector space with the basis vectors
  defined by the properties of the monitor
  phosphors.
• If the phosphors change the colour space changes.
• We cannot use RGB to universally define a colour.
• ? we require a device independent colour space.
• use XYZ, Lab, Luv, Pantone etc.

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Gamma

• Normally we assume that intensity ? RGB value.
• i.e. 0.5, 0.5, 0.5 is half as bright as the
  value 1.0, 1.0, 1.0
• Not True!
• Luminance is exponentially proportional to
  voltage
• ? must gamma correct the display

or
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Gamma Function and Correction
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Gamma Correction

• Implemented using a look-up table.
• Some display adapters store 3 256-entry LUTs, one
  per colour channel.
• These LUTs may be changed by the
  user/manufacturer as monitor characteristics
  change over time.
• Note gamma correction guarantees monitor
  luminance linearity, it does not guarantee images
  viewed on different monitors will appear equally
  bright.

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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.
• Typical frame buffer resolution
• 640 x 480 x 8
• 1280 x 1024 x 8
• 1280 x 1024 x 24

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Display Characteristics

• 2D Graphics displays are characterised by
• resolution number of independent pixels (e.g.
  1024 ? 768)
• colours/bits per pixel (e.g. 24bit ? 224 colours
  16,777,216)
• dot pitch no. of phosphors per inch. (dpi)
• refresh rate (Hz.)
• interlaced / non-interlaced
• phosphor wavelengths (nm.)
• these are usually specified using chromaticity
  values
• whitepoint (K)
• some monitors allow you to change the whitepoint

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Broadcast Standards

• European (except France) TV (PAL), introduced in
  1960
• Phase Alternating Line
• frame rate 25 Hz. and field rate 50 Hz.
• 625 scan lines
• US Japanese TV (NTSC), introduced in 1953
• National Television System Committee (or Never
  The Same Colour)
• frame rate 29.97 Hz. and field rate 59.94 Hz.
• 525 scan lines
• Both TV systems use interlacing.

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Display Memory Bandwidth

• CRT electron guns receive an analogue voltage
  signal from DACs, 1 per colour channel.
• Most current DACs are 8-bit per channel.
• ? 24 bits per pixel required
• 224 possible colours 16777216
• (humans can see many more than this!)
• Bandwidth requirements for a typical graphics
  system
• Resolution 1024 ? 768
• Refresh rate 75 Hz. non-interlaced
• 24 bits per pixel

? 176,947,200 bytes/sec. transfer required ? RAM
access time ? 5.65 ns.
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Solutions

• Use faster RAM VRAM
• Video RAM (dual ported)
• CPU and DACs can access RAM simultaneously
• no need for DMA or CPU latency
• more expensive than standard DRAM
• Reduce bandwidth
• use Colour LookUp Tables (CLUT)
• this is becoming very popular in systems using
  texture mapping it reduces the total memory
  requirements for textures ? CLUT per texture
• reduces memory per pixel
• total storage costs are now down, but slight
  increase in latency due to CLUT lookup, but this
  is usually negligible
• requires a fixed palette of colours

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24-bit Raster, No CLUT
In this case the value sent to the display is
the value in the pixel.
Each pixel requires 24-bits
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8-bit Raster with 24-bit CLUT
Each pixel requires 8-bits
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Typical Raster System Architecture
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Summary

• Basic Components of BW and Colour CRTs
• The relationship between elements in the frame
  buffer and pixels on the display
• Display Characteristics
• How Colour Tables work