Chapter 1: Overview of Graphics systems Video Display Devices Raster-Scan System! Random-Scan Systems Input Devices Graphics Software

Introduction:Graphics is the familiar word for all. Graphics means visual representation of things that mean giving the visual representation of whatever you are thinking. Very simple example for graphics is: which you are using in your daily life; visuals in mobile phone and computer display. Why you have to study graphics as part of in your curriculum. Graphics is the part of all computer software and applications. Without graphics, no computer software and application. So graphics is important for computer science. One more thing computer is the one of the system for graphics so its must for computer science.Computer graphics involves display, manipulation and storage of picture and experimental data for proper visualization using computer. Typical graphic system is consists of host computer with fast processor, large memory, frame buffer, input devices, output devices and interface devices.

Framework of GraphicsFrom the above you can understand the concept and framework of graphics system. In the above figure left side: application model and application program are belongs to the developer side. Software developers are developing the programs and application. That programs and applications are running in the graphics systems. Can get the output in devices like monitors and can give the input to the system using input devices like keyboard.

One small example for the graphics system: calculator application in computer. That calculator application is programed and developed by the software developer. That calculator application is running in the computer graphics system and it is producing then the result based on the inputs.

Applications:Lot of applications is available for graphics systems. Here I listed some important areas. 1. GUI (Graphical User Interface)Basically it is an interface, between the user and graphic system. It is help to interact the user and system vice versa by input and output. Some GUI components are: Menus Icons Cursors Scroll bars Dialog boxes Grids etc.,2. Plotting in business 3. Office automation 4. Desktop publishing 5. Plotting in science and technology6. Advertisements7. CAD/CAM design 8. Scientific visualization9. Entertainment10. Simulation studies11. Multimedia12. Process monitoring 13. Digital Image Processing14. Education in training

Application packages and standards:Why need standards in graphics:Standards are very important for graphics because the programmers have to use common syntax, rules and procedures to build the graphics system. GKS Graphics Kernel System by ISO(International Standard Organization) and ANSI(American National Standards Institute) SRGP Simple Raster Graphics Package PHIGS Programmers Hierarchical Interactive Graphics System OpenGL Open Graphic Library

Output Primitives:Four basic output primitives are available to draw any type of pictures. Those primitives are: POLYLINE Filled POLYGONS ELLIPSE TEXT

Major Area:Four major areas of computer graphics are: Display of Information Design and Modeling Simulation User InterfaceTypes:Two Types available Active Graphic SystemIn this type user can control the display with the help of GUI and Input devices.Example: Video games Passive Graphic SystemIn this type user cant control the display.Example: TV Channel

Video Display Devices:Video display devices are the most important output device in the graphics systems. Three type of display devices available. These devices are working based on the CRT (Cathode Ray Tube) only. These types are: Direct View Storage Tube (DVST) Calligraphic or Random Scan Display system Refresh and Raster Scan Display system

Direct View Storage Tube (DVST):Cathode Ray Tube is the important part in the DVST display. First we will see some properties of DVST. Storage tube is CRT with a long persistence phosphor. It provides the flicker free display. No refreshing is necessary A slow moving electron beam draws a line on the screen Screen has a storage mesh in which the phosphor is embedded Image is stored as a distribution of charges on the inside surface of the screen

Operation of electron gun with an accelerating anode:The below figure illustrate the basic operation of a CRT. A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward the specified positions on the phosphor screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly.Electron gun:The primary components of an electron gun in a CRT are the heated metal cathode and a control grid. Heat is supplied to the cathode by directing a current through a coil of wire, called the filament, inside the cylindrical cathode structure. This causes electrons to be 'killed off" the hot cathode surface. In the vacuum inside the CRT envelope, the free, negatively charged electrons are then accelerated towards the phosphor coating by a high positive voltage. Control Grid:Intensity of the electron beam is controlled by setting the voltage level in the control grid which one fit over the cathode. Applying high negative voltage to the control grid will shut off the beam from cathode by repelling concept. So by adjusting the voltage level in the control grid you can change the brightness of the display. Accelerating Anode:Accelerating anode is used in CRT to accelerate the electron to strike the screen by high speed.

Focusing Anode:Focusing anode is used in CRT to focus the electron beam in particular place in the screen. Focusing done by two ways in CRT: either using electric or magnetic field. Electrostatic focusing is commonly used in Television and computer monitors. This is a positive voltage so its captured the electrons and point tuned the electron beam to hit the particular place in the screen. From the above figure: electron beams are hitting screen in the center point only. But the screen size is square so electron beam must to hit the all places in screen then only can see the picture in the screen. Defection made in electron beam by two methods by Electrostatic deflection Magnetic deflection

Electrostatic deflection:Electrostatic deflection is done by the capacitive plates are fixed by horizontally and vertically to the screen. Vertical plates are used to move the beam vertically in the screen and horizontal plates are used to move the beam horizontally in the screen. Magnetic deflection:Magnetic deflection coils helps to focus the electron beam all over the screen.Drawbacks: Modifying any part of image requires redrawing the entire image again Change in the image requires to generate a new charge distribution in the DVST Slow process of drawing

Erasing takes about 0.5 seconds No animation is possible with DVST

Calligraphic or Random Scan Display:In this display system: CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random scan monitors draw a picture one line at a time and for this reason are also referred to as vector displays (or stroke-writing or calligraphic displays). Characters also made of sequence of strokes or short lines Beam is deflected from end point to end point Order of the deflection is dictated by arbitrary order of the display commands Phosphor has short persistence decays in 10-100us The display must refresh with regular intervals minimum of 30Hz(fps) for flicker free display Refresh buffer is available in this display system. It is used to store the display list or display program to draw the picture The display processor interprets the commands in the refresh buffer for plotting The display program has commands for point, line, and character plotting

Conceptual Block diagramApplication programs are running in the host CPU. Host CPU keeps the Display programs and display commands in the display buffer. Based on the program and command display controller will work and the picture processor sends the point coordinate values to the vector generator. Vector generator converts the digital coordinate value to analog voltage for beam deflection circuit. Beam deflection circuit displaces the electron beam for drawing on the CRT screen.

The above figure illustrates the operation of the electron beam deflection in the random scan display. In this display system: image is drawn in the CRT screen by line. In the first fig: its starting the image from one end point to another end point. Finally using 3 lines, it is finished the image of triangle. One more example for drawing the image in CRT using random scan display:

Random Scan System:The below figure illustrates the random scan system: application program and graphic packages stored in the system memory. Graphic commands are translated to display file in the system memory by package. Display processor is accessing that display file to refresh the screen.

Refresh and Raster Scan Display System:The most common type of graphics monitor employing a CRT is the raster-scan display, based on television technology. In a raster-scan system, the electron beam is swept across the screen, one row 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. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and "painted" on the screen one row (scan line) at a time. Each screen point is referred to as a pixel or pel (shortened forms of picture element).Drawing Image:

From the above figure: you can understand, how the raster scan system drawing image in the screen. In this system, display screen is matrix of pixels. So each pixel properties (coordination and intensity) are controlled by video controller. Each pixel is addressable in frame buffer ex: (5, 5) (here X and Y coordination values are 5 and 5). In this system cant draw the line directly. Its possible by point by point only so it is called point plotting. See the figure. Its drawing the line by point to point by scanning the screen from top to bottom row.

Architecture of simple raster-scan system: Interactive raster graphics systems typically employ several processing units. In addition to the central processing unit or CPU, a special-purpose processor called the video controller or display controller is used to control the operation of the display device. Organization of a simple raster system is shown in below figure.

Architecture of simple raster-scan systemIn the above figure: frame buffer is available in system memory and the video controller access the frame buffer to refresh the screen.

Architecture with fixed portion of system memoryIn the above figure: only one difference is: here frame buffer area is fixed size in the system memory so this architecture is called fixed portion of system memory. The below figure is two dimensional system. Here the coordinates are X and Y. these are used to fix the electron beam in the exact place in the screen. In the frame buffer two registers available, that is: register X and Y. these registers are used to keep the X and Y coordination: using these values video controller is drawing the image in the screen.

Input Devices:Various devices are available for data input on graphics systems. Most systems have a keyboard and one or more additional devices specially designed for interactive input. These include a mouse, trackball, space ball, joystick, digitizers, dials, and button boxes. Some other input devices used in particular applications are data gloves, touch panels, image scanners, and voice systems. Here we will discuss some of the important devices.

Keyboard:An alphanumeric keyboard on a graphics system is used primarily as a device for entering text strings. The keyboard is an efficient device for inputting such nongraphic data as picture labels associated with a graphics display. Keyboards can also be provided with features to facilitate entry of screen coordinates, menu selections, or graphics functions. Cursor-control keys and function keys are common features on general purpose keyboards. Function keys allow users to enter frequently used operations in a single keystroke, and cursor-control keys can be used to select displayed objects or coordinate positions by positioning the screen cursor.

Mouse:A mouse is small hand-held box used to position the screen cursor. Wheels or rollers on the bottom of the mouse can be used to record the amount and direction of movement. Another method for detecting mouse motion is with an optical sensor. For these systems: the mouse is moved over a special mouse pad that has a grid of horizontal and vertical lines. The optical sensor detects movement across the lines in the grid. Since a mouse can be picked up and put down at another position without change in cursor movement, it is used for making relative change in the position of the screen cursor. One, two, or three buttons are usually included on the top of the mouse for signaling the execution of some operation.

Trackball and Space ball:Track ball is a ball that can be rotated with the fingers or palm of the hand, to produce screen-cursor movement. Potentiometers, attached to the ball, it measures the amount and direction of rotation. Trackballs are often mounted on keyboards.Space ball does not actually move. Strain gauges measure the amount of pressure applied to the space ball to provide input for spatial positioning and orientation as the ball is pushed or pulled in various directions. Space balls are used for three-dimensional positioning and selection operations in virtual reality systems, modeling, animation, CAD, and other applications.Joysticks:A joystick consists of a small, vertical lever (called the stick) mounted on a base that is used to steer the screen cursor around. Most joysticks select screen positions with actual stick movement; others respond to the pressure on the stick. Some joysticks are mounted on a keyboard. The distance that the stick is moved in any direction from its center position corresponds to screen-cursor movement in that direction. Potentiometers mounted at the base of the joystick measure the amount of movement, and springs return the stick to the center position when it is released.

Data Glove:Data glove is used to grasp a virtual object. The glove is constructed with a series of sensors that detect hand and finger motions. Electromagnetic coupling between transmitting antennas and receiving antennas is used to provide information about the position and orientation of the hand. Input from the glove can be used to position or manipulate objects in a virtual scene.

Image Scanners:Drawings, graphs, color and black-and-white photos, or text can be stored for computer processing with an image scanner by passing an optical scanning mechanism over the information to be stored. The gradations of gray scale or color are then recorded and stored in an array. Once we have the internal representation of a picture, we can apply transformations to rotate, scale, or crop the picture to a particular screen area. We can also apply various image processing methods to modify the array representation of the picture. For scanned text input, various editing operations can be performed on the stored documents.