Transcript Introduction to Raster scan display

Introduction to Raster scan display
CAEDC
Computer Aided Engineering Design Centre
EXAMPLE RASTER GRAPHICS
ARCHITECTURE
Peripheral
devices
CPU
Sys tem bus
Dis play
proces s or
Dis play
proces s or
m em ory
Fram e
buffer
Sys tem
m em ory
Video
controller
Monitor
Raster Scan Displays (1)
Raster
 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.
Raster
 derived from TV systems for a row of pixels
 commonly referred to as a scan line
 does influence algorithms – reducing memory requirements,
parallelism, etc.
 is the derivation of rasterization, scan-line algorithms
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Raster Scan
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
Varies with intensity, individuals, phospher persistence,
lighting...
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.
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 Sec
1/60 Sec
1/60 Sec
1/60 Sec
1/60 Sec
Field 1
Field 2
Field 2
Field 1
Frame
Frame
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
non-interlaced
interlaced, cycle 1
interlaced, cycle 2
interlaced, 2 cycles
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
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.
Frame Buffer

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
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
True Color Display
24 bit planes, 8 bits per color gun.
224 = 16,777,216
N
N
N
Red
Green
Blue
Raster Displays

Cathode Ray Tubes (CRTs), most “tube” monitors you see.
Very common, but big and bulky: A cathode ray tube (CRT) is
a specialized vacuum tube in which images are produced when
an electron beam strikes a phosphorescent surface. Most
desktop computer displays make use of CRTs. The CRT in a
computer display is similar to the "picture tube" in a television
receiver.

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).
CRT Monitor
CRT Monitor
CRT
Shadow Mask
Electron Guns
Red Input
Green
Input
Blue Input
Deflection
Yoke
Red, Blue,
and Green
Phosphor Dots
Color CRT (Shadow Mask)
shadow mask
electron gun
screen
phosphor dot pattern
Different phosphor for each color !!!
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.
CRTs
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Strong electrical fields and
high voltage
Very good resolution
Heavy, not flat
Difficulties with the CRT
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Sometimes the convergence point is behind the screen.
 The picture appears to be blurred.
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The picture appears to be blurred.
 The Beam in focus at the center of the screen.
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Dynamic focusing
Liquid Crystal Displays (LCDs)
Small
Diffuser
LCD
Linear
fluorescent
Linear Module Color Polarizer
tubes
Polarizer
Filter
Wavefront
distortion
filter
Liquid Crystal Displays (LCDs)
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
Liquid Crystal Displays (LCDs)

Color is obtained by placing filters in front of each LCD
element.
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Usually black space between pixels to separate the filters.
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Because of the physical nature of the LCD matrix, it is
difficult to make the individual LCD pixels very small.
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Image quality dependent on viewing angle.
LCDs
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LCD resolution is often quoted as number of color elements
not number of RGB triads.
R
G
B
R
B
R
G
R
G
B
G
B
R
B
R
G
R
G
B
G
B
R
R
G
Example: 320 horizontal by 240 vertical elements = 76,800
elements
Equivalent to 76,800/3 = 25,500 RGB pixels
"Pixel Resolution" is 185 by 139 (320/1.73, 240/1.73)
LCDs (cont.)
 Passive LCD screens
 Cycle through each
element of the LCD
matrix applying the
voltage required for that
element.
 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 refresh
cycle.
 Higher contrast and
much faster response
than passive LCD
Advantages of LCDs
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Flat
Lightweight
Low power consumption
LCD vs. CRT
 Three times brighter
 Five times more contrast.
 TFT technology more efficient.
 Uses less electricity.
 TFT technology more efficient.
 Uses less electricity.
 Takes less space.
 Emits less radiation.
 Distortion free viewing.
 No flickering.
 Narrow viewing angle.
 Resolution