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Hardware Devices

Display Hardware

• Video display devices

• Hard-copy devices

Input devices

• Locator Devices

• Keyboard devices

• Valuator Devices

• Choice 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

Electron Guns

Red Input

GreenInput

Blue Input

Deflection Yoke

Shadow Mask

Red, Blue, and Green

Phosphor Dots

CRT

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CRT properties

1. Persistence

2. Resolution3. 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 to 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 picture.– Graphics monitors are usually constructed with 10 to

60 microseconds.

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Resolution

• 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.

Intensity distribution

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Addressability

• Addressability is a measure of the spacing between the centers of vertical and horizontal lines.

• The picture on a screen consists of intensified points.

• The smallest addressable point on the screen is called pixel or picture element

• In graphics mode there are 800x600

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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: vertical line with 3 points is equal in length to horizontal line of 4 points.

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Display Technologies: CRTs

1. Vector Displays

2. Raster Scan Displays

3. Liquid Crystal Displays (LCDs)

4. Plasma Panel

5. Organic LED Arrays

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Vector Displays

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Vector Displays or random scan display– 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

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Vector Displays

– Advantages: • Generates higher resolution than other

systems (Raster)• Produces smooth line drawings

– Disadvantage: Is not used for realistic shaded scenes

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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 اندماج frequency

• Typically 60 times/sec• Varies with intensity, individuals, phospher

persistence, lighting...

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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 (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 Sec1/60 Sec 1/60 Sec 1/60 Sec 1/60 SecField 1 Field 1Field 2 Field 2

Frame Frame

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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

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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– Discreet 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)

Electron Gun

1 bit 2 levels

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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 Display24 bit planes, 8 bits per color gun. 224 = 16,777,216

Green

Red

Blue

N

N

N

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

01000

011

67

100110100001

0

67

255

10011010 0001

R G B

RED

GREEN

BLUE

Pixel displayedat x', y'

Pixel inbit mapat x', y'

0 x0

y

xmax

maxy

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.

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Beam penetration• 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 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 & orange)

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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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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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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

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Display Technology: DMD / DLPDigital 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 ArraysOLED 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 ControllerCycles 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

Y address

Pixel value(s)

Raster scan generator

Data

Horizontal and vertical deflection signals

Intensity or color

Linear address

Set or increment

Set or decrement

M e m o r y

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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