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310482: Graphics Programming

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Title: 310482: Graphics Programming


1
310482 Graphics Programming
  • Seree Chinodom
  • seree_at_buu.ac.th
  • http//www.compsci.buu.ac.th/seree/lecture/310482

2
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

3
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

4
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

5
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

6
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.

7
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

8
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...

9
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
10
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

11
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
12
Display Technology Color CRTs
  • Color CRTs have
  • Three electron guns
  • A metal shadow mask to differentiate the beams

13
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)

14
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 90ยบ

15
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

16
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

17
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

18
Display Technology DMDs
  • Digital Micromirror Devices (projectors)
  • Microelectromechanical (MEM) devices, fabricated
    with VLSI techniques

19
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

20
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

21
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

22
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

23
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.
24
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

25
1-Bit Memory, Monochrome Display (Bitmap Display)
26
3-Bit Color Display
27
True Color Display
  • 24 bitplanes, 8 bits per color gun.
  • 224 16,777,216

8
8
Red
8
Blue
28
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
67
B
R
G
Pixel in
bit map
0
at x', y'
0
x
x
0
max
Frame buffer
Look-up table
Display
29
Pseudo color
28 x 24 Color Map LUT
30
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
31
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

32
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?

33
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)

34
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

35
Our Example
36
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

37
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

38
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
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