Overview of Graphics System
Objectives At the end of this chapter the reader will be able to: • Describe and distinguish raster and random scan displays • Describe various display devices. • Describe how color CRT works..
Overview of Graphics SystemStructure 1. Introduction 2. Refresh CRT 3. Random-Scan and Raster Scan Displays4. Random–Scan and Raster Scan System5. Graphics Monitor and Workstation6. Input Devices7. Hard Copy Devices 8. Graphics Software9. Summary
Video Display DevicesThe Primary Output Device of the Graphic System ??
How does this Operate???
Video Display DevicesVideo Monitor
Back
Cathode Ray Tube
Basic Design of Magnetic deflection CRT
Cathode Ray Tube
Basic Design of Magnetic deflection CRT
Operation of an Electron Gun with an Accelerating Anode
Electrostatic Deflection of the Electron Beam in a CRT
Characteristics of CRT oIntensity Distribution
oResolution
oAspect Ratio
oPersistence
Intensity Distribution
The intensity is greatest at the center of the spot, and decrease with Gaussian distribution out to the edges of the spot.
Resolution
The maximum number of points displayed without overlap on a CRT is referred to as the resolution.
A more precise definition of resolution is the number of points per centimeter that can be plotted horizontally and vertically, although it is often simply stated as the total number of points in each direction.
This depends on the type of phosphor used and the focusing and deflection system & the intensity to be displayed.
Resolution
Overlap
Aspect RatioAnother property of video monitors .
This number gives the ratio of vertical points to horizontal points necessary to produce equal-length lines in both directions on the screen.
An aspect ratio of 3/4 means that a vertical line plotted with three points has the same length as a horizontal line plotted with four points.
PersistancePersistence is defined as the time it takes the emitted light from screen to decay to one-tenth of its original intensity.
Color CRT MonitorsThese monitors displays color pictures by using combination of phosphors that emit different colored light.
There are two techniques for producing color displays on CRT. They are :
1. Beam Penetration Method
2. Shadow Mask Method
Beam Penetration MethodSimilar to simple CRT but contains different colored Phosphor layers(Red and Green) coated onto inside of the screen.
The displayed color depends on how far the electron beam penetrates into the layers.
The speed of electrons – beam acceleration voltage.Slow Electrons Red colorFast Electrons Green colorIntermediate Speed Electrons Yellow and orange colors
Quality of Picture is not so good and is inexpensive.
Shadow Mask Method
Delta Delta Shadow Mask Method
Shadow Mask MethodWorks on the principle of combining the basic colors - Red, green and Blue - in suitable proportions to get a combination of colors. Uses 3 electron gun placed one by the side of the other to form a triangle or a "Delta".
Each pixel point on the screen is also made up of 3 types of phosphors to produce red, blue and green colors. Before the phosphor screen is a metal screen, called a "shadow mask". This plate has holes placed strategically, so that the beams from the three electron guns are focused on a particular pixel.
Shadow Mask MethodWhen the beam passes through the mask they form a dot triangle.
Another configuration is inline arrangement of electron guns and RGB dots on screen are aligned on one scan line.
By varying intensity , we get different colors (White,Grey,Yellow,Cyan)
Beam Penetration Vs Shadow MaskBeam Penetration method Shadow Mask method
Where Used It is used with Random Scan System to display color.
It is Used With Raster Scan System to display color.
ColorsIt can displays Only four colors i.e. Red , Green, Orange and Yellow.
it can display Millions of colors.
Color DependencyLess colors are available because the colors in Beam Penetration depends on the speed of the electron beam.
Millions of colors are available because the colors in Shadow Mask depends on the type of the ray.
CostIt is Less Expensive as compared to Shadow Mask.
It is More Expensive than other methods.
Picture QualityQuality of picture is not so good i.e. Poor with Beam Penetration Method.
Shadow Mask gives realism in picture with shadow effect and millions of color.
Resolution It gives Low Resolution. It gives High Resolution.
Criteria
In Beam Penetration method, Color display depends on how far electron excites outer Red layer and then Green layer.
In Shadow Mask Method, there are no such criteria for producing colors. It is used in computers, in color TV etc.
Direct View Storage Tube
Direct View Storage TubeAn alternative method for maintaining a screen image is to store the picture information inside the CRT instead of refreshing the screen.
A direct-view storage tube (DVST) stores the picture information as a charge distribution just behind the phosphor-coated screen.
Two electron guns are used in a DVST. One, the primary gun, is used to store the picture pattern; the second, the flood gun, maintains the picture display .
A DVST monitor has both disadvantages and advantages compared to the refresh CRT. Because no refreshing is needed, very complex pictures can be displayed at very high resolutions without flicker.
Disadvantages of DVST systems are that they ordinarily do not display color and that selected parts of a picture cannot be erased. To eliminate a picture section, the entire screen must be erased and the modified picture redrawn. The erasing and redrawing process can take several, seconds for a complex picture. For these reasons, storage displays have been largely replaced by raster systems.
Raster Scan DisplayMost common method
Beam is swept across the screen one line at a time, pixel by pixel
Uses Frame Buffer (an array ,on and off state)
Screen is refreshed 60 times/sec –Interlacing (Odd numbered and Even numbered lines)
Progressive Scanning – alternate to interlacing,reduces Flickers- refreshes every line on screen
Random Scan DisplayAlso called Vector/Stroke/Calligraphic Displays
Beam is swept across the screen one line at a time, only intended point
Picture Definition – Display file (set of line drawing commands)
Only area where picture is drawn is refreshed
Raster Display VS Random Display It has Poor or Less Resolution
Electron beam is directed from top to bottom at a time on screen , but electron beam is directed to whole screen.
It is less expensive
Refresh Rate is 60 to 80 frames / second
Stores Picture Definition in Frame Buffer
Zig Zag line is produced
It uses Pixels along Scan lines for drawing an image
It has High Resolution
Electron beam is directed only to that part of screen is required to be drawn.
It is expensive
Refresh Rate depends on no.of lines to be displayed ,30 to 60 times/ second
Stores Picture Definition in Display files
Smooth line is produced
It uses various mathematical functions
Raster Scan SystemInteractive raster-graphics systems typically employ several processing units :
CPU
Special purpose register – Video/Display Controller
System Memory
Architecture of Simple RG System
CPUSystem
MemoryVideo
ControllerMonitor
System Bus
I/O Devices
How The Interactive Graphics Display Works
It consists of the three components :
(1)Frame Buffer
(2) Monitor like a TV set without the tuning and receiving electronics.
(3) Display Controller It passes the contents of the frame buffer to the monitor.
Architecture of RG Systemwith Frame Buffer
I/O Devices
CPUSystem Memory
Frame Buffer
Video Controller
Memory
System Bus
Basic Video Controller Refresh Operations
Memory Address
X Registe
r
Pixel Register
Y Register
Frame Buffer
Raster Scan Generator
Intensity
Architecture of RG System with Display Processor
CPUDisplay
Processor
Video Controller
Monitor
System Bus
I/O Devices
System Memory
Display Processor Memory
Frame buffer
Architecture of Simple Random Scan System
I/O Devices
System Bus
CPUSystem Memory
Display Processor
Monitor
Application programs are stored in system memory. Graphics commands in the program are translated by the graphics package into a display file stored in the system memory. This display file is accessed by the display processor to refresh the screen. Display processor in a random scan system is referred to as a display processing unit or graphics controller.
GRAPHICS MONITORS & WORKSTATIONS
Media Wall
Dual Monitor System
High Resolution Monitor
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Multiscreen Desks &Workstations
Workstation
Work station : a desktop computer terminal, typically networked and more powerful than a personal computer.
Input Devices1. Keyboard2. Mouse3. Trackball & Spaceball4. Joystick5. Data Glove6. Digitizers7. Image Scanners8. Touch Panels9. Light Pens10.Voice Systems
KeyboardA device for enter text strings.
MouseHand held device used to position the cursor.
Trackball It is a ball that is a ball that can be rotated
with fingers to produce cursor movements.
Potentiometers attached to the ball , measure the amount and direction of rotation.
Normally mounted on KB.
2d positioning device.
SpaceballSpaceball six-degree-of-freedom input devices.
The Spaceball may be used to rotate, translate, and scale molecules, using up to 6 control axes simultaneously.
The Spaceball can be used independently and simultaneously with the mouse.
Spaceball are used for 3D positioning and selection operations in virtual reality systems, modeling, animation, CAD, and other application.
Joystick A joystick is a input device consisting of a stick
that pivots on a base and reports its angle or direction to the device it is controlling.
Used to select screen positions with actual stick
movement, others respond to pressure on the stick.
Data Glove A glove equipped with sensors
that sense the movements of the hand and interfaces those movements with a computer.
Data gloves are commonly used in virtual reality environments where the user sees an image of the data glove and can manipulate the movements of the virtual environment using the glove.
Digitizers A device for drawing , painting , or
interactively selecting co-ordinate positions on an
object.
Conversion of a typically analog object, image or a signal into digital form.
Graphics Tablet It is a type of Digitizer.
It enables a user to hand-draw images, animations and graphics, similar to the way a person draws images with a pencil and paper.
These tablets may also be used to capture data or handwritten signatures. It can also be used to trace an image from a piece of paper which is taped or otherwise secured to the surface. Capturing data in this way, either by tracing or entering the corners of linear poly-lines or shapes is called digitizing.
Image Scanners Is a device that optically scans images,
printed text, handwriting, or an object, and converts it to a digital image.
Types : Drum FlatBed Film Hand Held Smart phone Scanner
Types of Image Scanners
Touch Panels Touch panels allow displayed objects or screen
position s to be selected with touch of a finger.
Types of Touch Panels :a. Resistive Film Touch
Panelsb.Capacitive Touch panelsc.Surface Capacitive Touch
Panelsd.Surface Acoustic Wave
Touch Panelse.Optical Touch Panelsf. Electromagnetic Induction
Touch Panels
Light Pens It is in the form of a light-sensitive
wand used in conjunction with a computer's CRT display.
It allows the user to point to displayed objects, or draw on the screen, in a similar way to a touch screen but with greater positional accuracy.
A light pen can work with any CRT-based display, but not with LCD screens, projectors and other display devices.
Voice SystemA device in which speech is used to input data or system commands directly into a system.
Such equipment involves the use of speech recognition processes, and can replace or supplement other input devices. Some voice input devices can recognize spoken words from a predefined vocabulary, some have to be trained for a particular speaker.
Hard Copy Devices
The quality of the picture obtained from the device depends on dot size and no of dots/inch.
Hard Copy Devices
Hard Copy Devices
Graphics SoftwareClassification :
a)General Programming PackageProvides functions that can be used in
HLP Lang.For Programmers.Eg : Graphics Library
b)Special Purpose Application PackageProvides functions for graphic DesignFor Non ProgrammersEg : CAD
5 Coordinate Representations in Graphics :General graphics packages are designed to be used with Cartesian coordinate representations (x,y,z).
Usually several different Cartesian reference frames are used to construct and display a scene:
Modeling coordinates are used to construct individual object shapes.
World coordinates are computed for specifying the placement of individual objects in appropriate positions.
Coordinate Representations
Normalized coordinates are converted from world coordinates, such that x,y values are ranged from0 to 1.
Device coordinates are the final locations on the output devices.
(xmc,ymc) (xwc,ywc) (xnc,ync) (xdc,ydc)
Coordinate Representation
The basic building blocks for pictures are referred to as Output Primitives. They include character strings, geometric entities.
Attributes are properties of O/P primitives. It describes how a primitive should be displayed.
Geometric transformation is changing the size ,position, orientation of an object within a scene.
Viewing transformations are used to specify the view that is to be presented & the portion of the o/p display area that is to be used.
Graphics Functions
Pictures can be sub divided into small parts called Structures or segments or objects. Each structure defines one logical unit of the picture.
I/P functions are used to control and process the data flow from these interactive devices.
Control operation is responsible for house keeping tasks.
Graphics Functions
The primary goal of standardized graphics software is portability.
First Graphic S/W standard : Graphical Kernel System (GKS)Second Graphic S/W standard : Programmers Hierarchical Interactive Graphics Standard (PHIGS)Third Graphic S/W standard : PHIGS+These standards were adopted by ISO and ANSI.Fourth Graphic S/W standard : Computer Graphics Interface (CGI)Fifth Graphic S/W standard : Computer Graphic Metafile (CGM)
Graphics Software Standard
