Introduction to Raster scan display

C A E D C Computer Aided Engineering Design Centre

EXAMPLE RASTER GRAPHICS ARCHITECTURE

Display processor memory

Frame buffer

Video controller

Monitor

System memory

CPUPeripheral

devices

System bus

Display processor

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.

Raster

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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2

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4

5

6

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

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 framesLeverages low-cost CRT technology (i.e., TVs)Bright! Display emits light

Cons:Requires screen-size memory arrayDiscreet 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 Display24 bit planes, 8 bits per color gun. 224 = 16,777,216

Green

Red

Blue

N

N

N

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

Electron Guns

Red Input

GreenInput

Blue Input

Deflection Yoke

Shadow Mask

Red, Blue, and Green

Phosphor Dots

CRT

Color CRT (Shadow Mask)shadow mask

screen

electron gun

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

Strong electrical fields and high voltage

Very good resolution Heavy, not flat

Sometimes the convergence point is behind the screen. The picture appears to be blurred.

The picture appears to be blurred. The Beam in focus at the center of the screen.

Dynamic focusing

Difficulties with the CRT

Liquid Crystal Displays (LCDs)

Small fluorescent tubes

Diffuser

Linear Polarizer

LCD Module Color

Filter

Linear Polarizer

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.

Usually black space between pixels to separate the filters.

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.

LCDs

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 = 76,800 elements

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

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

Passive LCD screensCycle 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

LCDs (cont.)

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

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