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CGMB214: Intro. To Computer Graphics

Chapter 2

Overview of Graphics System

Part I

Image from http://www.desktopwallpapers.in

What you need to know

• Understand and explain the basic feature of graphic hardware components

• Understand and explain the operations of a video display device

Video Display Device

• It is a primary output device in a graphics system

• Most video monitors is based on the standard cathode-ray tube (CRT) design

• Type of graphics monitor employing a CRT– Raster-Scan Display– Random-Scan Display

• Other monitor technologies– Flat-Panel Display– Plasma-Panel– Thin-film electro-luminescent Display

Cathode Ray Tube (CRT)


Basic design of a magnetic-deflection CRT


• Most widely used display– Scenes drawn by directing an electron beam that

excites phosphors on screen to produce a spot of light

– Electron are fired from an electron gun– Electron beam deflected by magnetism– Black and white/monochrome uses 1 gun,

colour uses three guns: red, green and blue (RGB)

– Shadow mask ensures that each gun hits the correct spot


Operation of an electron gun with an accelerating anode


• Brightness of display point is controlled by varying the voltage on control grid because the amount of light emitted by phosphor coating depends on the number of electrons string the screen.

• Focusing system forces electron beam to converge to a small cross section.

• Additional focusing hardware is used in high-precision system

• Deflection of electron beam is controlled with either electric or magnetic field

• Spots of lights are produced on the screen by the transfer of CRT beam energy to the phosphor; kinetic energy of electrons in the beam is absorbed by phosphor.


Electrostatic deflection of the electron beam in a CRT


• 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


• Two ways of beam been shot at the screen– Raster-Scan– Random-Scan


• Raster-Scan Display– Based on TV technology– The electron beam is swept across the screen

one row at a time, from top to bottom– Each row is referred as a scan line– As 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)


A raster scan system displays an object as a set of discrete points across each scan line


• Raster-Scan Display– 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• Defined as the number of pixel columns divided by

the number of scan lines that can be displayed by the system

• Can also be describes as the number of horizontal points to vertical points (or vice versa) necessary to produce equal-length lines in both directions on the screen


• Raster-Scan Display– 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 bitmap ; frame 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 performs refreshing at the rate of 60-80 Hz


• Random-Scan Displays– Designed for specialised line-drawing application e.g.

CAD– 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 also known as vector display or stroke-

writing display or calligraphic displays– Electron beam is focussed only where lines need to

be drawn– Higher resolution than raster-scan and draw a

smoother line because the path beam follows the line path


• Black and white monitor– Pixel intensity is either 1 (on) or 0 (off)– Frame buffer is called bitmap

• Color– Pixel intensity is an integer representing a

color (RGB value)– Frame buffer is called pixmap


• Colour CRT Monitors– 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


• Frame buffer memory requirements differ for black and white vs. color– E.g. 1024 x 1024 screen resolution

• Black and white– 1 bit per pixel– Memory required = 1,024 x 1,024 x 1 = 1,048,576 bits = 13,072

bytes• Color

– 8 bits per pixel (256 colors)– Memory required = 1,024 x 1,024 x 8 = 8,388,608 bits =

1,048,576 bytes» 1,048,576 / 1,024 bytes = 1024 Kb = 1Mb

– 24 bits per pixel (16,777,215 colors)– Memory required = 1,024 x 1,024 x 24 = 25,165,824 bits = 3Mb


• Frame buffer memory requirement is a trade-off between screen resolution (number of pixels) and the amount of color (number of bits per pixel)– 5,120 x 5,120 B & W required 3.125Mb– Compared to 3Mb for 1,024 x 1,024 with 24

bit color• Color system can produce realistic scenes

with shading

Graphics Workstation and Viewing System

• Graphics workstations range from small general-purpose computer systems to multi-monitor facilities, often with ultra-large viewing screen– Personal PC screen

• resolution range from 640 x 480 to 1280 x 1024

• diagonal screen lengths measure from 12 inches to over 21 inches

– High-definition graphics system• resolution up to 2560 x 2048

commonly used in medical imaging, air-traffic control, simulation and CAD

– High-end graphics workstations also include large viewing screen often with specialized features such as stereoscopic viewing etc

– Multi-panel display screens are used for applications that require “wall-size” viewing area

Input Devices

• Keyboards • Keypads• Touch pads• Mice • Trackballs• Data gloves• Scanner

– Reflective (print)– Transparency (film)

• Game pads • Joysticks• Light pens & sensors• Tablets • Force-feedback • Audio/Voice

– Microphone– MIDI (Musical Instrument Digital Interface)

Input Devices

• Barcode reader

– price scanner or point-of-sale (POS) scanner• Magnetic stripe reader

– Credit card reader , ATM – petrol station• MICR – magnetic ink character recognition

– Banking environment – check color copies of payroll checks or hand-altered characters

Input Devices

• OCR – optical character recognition– Read typewritten text - is used in high-volume financial

applications such as payment processing, check reconciliation, and billing. It is also commonly used for high-volume document management in the insurance and healthcare industries

Input Devices

• OMR – optical mark recognition– Paper marking - used for time and attendance, labor

tracking, inventory management, voting applications, exit surveys, polling, and all manner of questionnaires and evaluation studies

Input Devices

• Image scanning devices / digital camera– Ex. Educational environment , New car

technology

Hard-copy Devices

• Printer• Plotter• Film / Slide

Graphics Software

• Two broad classification– Special purpose packages

• Design for non-programmers who want to generate pictures

• They don’t have to worry about the graphics procedure

• E.g. CAD software, paintbrush– General programming packages

• Provides a library of graphics functions that can be used in a programming language such as C/C++, Java or Fortran

• E.g. OpenGL, VRML, Java2D, Java3D

cgmb214: intro. to computer graphics chapter 2 overview of graphics system part i image from

Documents

screen cathode ray tube

electron gunelectron

beam goesphosphor

beam intensity

transfer of crt beam

number of scan lines

system forces electron

scan lineas