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Display Devices An Introduction

Cathode-Ray Tubes

This is normally magnetic through electromagnetic field interaction with electron stream. Heated to produce electrons as a cloud Impart enough energy to produce light when they strike phosphor

Reduces number of electrons leaving the Cathode Brightness control Produce a dot rather than a smear, like lens in a camera. Inner surface of CRT is coated with a thin film of a substance which, when bombarded 08 Oct, 2008 Devices with electrons, emits visible light for a certain period afterDisplay the bombardment has ceased2

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Persistence & Refresh Rate y y

y

Light emitted by phosphor fades very rapidly To maintain screen picture, the picture is refreshed: redrawn repeatedly by directing electron beam to the same screen points Refresh Rate ◦ ◦ ◦ ◦

y

How often the picture is redrawn on the monitor? If the rate is low, the picture will appear to flicker 60 times per second is tolerable at low resolutions for most people 75 times per second or more is better and is necessary for high resolutions

Persistence: ◦ The time for the emitted light to decay to 10% of the initial intensity ◦ High persistence phosphors allow for a lower refresh rate to avoid flicker ◦ Low persistence phosphors are good for animation but require a high refresh rate to prevent flicker 08 Oct, 2008

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Raster-Scan Displays y y

A Raster Scan device scans the screen from top to bottom in a regular pattern. Image is a collection of dots which are painted on screen one row (scan line or raster line) at a time

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Aspect Ratio y y

Ratio of horizontal pixels to vertical pixels for an equal length line Aspect ratio of 8/6 implies ◦ Horizontal line with 8 points ◦ Vertical line with 6 points

y

For a resolution of 640 X 480 pixels ◦ horizontal --> 640/8 = 80 pixels/inch ◦ vertical --> 480/6 = 80 pixels/inch

y

For a resolution of 320 X 200 pixels: ◦ horizontal --> 320/8 = 40 pixels/inch ◦ vertical --> 200/6 = 33 1/3 pixels/inch

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Color Range y

Color Range depends upon: ◦ Type of phosphor ◦ No. of bits per pixel in frame buffer

y

Black & White System ◦ 1 bit per pixel ◦ 0: Off 1: On

y

High quality System ◦ 24 bits bi per pixel i l

Bitmap: Frame buffer with 1 bit/pixel y Pixmap: Frame buffer with multiple bits/pixel y

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Random-Scan Displays y y y y y

The electron beam directly draws the picture Pictures are generated as line drawings Picture definitions are stored as a set of line drawing commands in an area of memory known as display list Also termed as vector or stroke-writing or calligraphic displays Refresh rate depends upon no. of lines to be displayed

b a

Turn e beam off, move to a Turn e beam on, and draw to b Repeat move, draw sequence 08 Oct, 2008

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Random-Scan Displays y

Advantages: ◦ Very high resolution, limited only by monitor ◦ Easy animation, just draw at different positions ◦ Requires little memory (just enough to hold the display program)

y

Disadvantages: ◦ Requires intelligent electron beam, i.e. processor controlled ◦ Can't draw a complex image ◦ Limited color capability (very expensive) 08 Oct, 2008

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Color CRT Monitors y y

Use different phosphors, usually Red, Green, Blue then combine B Beam Penetration P (R,G): (R G) ◦ Coat the screen with layers of different colored phosphors. ◦ Emitted color depends on how far the electron beam penetrates into phosphors layers ◦ Beam Acceleration Voltage x Low speed e-beam: Red x Very high speed e-beam: Green x Intermediate e-beam:Yellow or Orange

◦ Inexpensive ◦ But Limited Colors ◦ Picture quality is not good as well

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Color CRT Monitors y

Shadow-mask ◦ Three electron guns: red, red green green, & blue ◦ 3 phosphor color dots at each pixel position. ◦ Wider color range

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Color CRT Monitors Phosphor dots arranged in triangular triad pattern Activate corresponding dot triangle

Guns are deflected together and converge on the same point RGB guns arranged in triangular pattern

One small hole for each triad

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Color CRT Monitors R

G

B

Color

0

0

0

Black

0

0

1

Blue

0

1

0

Green

0

1

1

Cyan

1

0

0

Red

1

0

1

Magenta

1

1

0

Yellow

1

1

1

White

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Flat-Panel Displays y y

Video devices with reduced volume, weight and power requirements E Emissive D l Displays ◦ Convert electrical energy into light ◦ Plasma Panels x Region between two glass plates is filled with mixture of gases

◦ Thin film electroluminescent x Region between two glass plates is filled with phosphor

◦ Light emitting diodes x Use a matrix of diodes to form pixel positions

y

Non-emissive Displays ◦ Use optical effects to convert light into graphics patterns ◦ LCD 08 Oct, 2008

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LCD Screen LCDs are organic molecules that, in the absence of external forces forces, tend to align themselves in crystalline structures. y But, when an external force is applied they will rearrange themselves as if they were a liquid. Some liquid crystals respond to heat others respond to electromagnetic forces y

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LCD Screen Made of two layers of a polarizing material with a liquid crystal solution in between y An electrical signal makes the crystals line up in a way that keeps light from going through entirely or just partly y A black screen has all the crystals lined up so that no light gets through y

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Polarizer

Nematic liquid crystal

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Polarizer

ON STATE

OFF STATE

Passive Matrix LCD? Active Matrix LCD? 08 Oct, 2008

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Three-Dimensional Viewing Devices Reflecting a raster image from vibrating, flexible mirror y Vibrations sync. with the screen display y Each point reflected to a position relative to its depth y

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Stereoscopic & Virtual-Reality Systems y

Stereoscopic ◦ 2 views: 1 for left eye and 1 for right eye. ◦ Two views are displayed on alternate refresh cycles.

y

Virtual Reality Systems ◦ User can also interact with the environment. ◦ Stereoscopic views ◦ Input p devices to locate and manipulate p objects j in scene ◦ Sensing system to locate viewer’s position.

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Architecture of a Raster-Graphics System y y

Frame buffer is anywhere in the system memory Video Controller accesses the frame buffer to refresh the screen.

CPU

Frame buffer Cartesian Coordinates

System Memory M

Video Controller C t ll

Monitor

System Bus 08 Oct, 2008

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Architecture of a Raster-Graphics System y y

Frame buffer is assigned a fixed area in system memory Video Controller is given direct access to frame-buffer memory.

CPU

System Frame M Memory B ff Buffer

Frame buffer Cartesian Coordinates Video C t ll Controller

Monitor

System Bus 08 Oct, 2008

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Cartesian Reference Frame Origin (0,0)

y

x

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Video Controller Refresh Operation Horizontal & Vertical Deflection Voltages

Raster-Scan Generator

x Register

y Register

Store coordinate values for pixels

Pixel Register

Memory Address

Intensity

Frame Buffer 08 Oct, 2008

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Video Controller Refresh Operation Horizontal & Vertical Deflection Voltages

Raster-Scan Generator

x Register

y Register

0

Top scan line

1: If x equals maxx and y equals maxy Set x to 0 Set y to Top scan line 2: Else If x equals maxx Set x to 0 Increment y 3: Else Increment x

Pixel Register

Memory Address

Intensity

Frame Buffer 08 Oct, 2008

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Video Controller Refresh Operation Need to process at the rate of 60 frames per second at least y Retrieval of Multiple adjacent pixels’ values on each pass. y Retrieval of pixel values from different memory areas on different refresh cycles. y Lookup L k table bl y

◦ Frame buffer accesses the LUT instead of controlling intensity directly 08 Oct, 2008

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Raster-Scan Display Processor Display Processor Memory

CPU

Video Controller

Frame Buffer

Display Processor P

M it Monitor

System Memory M

System Bus 08 Oct, 2008

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Raster-Scan Display Processor y

Scan Conversion

◦ Digitization from picture definition in an app. program into a set of pixel i el values al es in frame buffer. b ffer ◦ For example, straight line scan conversion: x Define pixel positions x Define color for each position

y

Run-length Encoding

◦ Store each scan line as a set of number pairs. x First number: Color Value x Second number: Number of adjacent pixels

y

Cell Encoding

◦ Storage as a set of rectangular areas.

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y

Personal computers ◦ 640x480 – 1280 x 1024

y

Desktop workstation specifically designed for graphics apps. ◦ 1280x1024 – 1600x1200

y

High-definition graphics systems ◦ 2560x2048 ◦ Apps: Medical Imaging Imaging, Air-traffic Air traffic control, CAD

Multi-panel display systems y Curved Viewing Screens y

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