Computer Graphics Hardware

1
Computer Graphics Hardware

• Input/Output Technologies

2
(No Transcript)
3
Display Hardware

• Hard Copy devices
• Printers, plotters
• Transient displays
• LCD Monitors, CRT Monitors, projectors
• . Wooden Mirrors?!

4
Cathode ray tube

• Most common is Cathode Ray Tube (CRT) monitor
• Horizontal and vertical deflectors focus an
  electron beam on any spot on a phosphor coated
  screen

Electrons hit the screen phosphor molecules and
excite them
Karl Ferdinand Braun 1897 Cathode Ray
Oscilloscope
5
Phosphors

• Most phosphors relax back to the ground state by
  emitting a photon of light which is called
  flourescence, which decays in under a millisecond
• Some molecules may be further excited, and emit a
  light call phosphorescence, which decays slower,
  but still rapidly (15-20 milliseconds)
• Therefore, the screen must be refreshed by
  redrawing the image
• They also are characterized by their persistence
  (time to decay of emitted light)
• High persistence cheap and good for text, bad for
  animation
• Low persistence, good for animation, but need
  high refresh rate

6
Colour Systems

• Phosphors have a colour. Colour systems have
  groups of 3 different phosphors, for red, green
  and blue. Additive Colour.
• 3 Electron guns used, for R G and B
• Each pixel consists of 3 dots of phosphor,
  arranged as triangle
• Combining different intensities of phosphors can
  generate different colours

Hitachi EDP
Standard Dot-trio
SONY Trinitron CRT
NEC Hybrid Mask
7
Shadow Masks

• Shadow maskholes are arranged so that each beam
  can only excite its own color phosphor

8
Shadow Masks (2)
9
Vector Display Devices

• A.K.A. Vector Scan Displays, Random Scan Devices,
  Line Plotters
• The electron beam directly draws the picture e.g.

DrawLine(A, B) Turn beam off, move to A. Turn
beam on, move to B.
10
Vector Graphics

• Although Vector Displays no longer as widely
  used, it is still common practice to deal with
  certain types of images in terms of vector
  graphics.
• A Vector File contains a list of entries each of
  which describes an element of a picture.
• How a picture element is described depends on
  what type of element it is. e.g. a line segment
  can be described in terms the co-ordinates of its
  two end points, its thickness, and its style
  (solid, dotted, etc.) Also curves and shapes.
• Example postscript files (PS/EPS)
• To display on a RASTER device the graphic needs
  to be rasterized

11
/ok_Ellipses on stack xCenter yCenter
xOffset yOffset width height weight howMany
color array aload pop setcmykcolor /ok_HowMany
exch def /ok_Weight exch def /ok_Height exch
def /ok_Width exch def /ok_yOffset exch
def /ok_xOffset exch def /ok_yCenter exch
def /ok_xCenter exch def /ok_Angle 360 ok_HowMany
div def /ok_xR ok_Width 2 div def /ok_yR
ok_Height 2 div def /ok_x1 ok_xR ok_xR .552292
mul sub def /ok_y1 ok_yR ok_yR .552292 mul sub
def ok_Weight setlinewidth gsave ok_xCenter
ok_yCenter translate ok_HowMany ok_Ellipse ok_An
gle rotate repeat stroke grestore def
!PS-Adobe-3.0 EPSF-3.0 BoundingBox0 0 288
288 ColorUsage Color DocumentProcessColors
Cyan Magenta Yellow Black /ok_Ellipse ok_xOffset
ok_yOffset moveto ok_x1 0 ok_xR ok_y1 ok_xR
ok_yR rcurveto 0 ok_y1 ok_x1 neg ok_yR ok_xR neg
ok_yR rcurveto ok_x1 neg 0 ok_xR neg ok_y1 neg
ok_xR neg ok_yR neg rcurveto 0 ok_y1 neg ok_x1
ok_yR neg ok_xR ok_yR neg rcurveto closepath def
12
Raster Graphics

• An image made up of many small regularly placed
  cells (pixels)
• Stored as an array of numerical values commonly
  called a pixelmap (or bitmap)

13
Raster Scan Devices

• Scans the screen from top to bottom in a regular
  pattern (common TV technology)
• A Raster is a matrix of pixels (picture elements)
  covering the screen
• The electron beam is turned on/off so the image
  is a collection of dots painted on screen one row
  (scan line) at a time.

14
Frame Buffer

• An image is stored in a special graphics memory
  area called a frame buffer (or bit map) where
  each memory location corresponds to a pixel
• A display processor scans this memory and
  controls the electron beam at each pixel
  accordingly
• For a monochrome system, each pixel is either on
  or off, so only one bit per pixel is required,
  the electron beam is either on or off
• For grayscale images, 8 bits per pixel gives 256
  different intensities of gray
• For an ideal colour display there should be 8
  bits/pixel giving a total of 24 bits per pixel
  and about 16,000,000 displayable colours. Such a
  display is called a true colour display

15
Bits Per Pixel

• 1 Bit Per Pixel
• 2 Bits Per Pixel
• or 2 binary digits
• 8 Bits Per Pixel
• or 8 digits in binary
• 24 Bits Per Pixel

• 1 or 0 in binary (on/off)
• 22 4 possible combinations 00, 01, 10, 11
• 00000000 to 11111111. 28 255 different possible
  values
• 000000000000000000000000 to 1111111111111111111111
  11. i.e. 224 or 16 million possible values

16
Memory Usage Example
Black-and-white
Greyscale
True-colour
8x8x1 64 bits
8x8x8 512 bits
8x8x24 1536 bits
17
Colour Lookup table (CLUT)

• In many colour raster systems, the display
  controller includes a colour lookup table.
• The value of a pixel in the frame buffer is not
  used to directly control the beam, but is an
  index into the LUT
• The entry in the LUT is used to directly control
  the colour of the pixel
• Eg. If we use 1 byte (8 bits) per pixel in frame
  buffer, 6 bits for each of R,G and B in LUT, then
  an application can choose 256 (28) colours out of
  262,144 (26x26x26) available colours.

8x8x8 512 bits But a large range of colours
18
Direct Color
Indirect Color with Colour Lookup Table
256 shades
DC 24 bits per pixel required 7.2M CLUT256
entries 3.6M
32 shades
DC 18 bits per pixel required 5.4 CLUT64
entries 1.8M
16 shades
DC 12 bits per pixel required 3.6M CLUT16
entries 1.2M
19
Rasterization

• Geometric and Mathematical Data structures
  typically in vertex coordinates not dependent on
  resolution
• We must convert from typical continuous
  representation to discrete

20
Rasterization

• Mostly based on Interpolation
• X and Y projected coordinates
• Red, green and blue values
• Intensity values
• Alpha values
• Z values (depth)
• Colour index values
• Surface Normals
• Texture map coordinates

21
Anti Aliasing
22
RAMDAC

• Random Access Memory Digital-to-Analog Converter,
  
• A single chip on video adapter cards.
• The RAMDAC's role is to convert digitally encoded
  images into analog signals that can be displayed
  by a monitor.
• A RAMDAC actually consists of four different
  components
• SRAM to store the color map
• three digital-to-analog converters (DACs), one
  for each of the monitor's red, green, and blue
  electron guns.

23
Raster System Architecture
24
Raster Scan Systems Conclusion

• Advantages of Raster Scan systems
• Low cost (cheap ram used for bitmaps)
• Refresh rate independent of image complexity
• Handles colour and filled areas images -gt high
  refresh
• Regular repetitive gt easier and cheaper to
  implement.
• Disadvantages
• Models must be scan converted. Often this cant
  be reused so must do this every frame.
• Aliasing
• Requires large refresh buffers even for small or
  simple images.
• Images bound to a certain resolution

25
Vector Displays

• Advantages
• High resolution and not discretized
• Less Storage Space
• Less Transfer Time (usually)
• Disadvantages
• Limited colour capability. Problems with filled
  areas and shading.
• Flicker occurs as complexity of image increases.
• Vector data needs some processing before display
• Processing required before obtaining the Vector
  representation
• Wastage in Overlapping areas

26
LCD Technology

• Liquid Crystal Display
• A transmissive technology
• Works by letting varying amounts of a
  fixed-intensity white backlight through an active
  filter
• Organic crystals that are aligned in a certain
  way in layers affect light passing through them
• When external force is applied they realign
  themselves
• This is used to change polarisation and filter
  light

27
LCD displays

• Two layers of polarizers are placed at right
  angles to each other.
• With layers of LCD between the two polarizing
  layers.
• LCD crystals are aranged in a twisted spiral
  layout.

• After light passes through first layer, the LCD
  crystals change the plane of the lights
  vibration so that it can pass through the second
  layer.

28
LCD displays

• When an electric field is passed through the LCD
  layers, they align themselves with the field and
  untwist.
• Polarised light is let through the crystals
  unhindered but becomes blocked by the second
  polarizer layer.

• The relevant pixel then becomes darkened out

29
Passive Matrix LCD

• Traditional LCD systems used an array of LCD
  nodes that could be twisted and untwisted based
  on simple grid controlled by column and row
  inputs that turned the relevant pixel on and off
• Simple but slow refresh
• And neighbouring pixels become affected to some
  degree causing fuziness

Positive voltage applied
Ground line activated
Sub-Pixel corresponding to relevant row and
column becomes disactivated.
30
TFT Displays

• Thin film transistor (TFT) displays also called
  Active Matrix LCDs are a more flexible variant of
  the LCD technology.
• Each pixel is controlled individually by using
  one or more transistors to control electric field
  for each pixel.
• Besides the polarising sheets and cells of liquid
  crystal, there is a matrix of thin-film
  transistors (TFTs) which store the electrical
  state of each pixel on the display while all the
  other pixels are being updated
• Advantage of TFTs Light Weight, Very Good Image
  Quality, wide colour range and good response time.

31
CRT Displays

• Disadvantages
• Large and heavy (typ. 70x70 cm, 15 kg)
• High power consumption (typ. 140W)
• Harmful DC and AC electric and magnetic fields
• Flickering at 50-80 Hz (no memory effect)
• Geometrical errors at edges

• Advantages
• Fast response (high resolution possible)
• Full colour (large modulation depth of E-beam)
• Saturated and natural colours
• Inexpensive, matured technology
• Wide angle, high contrast and brightness

32
LCD Displays

• Advantages
• Small footprint (approx 1/6 of CRT)
• Light weight (typ. 1/5 of CRT)
• Low power consumption (typ. 1/4 of CRT)
• Completely flat screen - no geometrical errors
• Crisp pictures - digital and uniform colors
• No electromagnetic emission
• Fully digital signal processing possible
• Large screens (gt20 inch) on desktops

• Disadvantages
• High price (presently 3x CRT)
• Poor viewing angle (typ. /- 50 degrees)
• Low contrast and luminance (typ. 1100)
• Low luminance (typ. 200 cd/m2)

33
Input DEVICES
34
Logical Input Devices

• A Classification of Input Devices to enable some
  level of abstraction so they can be supported by
  various graphics systems
• Diverse variations of input devices exist
• It is useful to classify object in terms of whatA
  it does
• This provides level of abstraction
• Enhances portability (device independent design
  of interface)
• Shields application from physical details

35
Classes of Logical Input Devices

• Locator/ Pick
• to indicate a position or orientation
  (subclasses)
• to select a displayed entity
• Valuator
• to input a single real number
• String
• To input a character string
• Returns key with specific meaning
• Letters, Numbers etc.
• Choice
• To select from a set of possible actions or
  choices
• Often return sensory feedback e.g. lights, clicks

36
Physical Input Devices (1)

• Keyboard string/choice input
• Gamepad choice
• Mouse pick/locator device with relative
  positioning and indirect input.
• Tablet pick/locator device with absolute
  positioning and indirect input.
• Joystick/Trackball locator/valuator
• Knobs (e.g. Volume control) valuator devices

37
Drivers

• Nowadays days we have a lot more diverse input
  hardware, some of which dont fit well into the
  Logical Input Device Classes
• e.g. bat, blow-suck tube, brain-computer
  interface, chording, dataglove, electronic ink,
  eye-tracking, foot pedal, gesture, joystick,
  light pen, motional input device, mouthstick,
  OCR, paddle, pointing device, puck, stroke
  recognition, tongue-activated joystick, touch
  interactive display, touch tablet, touchpad,
  trackball
• Logical Input Devices were invented for the GKS
  standard graphics system. These days dedicated
  drivers for all pieces of hardware can be written
  that talk more directly to your Operating system.

38
Physical Input Devices
39
Data Gloves
40
3D Model Acquisition

• 3D laser scanner
• Digitizer
• Video based modelling

41
Motion Capture
VICON Optical Motion Capture System
42
Force Feedback Devices

• Combine input and some degree of output
• Useful for navigating simulated virtual
  environments
• Range of feedback types
• Tactile feedback
• Haptic Feedback
• Force Feedback

43
Eye Trackers
EYELINK head and Eye-tracking System
44
CAVE Cave Automatic Virtual Environment
HMD (Head Mounted Display)
HITLabs/Microvision Googgles