CGMB214: Introduction to Computer Graphics

TOPIC 2OVERVIEW OF GRAPHICS SYSTEM

CGMB214: Introduction to Computer Graphics

Objectives

To provide an understanding on the basic feature of graphic hardware components.

To provide an understanding on the operations of a video display device.

CATHODE RAY TUBE TECHNOLOGY

Video Display Devices



CRT Technology Flat Panel Displays

Refresh Cathod

e Ray Tubes

Raster Scan

Display

Random Scan Display

Colour CRT

Monitor Non Emissive Display

Emissive Display

Refresh Cathode Ray Tubes

Connector Pins

Base

Electron Gun

Focusing System

Magnetic Deflection

Coils

Phosphor-Coated Screen

Refresh Cathode Ray Tubes: Electron Gun

Heating Filament

Cathode

Control Grid

Focusing Anode

Accelerating Anode



Heating Filament Cathode

Control Grid

Focusing Anode

Accelerating Anode

Current is directed through filament to supply heat

Electron is being emitted from the hot cathode surface

Control grid is a metal cylinder that is controlled by voltage.

A high negative voltage, will repel electron and stop them from

passing

A smaller negative decrease the number of electron passing

through


Heating Filament Cathode

Control Grid

Focusing Anode

Accelerating Anode

Once an electron is being released and the correct amount of voltage

is being supplied by the control grid, the electron will move in a straight line through the control

grid

The focusing anode which is charged with hundreds of positive volts, will force the

electron beam to converge to it. However, since there are two anode attracting the

electron, the electron will continue moving in a straight line

The accelerating anode which is charged with thousand of positive volts will have greater force on attracting the electron

beam to converge to it. As focusing anode, the built of the accelerating anode make

sure the electron will continue moving in a straight line


Connector Pins Base Electron

GunFocusing System

Magnetic Deflection

Coils

Phosphor-Coated Screen

Once the electron beam is emitted by the electron, it will pass through focusing and deflection system that direct the beam

toward a specific position on the phosphor-coated screen.The phosphor emits a small spot of light at each position contacted

by electron beamThe brightness of the of the display point is controlled by the

voltage provided by the control grid in the electron gun


Resolution A max number of points that can be displayed without

overlap on CRT Typically 1028X1024 High resolution means high definition

Persistence How long the phosphors will emit light after beam

goes Phosphor with low persistence require higher refresh

rate to prevent flicker


Raster Scan Displays

The most common type of graphics monitor that employs CRT

The electron beam is swept across the screen one row at a time, from top to bottom

Each row is referred as a scan lineAs the electron beam moves across a scan

line, the beam intensity is turned on and off to create a pattern of illuminated spots

Screen is divided into smallest element of a rectangular array called pixel or pel (picture element)



Screen Display

Pixels/pels


Raster system commonly characterized by their resolution, which is the number of pixel positions that can be plotted

Another property of video monitors is aspect ratio: number of pixel columns divided by the number of

scan lines that can be displayed by the system number of horizontal points to vertical points (or vice

versa) necessary to produce equal-length lines in both directions on the screen


Picture definition is stored as set of color values for screen points in memory area called refresh buffer or color buffer or frame buffer (frame refers to total screen area).

Frame buffer with one bit per pixel is called bitmapFrame buffer with multiple bits per pixel is a pixmap.Refresh rates are described in units of cycles per

second (Hertz) where a cycle corresponds to one frame. Raster scan displays normally perform refreshing at the rate of

60-80 Hz


Zoom in

Random Scan Display

Electron beam is focussed only where lines need to be drawn

Random-scan also known as vector display or stroke-writing display or calligraphic displays

Scene definition stored in a display list as a set of line drawing instruction

Scene is drawn by cycling through the display list drawing one line at a time

Random Scan Display

Higher resolution than raster-scan and draw a smoother line because the path beam follows the line path

Designed for specialised line-drawing application e.g. CAD

Random Scan Displays

Random Scan Display

Zoom in

Colour CRT Monitor

Two ways of displaying colour: Beam penetration

2 layers of phosphors (red and green) Colour depends on the speed of the electron beams Non-expensive way to get colour

Shadow mask Used in raster scan Use 3 phosphor colour dots at each pixel (RGB) CRT needs to have 3 electron guns

Colour CRT Monitor: Beam Penetration

slow

Beam of SLOW electron excited only the outer layer producing red

moderatefast

Beam of FAST electron penetrates through the

red layer and excites the inner green layer

Moderate beam speeds will emits light

combination of red and green: orange and yellow

Red phosphor layer

Green phosphor layer

Colour CRT Monitor: Shadow Mask

Each pixel position has three phosphor colour dot: Red, Green and Blue (RGB). Each phosphor emits lights according to

their colour.

Shadow mask CRT needs 3 electron guns, one for each colour dot.

Electron guns

Shadow mask

Screen

The three electron beams are deflect and focused as a group at a shadow mask. Here

it activated a dot triangle

The phosphor dots in the triangles are arranged so that each electron beam can only activate its corresponding colour dot

when it passes the shadow mask

Once all the electrons has been arranged in the shadow mask, it will be released on

the correct phosphor triangle on the screen

Colour variation is produced by varying the intensity level of the electron beams:Eg. To get yellow, blue gun need to be

switch off.

Documents

CGMB214: Introduction to Computer Graphics