Download Introduction to Computer Graphics

Video Display Devices
Display Hardware
• Video display devices
Input devices
• Locator Devices
• Keyboard devices
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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 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)
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CRT Monitor
CRT
Shadow Mask
Electron Guns
Red Input
Green
Input
Blue Input
Deflection
Yoke
Red, Blue,
and Green
Phosphor Dots
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List the properties of CRT
1. Persistence
2. Resolution
3. Addressability
4. Aspect ratio
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Persistence
How long small spots continue to emit light after the
beam is moved. How long it takes for the emitted light
from the screen to decay to one-tenth of its original
intensity.
– Lower persistence requires high refresh rate & it is
good for animation.
– High persistence is useful for displaying highly
complex static pictures.
– Graphics monitors are usually constructed with 10 to
60 microseconds.
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Resolution
Intensity distribution
• The maximum number of points that can be displayed
without an overlap on a CRT is referred to as the resolution.
• 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.
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Addressability
• Addressability is the number of individual dots
per inch (d.p.i.) that can be created. If the
address of the current dot is (x, y) then the next
dot will be (x + y), (x + y + 1)
• The picture on a screen consists of intensified
points.
• The smallest addressable point on the screen
is called pixel or picture element
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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
that a vertical line plotted with 3 points is equal in length
to a horizontal line plotted with 4 points.
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Display Technologies: CRTs
1.
2.
3.
4.
5.
Raster Scan Displays
Vector Displays
Liquid Crystal Displays (LCDs)
Plasma Panel
Organic LED Arrays
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Raster Scan Displays
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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
system
– Scan line: A row of pixels
– Picture elements are stored in a memory called frame
buffer
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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 frequency
• Typically 60 times/sec
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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.
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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
Cons:
– Requires screen-size memory array
– Discrete sampling (pixels)
– Practical limit on size
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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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Monochrome Display
(Bit-map Display)
1 bit
2 levels
Electron
Gun
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3-Bit Color Display
3
red
green
blue
COLOR: black red green blue yellow cyan magenta white
R
G
B
0
0
0
1
0
0
0
1
0
0
0
1
1
1
0
0
1
1
1
0
1
1
1
1
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True Color Display
24 bit planes, 8 bits per color gun.
224 = 16,777,216
N
N
Red
N
Green
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.
y
RED
max
GREEN
255
y
0
0
1
0
0
1
0
BLUE
1
67
1001 1010 0001
67 100110100001
R
Pixel in
bit map
at x', y'
0
0
x
Bit map
G
Pixel displayed
at x', y'
B
0
x
max
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.
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2 Basic Techniques for Color Display
Beam-Penetration
Method
Shadow-Mask
Method
Beam-Penetration Method
• Used with random scan monitors
• The screen has two layers of phosphor: usually red
and green
• The displayed color depends on how far the electron
beam penetrates 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 &
orange)
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Shadow-Mask Method
Color CRTs have
– Three electron guns
– A metal shadow mask to differentiate the beams
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• The Shadow mask in the previous image is known as the
delta-delta shadow-mask.
• The 3 electron beams are deflected and focused as a
group onto the shadow mask, which contains a series of
holes aligned with the phosphor-dot patterns.
• The 3 beams pass through a hole in the shadow mask
and activate a dot triangle, which appears as a small
color spot on the screen.
• A second arrangement of the 3 electron guns is in-line
Where the corresponding red-green-blue color dots on the
screen are aligned along one scan – line instead of a
triangular pattern.
Controlling Colors in shadow mask
• Different colors can be obtained by varying the intensity levels of the
three electron beams.
• Example: Simply turning off the red and green guns, we get only the
color coming from the blue phosphor.
• Yellow = Green + Red
• Magenta = Blue + Red
• Cyan = Blue + Green
• White is produced when all the 3 guns possess equal amount of
intensity.
Application:
• Color CRT’s in graphics systems are designed as RGB
monitors, which employ the shadow mask technique.
• The color CRT takes the intensity level for each electron
gun ( red, green and blue) directly from the computer
system without any intermediate processing.
• An RGB color system with 24 bits of storage per pixel is
generally referred to as full-color system or a truecolor system. It allows 256 voltage settings for each
electron gun and nearly 17 million colors.
Flat-Panel Displays
• Although CRT monitors are still in use and form 70% of
the users, still other technologies are emerging in the
market to replace the classic CRT model.
• So What does Flat-Panel mean?
It refers to a class of video devices
that have a:reduced volume,
reduced weight and
reduced power requirements
compared to a standard CRT
In existence since
1964’s,Flat panel displays
are much thinner and
flatter than traditional TVs
Very Light in weight
• Thinner than CRTs and can be hung on walls.
• Also available as pocket notepads on which notes can
be scribbled and can be carried around easily.
• Current usage:
– Small and big TV monitors
– Calculators
– Pocket video games
– Laptop computers
– Advertisement boards in elevators and showrooms.
– Portable monitors.
Emissive and Non-Emissive
• Emissive displays (also called emitters) are devices that convert
electrical energy into light.
• Example:
– Plasma panels,
– thin–film electroluminescent displays,
– LEDs (Light emitting diodes)
Flat CRTs have also been tried to be devised where the electron
beams are accelerated parallel to the screen and then deflected
900.
Sadly the above was not successful at all.
Non – Emissive Flat-Panel Displays
• Non-emissive displays or non-emitters use optical effects
to convert sunlight or light from some other source into
graphics patterns.
• Example:
LCD (Liquid – crystal device)
Plasma Panels
• It is an example of an emissive
flat panel display.
• Also called as a gas-discharge display.
• Can be wall mounted.
• Working:
• A plasma display is comprised of two parallel sheets of
glass, which enclose a gas mixture usually composed of
neon and xenon (some manufacturers also use helium in
the mix) that is contained in millions of tiny cells
sandwiched in between the glass.
Working of Plasma Panel
• Electricity, sent through an array of electrodes that are in
close proximity to the cells, excites the gas, resulting in a
discharge of ultraviolet light.
•
The light then strikes a phosphor coating on the inside
of the glass, which causes the emission of red, blue or
green visible light. (Each cell, or pixel, actually consists
of one red, one blue and one green sub-pixel). The three
colors in each pixel combine according to the amount of
electric pulses fed to each sub-pixel, (which varies
according to the signals sent to the electrodes by the
plasma display’s internal electronics), to create visible
images.
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Working of Plasma Panel (pic)
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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
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Plasma Panel (2)
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 to
some extent in order to make up for the less
illumination.
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Backlight
• A backlight is a form of illumination used in liquid crystal displays
(LCDs).
• As LCDs do not produce light themselves (unlike for example
Cathode ray tube (CRT) displays), they need illumination (ambient
light or a special light source) to produce a visible image.
• Backlights illuminate the LCD from the side or back of the display
panel, unlike front lights, which are placed in front of the LCD.
• Backlights are used in small displays to increase readability in low
light conditions, and in computer displays and LCD televisions to
produce light in a manner similar to that of a CRT display.
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Liquid & A crystal?????
• How many states exist in science?
• What are these things called liquid crystals?
• The name "liquid crystal" sounds like a contradiction. We
think of a crystal as a solid material like quartz, usually
as hard as rock, and a liquid is obviously different. How
could any material combine the two?
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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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Working of LCD
• An LCD works entirely differently compared to Plasma
Panel.
• An active matrix LCD’s light source is generated by small
fluorescent bulbs. The white light from these bulbs is
diffused to create a uniform light source by shining it
through a polarizer located in the back of the display,
which allows light to go through in only one direction.
• Individual LCD cells in the panel are then turned “on”
and “off” by applying a small electric charge to the thin
film transistors (TFT), located in each sub-pixel.
• This charge causes the liquid crystals to twist, allowing
white light to be passed through red, green and blue
color filters and a front polarizer in front of the LCD cells.
The image is formed according to which crystals twist to
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let light through or block it.
Working of LCD (pic)
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Display Technology: DMD / DLP
Digital Micromirror Devices (projectors) or Digital Light
Processing
Microelectromechanical (MEM) devices, fabricated with
VLSI techniques
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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
vacuum.
• Dope emissive layers with fluorescent
molecules to create color.
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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.
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Display Processor
• Also called either a Graphics Controller or Display
CoProcessor
• 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.
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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
M
e
m
o
r
y
Linear
address
Raster scan
generator
Y address
Data
Set or increment
Pixel
value(s)
Set or decrement
Horizontal
and vertical
deflection
signals
Intensity
or color
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Hard-copy devices
ο Ink-jet printer
ο Laser printer
ο Film recorder
ο Electrostatic printer
ο Pen plotter
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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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