310482: Graphics Programming

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310482 Graphics Programming

• Seree Chinodom
• seree_at_buu.ac.th
• http//www.compsci.buu.ac.th/seree/lecture/310482

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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

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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 phospher dots
• When the beam hits a phospher 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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Display Technologies CRTs

• Vector Displays
• Early computer displays basically an
  oscilloscope
• Control X,Y with vertical/horizontal plate
  voltage
• Often used intensity as Z
• Name two disadvantages
• Just does wireframe
• Complex scenes ? visible flicker

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Display Technologies CRTs

• Raster Displays
• Raster A rectangular array of points or dots
• Pixel One dot or picture element of the raster
• Scan line A row of pixels

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Display Technologies CRTs

• Raster Displays
• Black and white television an oscilloscope with
  a fixed scan pattern left to right, top to
  bottom
• To paint the screen, computer needs to
  synchronize with the scanning pattern of raster
• Solution special memory to buffer image with
  scan-out synchronous to the raster. We call this
  the framebuffer.

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Display Technologies CRTs

• Phosphers
• Flourescence Light emitted while the phospher is
  being struck by electrons
• Phospherescence Light emitted once the electron
  beam is removed
• Persistence The time from the removal of the
  excitation to the moment when phospherescence has
  decayed to 10 of the initial light output

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Display Technologies CRTs

• Raster Displays
• 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 frequency
• Typically 60 times/sec
• Varies with intensity, individuals, phospher
  persistence, lighting...

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Display Technologies CRTs

• Raster Displays
• 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 2
Field 1
Frame
Frame
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Display Technologies CRTs

• Raster Displays
• 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

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Display Technology Color CRTs

• Color CRTs are much more complicated
• Requires manufacturing very precise geometry
• Uses a pattern of color phosphors on the screen
• Why red, green, and blue phosphors?

Delta electron gun arrangement
In-line electron gun arrangement
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Display Technology Color CRTs

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

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Display Technology Raster

• Raster CRT pros
• Allows solids, not just wireframes
• Leverages low-cost CRT technology (i.e., TVs)
• Bright! Display emits light
• Cons
• Requires screen-size memory array
• Discreet sampling (pixels)
• Practical limit on size (call it 40 inches)
• Bulky
• Finicky (convergence, warp, etc)

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Display Technology LCDs

• 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

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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

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Display Technology Plasma

• 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

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Display Technology

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

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Display Technology DMDs

• Digital Micromirror Devices (projectors)
• Microelectromechanical (MEM) devices, fabricated
  with VLSI techniques

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Display Technology DMDs

• DMDs are truly digital pixels
• Vary grey levels by modulating pulse length
• Color multiple chips, or color-wheel
• Great resolution
• Very bright
• Flicker problems

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Display Technologies Organic LED Arrays

• Organic Light-Emitting Diode (OLED) Arrays
• The display of the future? Many think so.
• OLEDs function like regular semiconductor LEDs
• But with thin-film polymer construction
• Thin-film deposition or vacuum deposition
  processnot grown like a crystal, no
  high-temperature doping
• Thus, easier to create large-area OLED sheet

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Display Technologies Organic LED Arrays

• OLED pros
• Transparent
• Flexible
• Light-emitting, and quite bright (daylight
  visible)
• Large viewing angle
• Fast (lt 1 microsecond off-on-off)
• Can be made large or small
• OLED cons
• Not quite there yet (96x64 displays)
• Not very robust, display lifetime a key issue

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Raster Scan Displays

• Beam of electrons deflected onto a phosphor
  coated screen
• Phosphors emit light when excited by the
  electrons
• Phosphor brightness decays -- need to refresh
  the display
• Phosphors make up screen elements called pixels

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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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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 (Bitmap Display)
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3-Bit Color Display
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True Color Display

• 24 bitplanes, 8 bits per color gun.
• 224 16,777,216

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8
Red
8
Blue
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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.

RED
y
max
GREEN
255
BLUE
y
Pixel displayed
7
1001
1010
0001
at x', y'
6
100110100001
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B
R
G
Pixel in
bit map
0
at x', y'
0
x
x
0
max
Frame buffer
Look-up table
Display
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Pseudo color
28 x 24 Color Map LUT
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Drawing

• Most computer generated images made up of
  geometric primitives (polygons, lines, points)
• Application draws them by setting bits in the
• framebuffer
• Most graphics applications involve scene
  database management

Graphic Application
Device Driver
Scene Database
Framebuffer
Display
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A DDA Line Drawing Function

• Line(int x1, int y1, int x2, int y2)
• int dx x1 - x2, dy y2 -y1
• int n max(abs(dx),abs(dy))
• float dt n, dxdt dx/dt, dydt dy/dt
• float x x1, y y1
• while (n--)
• DrawPoint( round(x), round(y) )
• x dxdt
• y dydt

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We Can Do Better Than That...

• Get rid of all those nasty floating point
  operations
• The idea find the next pixel from the current
  one
• So which of the green pixels is next?

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The Key

• Were only ever going to go right one pixel, or
  up and right one pixel (if the slope of the line
  is between 0 and 1). Call these two choices E
  and NE
• Lets think of pixels as lattice points on a
  grid
• Given an X coordinate, we only need to choose
  between y and y1 (y is the Y coordinate of the
  last pixel)

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The Midpoint Test

• Look at the vertical grid line that our line
  intersects
• On which side of the midpoint (y1/2) does the
  intersection lie?
• If its above the midpoint, go NE, otherwise go E

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Our Example
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Implicit Functions

• Normally, a line is defined as y mx b
• Instead, define ,F(x,y) ax by c and let the
  line be everywhere where F(x,y) 0
• Now, if F(x,y) gt 0 , were above the line, and
  if F(x,y) lt 0 , were below the line

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Who Cares?

• We can evaluate the implicit line function at the
• midpoint to figure out where to draw next!
• In fact, we can use the last function evaluation
  to find the next one cheaply!
• For ANY x, y
• F(x 1,y) - F(x,y) a
• F(x 1,y 1) - F(x,y) a - b

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Midpoint Algorithm

• Line(int x1, int y1, int x2, int y2)
• int dx x2 - x1, dy y2 -y1
• int e 2dy - dx
• int incrE 2dy, incrNE 2(dy-dx)
• int x x1, y y1
• DrawPoint( x, y )
• while (x lt x2)
• x
• if ( e lt 0 ) e incrE
• else y e incrNE
• DrawPoint( x, y )

• e holds the implicit function evaluation at
  each x value (actually, its multiplied by 2,
  but all we care about is the sign).
• Easy extension for lines with arbitrary slopes