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Computer Graphics

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Dr.S.Sivakumar,Principal

C.P.A College, Bodinayakanur

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Overview of Graphics System

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Basic Graphics System

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Refresh Cathode-Ray Tube

Electron Gun

Focusing System

Horizontal Deflection Plates

Vertical Deflection Plates

Phosphor-Coated Screen

Electron Bean

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Refresh Cathode-Ray Tube

  • Focusing system
    • Electrostatic focusing
    • Positively charged metal cylinder
  • Deflection of electron beam
  • Persistence
    • How long phosphors continue to emit light
    • Low persistence is useful for animation
  • Resolution
    • Number of points per centimeter can be plotted horizontally and vertically

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Refresh Cathode-Ray Tube

  • Intensity distribution of a phosphor spot
    • Adjacent spots appear distinct: 60%
    • Spot size also depends on intensity
  • Deflection of electron beam
  • Resolution of a CRT depends
    • Type of phosphor
    • Intensity to be displayed
    • Focusing and deflection system
  • Aspect ratio
    • The ratio of vertical points to horizontal points necessary to produce equal-length lines

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Video Display Devices

  • Raster-scan displays
  • Random-scan displays
  • Color CRT Monitors
  • Direct-view storage tubes
  • Flat-panel displays
  • Three-dimensional viewing devices
  • Stereoscopic and virtual-reality systems

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Raster-Scan Display

The electron beam is swept across the screen, one row at a time from top to bottom

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Raster-Scan Display

  • Frame buffer (refresh buffer)

Picture definition is stored in the memory area

  • Pixel Picture each screen point is referred as pixel or pel
  • Scan line
  • Pixmap (bitmap) Picture for black-and-white system with one bit per pixel, the frame buffer is called a bitmap. For systems with multiple bits per pixel the frame buffer is called a pixmap
  • Refreshing (60 - 80 frames/sec)
  • Horizontal retrace
  • Vertical retrace
  • Interlacing each frame is displayed in two pass
  • Scan conversion

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Interlaced refresh

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Random-Scan Display

A CRT has the electron beam directed only to the parts of the screen where the picture is to be drawn

draw a picture one line.

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Random-Scan Display

  • Random-scan (vector, stroke-writing, calligraphic) displays
    • Draw a picture one line at a time
  • Refresh rate
    • Depends on the number of lines to be displayed
  • Picture definition
    • A set of line-drawing commands
    • Refresh display file
  • Designed for line drawing applications
    • Can not display realistic shaded scenes

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Color CRT Monitors

  • Beam-penetration method
    • Used with random-scan monitors
    • Two layers of phosphor: red and green
    • The displayed color depends on how far the electron beam penetrates into the phosphor layers.
    • Only four colors are possible: red, green, orange, and yellow.

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Color CRT Monitors

  • Shadow-mask method
    • Three phosphor color dots at each pixel position
    • Three electron guns
    • Color variations: Varying the intensity levels of the three electron beams
    • Full-color (true-color) system: 24 bits of storage per pixel

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Direct-View Storage Tubes

  • Store the picture information inside the CRT
    • A charge distribution just behind the phosphor-coated screen
    • Two electron guns: primary gun and flood gun
  • Advantage
    • Very complex pictures can be displayed at very high resolutions
  • Disadvantages
    • Do not display color
    • Selected parts of a picture cannot be erased

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Flat-Panel Displays

  • Video devices have reduced
    • volume,
    • weight, and
    • power requirements
  • compared to a CRT.
  • A significant feature - thinner than CRT
  • Classified into two categories:
    • Emissive displays
    • Nonemissive displays

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Three-Dimensional Viewing Devices

  • Three-dimensional display system
    • Reflects a CRT image from a vibrating, varifocal mirror.
    • The mirror changes focal length to match the depth of points in a scene.
  • Genisco SpaceGraph System
    • Project objects into a 25X25 X25 cm3 volume
    • Applications
      • Medical
      • Geological
      • Molecules

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Stereoscopic and Virtual-Reality Systems

  • Stereoscopic view
    • Present a different view to each eye of an observer
    • Need to obtain two views (left and right) of a scene
    • Shutter glasses
    • Infrared synchronizing emitter
  • Head-mounted displays
    • Two LCD displays
    • Tracking devices
    • Earphone

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Raster-Scan Systems

Architecture of a simple raster graphics system

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CPU

System Memory

Video Controller

Monitor

System Bus

I/O Devices

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Raster-Scan Systems

A fixed area of system reserved for frame buffer

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CPU

System Memory

Video Controller

Monitor

System Bus

I/O Devices

Frame buffer

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Video Controller

  • Coordinate system
    • Frame-buffer locations are referenced in Cartesian coordinates.
    • Coordinate origin
      • Lower left screen corner
      • Upper left screen corner
  • Refresh operations of video controller
    • Top-to-bottom, left-to right
    • x register ( initial value = 0)
    • y register (initial value = ymax)

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Video Controller

Raster-scan Generator

X Register

Pixel Register

Frame Buffer

Y Register

Memory Address

Horizontal and Vertical Deflection Voltages

Intensity

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Raster-Scan Display Processor

  • Display processor (graphics controller, display coprocessor)
    • To free the CPU from the graphics chores
    • Digitize a picture definition into a set of pixel-intensity values
  • Scan conversion: The digitization process is called.
    • Straight-line segments
    • Curved lines and polygon outlines
    • Characters

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Raster-Scan Display Processor

CPU

Display Processor

Video Controller

Monitor

System Bus

I/O Devices

System Memory

Display Processor Memory

Frame buffer

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Raster-Scan Display Processor

  • Display processor functions
    • Generating various line styles
    • Displaying color areas
    • Performing certain transformations
    • Manipulations on display objects
  • Run-length encoding
    • Reduce memory requirements
    • Organizing the frame buffer as a linked list
    • Store a scan line as a set of integer pairs
      • An intensity value
      • The number of adjacent pixels with the same intensity

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Random-Scan Systems

Graphics commands are translated into a display file stored in the system memory

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CPU

System Memory

Display Processor

Monitor

System Bus

I/O Devices

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Input/Output Devices

  • Input Devices
    • Keyboards
    • Mouse/3D mouse
    • Digitizer
    • Trackball and Spaceball
    • Joysticks
    • Image scanner
    • Data Glove
    • Touch panels
    • Light pens

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  • Output Devices
    • Color graphics display/Head mounted display
    • Photo-realistic color printer
    • Film recorder
    • Video output (Video tape, LD recorder)
    • Pen plotter

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Graphics Software

  • Classification
    • General programming package
    • Special-purpose applications package
  • Standards Independent of any language
    • GKS - Graphics Kernel System (ISO)
    • PHIGS - Programmer’s Hierarchical Interactive Graphics Standard (extension of GKS) PHIGS+
  • Language Binding defined for particular programming language
  • OpenGL SGI (Silicon Graphics Inc.)
  • Direct 3D Microsoft
  • Animation Packages
    • ALIAS/WAVEFRONT (SGI)
    • 3D Studio Max
    • Maya
  • VR Packages
    • Division
    • Sense 8
    • VRML

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