Lcd Full Notes and Crt

8/14/2019 Lcd Full Notes and Crt

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Larry F. Hodges (modified by H.Quynh Dinh 11/02)1

Raster Graphics

Hardware


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EXAMPLE RASTER

GRAPHICSARCHITECTURE

Display

processor

memory

Frame

buffer

Video

controllerMonitor

System

memory

CPUPeripheral

devices

System bus

Displayprocessor

Fig. 4.22 Raster system architecture with a display processor.

(Used by permission from Computer Graphics: Principles and Practice.)


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BASIC

DEFINITIONSRASTER: 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 devicesdisplay an image bysequentially drawingout the pixels of thescan lines that formthe raster.


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Pixels

Pixel - The most basic addressable image element in a

screen

CRT - Color triad (RGB phosphor dots)

LCD - Single color element

Screen Resolution - measure of number of pixels on a

screen (m by n)

m - Horizontal screen resolution

n - Vertical screen resolution


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Color

There are no commercially available small

pixel technologies that can individually

change color. Color is encoded by placing different-

colored pixels adjacent to each other.

Field sequential color uses red, blue andgreen liquid crystal shutters to change color

in front of a monochrome screen.


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Larry F. Hodges (modified by H.Quynh Dinh 11/02)

6

Raster Displays

Cathode Ray Tubes (CRTs), most tube monitors

you see. Very common, but big and bulky.

Liquid Crystal Displays (LCDs)- there are two types:

1) transmissive (Shine light through the image-forming

element, e.g. laptops, those snazzy new flat panel

monitors)

2) reflective (Bounce light off the image-forming element

e.g. wrist watches).


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CRT Monitor


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CRT Monitor

Electron Guns

Red Input

Green

Input

Blue Input

Deflection

Yoke

Shadow Mask

Red, Blue,

and Green

Phosphor Dots

CRT


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CRT Phosphor Screen

The screen is coated with

phosphor, 3 colors for a color

monitor, 1 for monochrome.

For a color monitor, three guns

light up red, green, or blue

phosphors.

Intensity is controlled by the

amount of time at a specific

phosphor location.


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Color CRT

G R B G

B G R B

G R B G

FLUORESCENCE- Light emitted while the phosphor is being struck by electrons.

PHOSPHORESCENCE- Light given off once the electron beam is removed.

PERSISTENCE- Is the time from the removal of excitation to the moment whenphosphorescence has decayed to 10% of the initial light output.

Red, Green and Blue electron guns.

Screen coated with phosphor triads.

Each triad is composed of a red, blue

and green phosphor dot.

Typically 2.3 to 2.5 triads per pixel.


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Scanning An

ImageFrame: The image to be scanned out on the CRT.Some minimum number of frames must be displayed each second to eliminateflicker in the image.

CRITICAL FUSION FREQUENCY

Typically 60 times per second forraster displays.

Varies with intensity, individuals,phosphor persistence, room lighting.


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13

Scannin

g

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 Time needed to get from the end of thecurrent scan line to the start of the next scan line.


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Interlaced Scanning

1 /30 SE C

1/60 SEC

F IE L D 1 F IE L D 2

F R A M E

1/60 SEC

1/30 SE C

1 /60 SE C

F IE L D 1 F IEL D 2

F R A M E

1/60 S EC

Scan frame 30 times per second

To reduce flicker, divide frame intotwo fieldsone consisting of theeven scan lines and the other of the

odd scan lines.Even and odd fields are scanned out

alternately to produce an interlacedimage.

Image from

http://www.anchorbaytech.com/_media/images/s

upport/interlaced-scan.jpg


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Video

Formats NTSC - 525x480, 30f/s, interlaced PAL - 625x480, 25f/s, interlaced VGA - 640x480, 60f/s, noninterlaced

SVGA 800x600, 60f/s noninterlaced RGB - 3 independent video signals and

synchronization signal, vary in resolution and refreshrate

Time-multiplexed color- R,G,B one after another on asingle signal, vary in resolution and refresh rate


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Liquid Crystal Displays (LCDs)

Also divided into pixels, but without an electron

gun firing at a screen, LCDs have cells that either

allow light to flow through, or block it.


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Liquid crystal displays use small flat chips which

change their transparency properties when a

voltage is applied. LCD elements are arranged in an n x m array call

the LCD matrix

Level of voltage controls gray levels.

LCDs elements do not emit light, use backlights

behind the LCD matrix


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Color is obtained by placing filters in front of each

LCD element

Usually black space between pixels to separate thefilters.

Because of the physical nature of the LCD matrix, it is

difficult to make the individual LCD pixels very small.

Image quality dependent on viewing angle.


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LCDs(cont.)

LCD resolution is often quoted as number of

color elements not number of RGB triads.

R G B R BG R G

B R G B R G B R

R G B R BG R G

Example: 320 horizontal by 240 vertical elements = 76,800elements

Equivalent to 76,800/3 = 25,500 RGB pixels

"Pixel Resolution" is 185 by 139 (320/1.73, 240/1.73)


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LCDs(cont.)

Passive LCD screens Cycle through each

element of the LCD

matrix applying the

voltage required for thatelement.

Once aligned with the

electric field the

molecules in the LCD

will hold their

alignment for a short

time

Active LCD screens Each element contains

a small transistor that

maintains the voltage

until the next refreshcycle.

Higher contrast and

much faster response

than passive LCD


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Advantages

of LCDs Flat

Lightweight

Low power consumption


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CRTs (cont.)

Strong electrical fields and high voltage

Very good resolution

Heavy, not flat


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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 onepixel.

Bit Planes orBit 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. MonochromeDisplay

(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 ColorDisplay24 bitplanes, 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.

Fig. 4.LUT Video look-up table organization. A pixel with value 67(binary 01000011) is displayed on the screen with the red electrongun at 9/15 of maximum, green at 10/15, and blue at 1/15. This look-uptable is shown with 12 bits per entry. Up to 24 bits per entry arecommon.

01000

011

67

100110100001

0

67

255

10011010 0001

R G B

RED

GREEN

BLUE

Pixel displayed

at x', y'

Pixel in

bit map

at x', y'

0 x

0

y

xmax

maxy

Bit map Look-up table Display


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Pseudo Color: 28 x 24 Color MapLUT

0

1

2

3

254

255

RED GREEN BLUE

256 colors chosen from a

palette of 16,777,216.

Each entry in the color map

LUT can be user defined.

Could be used to define 256 shades of

green or 64 shades each of red, blue,green and white, etc.


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Examples of Pseudo Color Application

Image fromwww.miram

etrics.com/br

ief_contour.h

tm

Image from

www.catenary.com/howto/

pseudo.html

Image from

hinode.nao.ac.jp

/news_e/200611

27_press_e


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Display Processor

Also called either a Graphics Controller or Display CoProcessor or

Graphics Accelerator or Video Card

Specialized hardware to assist in scan converting output primitives intothe frame buffer.

Fundamental difference among display systems is how much the displayprocessor 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 thememory are used to 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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Projection

Displays Use bright CRT or LCD

screens to generate an image

which is sent through an

optical system to focus on a(usually) large screen.

Full color obtained by

having separate

monochromatic projector foreach of the R,G,& B color

channels


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Basic Projector Designs(Images from Phillips Research)

Reflective Projection System Transmittive ProjectionSystem


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Advantages/D

isadvantagesof ProjectionDisplay Very large screens can provide large field of view and can

be seen by several people simultaneously.

Image quality can be fuzzy and somewhat dimmer thanconventional displays.

Sensitive to ambient light.

Delicate optical alignment.

Less eye strain Very immersive

Very expensive


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Displays in Virtual Reality

Head-Mounted Displays (HMDs)

The display and a position tracker are attached

to the users head

Head-Tracked Displays (HTDs)

Display is stationary, tracker tracks the usershead relative to the display.

Example: CAVE, Workbench, Stereo monitor


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Image Quality Issues

Screen resolution

Color

Blank space betweenthe pixels

Intentional image

degradation

Brightness

Contrast

Refresh rate Sensitivity of display

to viewing angle


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Input

Devices Locator Devices: to indicate a position and/or orientatione.g. Tablet, Mouse, Trackball, Joystick, Touch Panel, Light Pen

Keyboard devices: to input a character string

e.g. Alphanumeric keyboard

Scanner

Image Scanners, e.g. Flatbed, etc

What type of data is returned? Bitmap

Laser Scanners, e.g. Deltasphere

Emits a laser and does time of flight. Returns 3D point

Documents

Lcd Full Notes and Crt