lcd full notes and crt
TRANSCRIPT
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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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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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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 (LCDs)
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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Liquid Crystal Displays (LCDs)
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