Introduction to Computer Graphics - PowerPoint by maclaren1


									Hardware Devices
Display Hardware
• Video display devices
• Hard-copy devices
Input devices
• Locator Devices
• Keyboard devices
• Valuator Devices
• Choice Devices

Display Technologies
Cathode Ray Tubes (CRTs)
 – Most common display device today
 – Evacuated glass bottle
 – Extremely high voltage
 – Heating element (filament)
 – Electrons pulled towards
   anode focusing cylinder
 – Vertical and horizontal deflection plates
 – Beam strikes phosphor coating on front of tube

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
•The picture is repeatedly repainted (refresh CRT)
CRT Monitor

                                    Shadow Mask

              Electron Guns

  Red Input


 Blue Input

                     Yoke                     Red, Blue,
                                              and Green
                                            Phosphor Dots

CRT properties
1. Persistence
2. Resolution
3. Addressability
4. Aspect ratio

How long small spots continue to emit light after the
beam is moved. How long it takes to the emitted light
from the screen to decay to one-tenth of its original
 – Lower persistence requires high refresh rate & it is
   good for animation
 – High persistence is useful for displaying highly
   complex static picture.
 – Graphics monitors are usually constructed with 10 to
   60 microseconds.


                                               Intensity distribution

•  Resolution is the number of pointes per inch or centimeter
  that can be plotted horizontally & vertically.
• The smaller the spot size, the higher the resolution.
• The higher the resolution, the better is the graphics system
• High quality resolution is 1280x1024
• The intensity distribution of spots on the screen have
  Gaussian shape.
• Adjacent points will appear distinct as long as their
  separation is greater than the diameter at which each spot
  has intensity of about 60% of that at the center of the spot.

• Addressability is a measure of the spacing
between the centers of vertical and
horizontal lines.
• The picture on a screen consists of intensified
• The smallest addressable point on the screen
is called pixel or picture element
• In graphics mode there are 800x600
Aspect ratio
This number gives the ratio between vertical points and
horizontal points necessary to produce equal length lines
in both directions on the screen. Aspect ratio = ¾ means:
vertical line with 3 points is equal in length to horizontal
line of 4 points.

Display Technologies: CRTs
1.   Vector Displays
2.   Raster Scan Displays
3.   Liquid Crystal Displays (LCDs)
4.   Plasma Panel
5.   Organic LED Arrays

Vector Displays

Vector Displays or random scan
– The electron beam is directed only to the parts of the
  screen where a picture is to be drawn.
– Like plotters it draws a picture one line at a time
– Used in line drawing and wireframe displays
– Picture definition is stored as a set of line-drawing
  commands stored in a refresh display file.
– Refresh rate depends on number of lines
– Refresh cycle is 30 to 60 times each second
– Capable of drawing 100,000 short lines at this refresh rate

Vector Displays
 – Advantages:
    • Generates higher resolution than other
      systems (Raster)
    • Produces smooth line drawings
 – Disadvantage: Is not used for realistic shaded

Raster Scan Displays (1)
– Raster: A rectangular array of points or dots
– Pixel: One dot or picture element of the raster. Its
  intensity range for pixels depends on capability of the
– Scan line: A row of pixels
– Picture elements are stored in a memory called frame

Raster Scan Displays (2)

– Frame must be “refreshed” to draw new images
– As new pixels are struck by electron beam, others
  are decaying
– Electron beam must hit all pixels frequently to
  eliminate flicker
– Critical fusion ‫ اندماج‬frequency
   • Typically 60 times/sec
   • Varies with intensity, individuals, phospher
     persistence, lighting...

Raster Scan Displays (3)
– Intensity of pixels depends on the system for
  example black and white screens each point can
  be on or off thus it needs one bit of memory to
  represent each pixel.
– To paint color screen additional bits are needed.
  If three bits are used, then number of different
  colors are 2*2*2.
– A special memory is used to store the image with
  scan-out synchronous to the raster. We call this
  the frame buffer.
Raster Scan Displays (4)

  – Interlaced Scanning
  – Assume can only scan 30 times / second
  – To reduce flicker, divide frame into two “fields” of
    odd and even lines

          1/30 Sec                     1/30 Sec
    1/60 Sec     1/60 Sec       1/60 Sec       1/60 Sec
    Field 1       Field 2        Field 1         Field 2
           Frame                         Frame

Raster Scan Displays (5)
Scanning (left to right, top to bottom)
   – 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: The time needed to get from the
     end of the current scan line to the start of the next
     scan line

Raster Scan Displays (6)
Raster CRT pros:
  – Allows solids, not just wire frames
  – Leverages low-cost CRT technology (i.e., TVs)
  – Bright! Display emits light
  – Requires screen-size memory array
  – Discreet sampling (pixels)
  – Practical limit on size

   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                                              20
Monochrome Display
(Bit-map Display)

                1 bit
             2 levels


3-Bit Color Display



    COLOR: black red green blue yellow cyan magenta white

       R       0     1     0      0          1   0      1   1
       G       0     0     1      0          1   1      0   1
       B       0     0     0      1          0   1      1   1

True Color Display
24 bit planes, 8 bits per color gun.
224 = 16,777,216


      N                            Red


Color Map Look-Up Tables
Extends the number of colors that can be displayed by a given number of bit-planes.

y                                                                                  RED
     y                    0
                     0         67                                                          Pixel displayed
                   1                                                   1001 1010 0001
                                                  67 100110100001                          at x', y'
                                                                        R    G    B
              Pixel in
              bit map                              0
     0        at x', y'
          0        x                   x

                 Bit map                               Look-up table                       Display

     Video look-up table organization: each table entry is a 12 bit per entry.
     A pixel with value 67 is displayed on the screen with the red electron gun
     at 9/15 (binary 1001) of maximum, green at 10/15, and the blue is 1/15.
 Beam penetration
• Used with random scan monitors
• The screen has two layers of phosphor: usually red and
• The displayed color depends on how far the electron
  beam penetrate through the two layers.
• A beam of slow electrons excites only the outer of the
  red layer, a beam of fast electrons penetrates through
  the red layer and excites the inner green layer, and at
  intermediate beam speeds, combinations of the two
  colors are emitted to show other colors (yellow &
Display Technology: Color CRTs

Color CRTs have
 – Three electron guns
 – A metal shadow mask to differentiate the beams

Liquid Crystal Displays (LCDs)

– LCDs: organic molecules, naturally in crystalline
  state, that liquefy when excited by heat or E field
– Crystalline state twists polarized light 90º.

Display Technology: LCDs
Transmissive & reflective LCDs:
   – LCDs act as light valves, not light emitters, and
     thus rely on an external light source.
   – Laptop screen: backlit, transmissive display
   – Palm Pilot/Game Boy: reflective display

Plasma Panel
Plasma display panels
   – Similar in principle to
     fluorescent light tubes
   – Small gas-filled capsules
     are excited by electric field,
     emits UV light
   – UV excites phosphor
   – Phosphor relaxes, emits
     some other color

Plasma Panel (2)
Plasma Display Panel Pros
   – Large viewing angle
   – Good for large-format displays
   – Fairly bright
   – Expensive
   – Large pixels (~1 mm versus ~0.2 mm)
   – Phosphors gradually deplete
   – Less bright than CRTs, using more power

Display Technology: DMD / DLP
Digital Micromirror Devices (projectors) or Digital Light
Microelectromechanical (MEM) devices, fabricated with
  VLSI techniques

Organic LED Arrays
• Organic Light-Emitting Diode (OLED) Arrays
  – The display of the future? Many think so.
  – OLEDs function like regular semiconductor LEDs
  – But they emit light
     • Thin-film deposition of organic, light-emitting
       molecules through vapor sublimation in a
     • Dope emissive layers with fluorescent
       molecules to create color.

Organic LED Arrays
OLED pros:
   – Transparent
   – Flexible
   – Light-emitting, and quite bright (daylight visible)
   – Large viewing angle
   – Fast (< 1 microsecond off-on-off)
   – Can be made large or small
   – Available for cell phones and car stereos
OLED cons:
   – Not very robust, display lifetime a key issue
   – Currently only passive matrix displays
       • Passive matrix: Pixels are illuminated in scanline order, but the
         lack of phospherescence causes flicker
       • Active matrix: A polysilicate layer provides thin film transistors
         at each pixel, allowing direct pixel access and constant illum.

Display Processor
• Also called either a Graphics Controller or Display

• 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.

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.

                                X address        Set o r incremen t
     M         Li near                            Raster scan
                                                                      and vertical
     e        address                              generato r
                                                                      signa ls
     o                          Y address        Set o r decrement

                 Data             Pi xel                              Intensi ty
                                 val ue(s)                            or col or

Hard-copy devices
ο Ink-jet printer
ο Laser printer
ο Film recorder
ο Electrostatic printer
ο Pen plotter

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


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