Chapter 2

Computer Graphics Hardware

Agenda

•CRT Monitor

•Color CRT Screen

•Flat-Panel Displays

− Plasma Panel

− Electroluminescent Display

− Light-Emitting Diode (LED)

− Liquid-Crystal Device (LCD)

•Random-Scan System

•Raster-Scan System 2

Cathode Ray Tubes (CRTs)• The cathode ray tube (CRT) is a vacuum tube containing one

or more electron guns, and a phosphorescent screen used to view images.

• The images may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), radar targets or others.

• CRTs have largely been superseded by newer display technologies such as LCD, plasma display, and OLED, which have lower manufacturing costs, power consumption, weight and bulk.

CRT Monitor (1)

4

CRT Monitor (2)

5

CRT Monitor (3)

  Component   Description

1 Filament   It generates heat

2 Cathode   It emits electrons

3 Control grid  It controls the intensity of the electron beam by setting voltage levels.

4 Focusing system

 It is needed to force the electron beam to converge into a small spot as it strikes the phosphor.

5 Deflection system

 It directs the beam toward specified positions on the phosphor-coated screen.

6 Phosphor coating

 It emits a small spot of light at each position contacted by the electron beam

CRT Monitor (4)

• The phosphor than emits a small spot of light to each position contacted by the electron beam.

• The light emitted by the phosphor fades very rapidly.

• One way to keep the phosphor glowing is to redraw the picture repeatedly. This type of display is called a Refresh CRT

CRT Monitor (5)• Intensity of the electron beam is controlled by

setting voltage levels on the control grid• Amount of light emitted by the phosphor

coating depends on the number of electrons striking the screen, we control the brightness of a display by varying the voltage on the control grid

• Focusing deflection of the electron beam can be controlled either with electric fields or with magnetic fields

CRT Monitor (6)• Different kind of phosphors are available for

use in a CRT, besides color a major difference between phosphors is their persistence: how long they continue to emit light after the CRT beam is removed.

• Persistence is defined as the time it takes the emitted light from the screen to decay to one tenth of its original intensity.

• Lower persistence phosphors require higher refresh rates to maintain a picture on the screen without flicker.

10

Color CRT Monitor (1)The two basic techniques for producing color displays with a CRT are the beam-penetration method and the shadow-mask methods.• The beam-penetration method for displaying color

pictures has been used with random-scan monitors.

• Shadow-mask methods are commonly used in raster-scan systems (including color TV) since they produce a much wider range of colors than the beam-penetration method. – This approach is based on the way that we seem to

perceive colors as combinations of red, green, and blue components, called the RGB color model.

11

Color CRT Monitor (2)

12

Color CRT Monitor (3)

Color CRT Monitor (4)• The three electron beams are deflected and focused

as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns.

• When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen.

• The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. 13

14

Color CRT Monitor (5)• We obtain color variations in a shadow-mask CRT by

varying the intensity levels of the three electron beams. • By turning off the three guns, we get only the color coming

from the single activated phosphor (red, green, or blue).

• The color depends on the amount of excitation of the red, green, and blue phosphors.

• When all three dots are activated with equal beam intensities, we see a white color.

• Yellow is produced with equal intensities from the green and red dots only, magenta is produced with equal blue and red intensities, and cyan shows up when blue and green are activated equally.

Color CRT Monitor (5)• High-quality raster-graphics systems have 24

bits per pixel in the frame buffer, allowing 256 voltage settings for each electron gun and nearly 17 million color choices for each pixel.

(2^24 = 16777216 colors).

• An RGB color system with 24 bits of storage per pixel is generally referred to as a full-color system or a true-color system.

15

Flat-Panel Displays• The term flat-panel display refers to a class of

video devices that have reduced volume, weight, and power requirements compared to a CRT.

• A significant feature of flat-panel displays is that they are thinner than CRTs, and we can hang them on walls or wear them on our wrists. Since we can even write on some flat-panel displays, they are also available as pocket notepads.

16

Use of Flat-Panel Displays

Here are some additional uses for flat-panel displays:

• Small TV monitors• Calculator screens• Pocket video-game screens• Laptop computer screens• Advertisement boards in elevators• Portable monitors

…17

18

Emissive and non-emissive displays• We can separate flat-panel displays into two categories:

Emissive displays and non-emissive displays

• Emissive displays (or emitters) are devices that convert electrical energy into light.

Examples:– Plasma Panel, Thin-Film Electroluminescent Display,

Light-Emitting Diode (LED)

  • Non-emissive displays (or non-emitters) use optical effects to

convert sunlight or light from some other source into graphics patterns.

Example:– Liquid-Crystal Device (LCD)

19

Plasma Panel• A plasma display panel (PDP) is a type of flat

panel display common to large TV displays 30 inches (76 cm) or larger.

• They are called "plasma" displays because they use small cells containing electrically charged ionized gases, which are plasmas.

Electroluminescent Displays• They are a type of Flat panel display created

by sandwiching a layer of electroluminescent material such as GaAs between two layers of conductors.

• When current flows, the layer of material emits radiation in the form of visible light.

• Electroluminescence (EL) is an optical and electrical phenomenon where a material emits light in response to an electric current passed through it, or to a strong electric field. 20

Light-Emitting Diode (LED)• A matrix of diodes is arranged to form the pixel

positions in the display, and picture definition is stored in a refresh in a refresh buffer.

• As in scan-line refreshing of a CRT, information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display.

21

22

Liquid-Crystal Device (1)• It is a flat panel display, electronic visual

display, or video display that uses the light modulating properties of liquid crystals.

• The LCD technique is used in small systems such as calculators and laptop computers.

Liquid-Crystal Device (2)An LCD device contains the following components:– Two glass plates, each containing a light polarizer that is

aligned at a right angle to the other plate, sandwich the liquid-crystal material.

– Rows of horizontal, transparent conductors are built into one glass plate.

– Columns of vertical transparent conductors are put into the other plate.

– The region between the glass plates is filled with a liquid called crystalline.

– The intersection of two conductors defines a pixel position.– Originally, all pixels in the screen are turned on thanks to

polarizer.– To turn off a pixel, we apply a voltage to the intersecting

conductors so that the crystalline prevents light from passing through it. 23

24

Random-Scan display• In Random Scan System, an electron beam is directed to only

those parts of the screen where a picture is to be drawn. • The picture is drawn one line at a time, so also called vector

displays or stroke writing displays. • After drawing the picture the system cycles back to the first

line and design all the lines of the picture 30 to 60 time each second.

25

Raster-Scan Systems (1)• In a raster-scan system, the electron beam is

swept across the screen, one row (Scan Line) at a time from top to bottom.

• As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots.

• Picture definition is stored, in a memory area called the refresh buffer or frame buffer, where the term frame refers to the total screen area.

26

Raster-Scan Systems (2)

Simple raster-graphics system

27

Raster-Scan System with a Display Processor

28

Raster-Scan SystemsArchitecture of a raster system with a fixed portion of the system

memory reserved for the frame buffer

29

Video Controller

30