UNIT 1 INTRODUCTION TO COMPUTER

Transcription

1 UNIT 1 INTRODUCTION TO COMPUTER Introduction to Computer Structure 1.1 Introduction Objectives 1.2 Display Devices Cathode Ray Tube Technology (CRT) Random Scan Display Raster Scan Display Liquid Crystal Display 1.3 Input Device Logical Devices Other Input Devices 1.4 Output Devices 1.5 Summary 1.1 INTRODUCTION An understanding of the nature of colour and visual communication, and an appreciation of what makes an effective image is necessary for better understanding and presentation of the subject when we start with Computer Aided Design. The knowledge gained in this unit is related later to the theoretical concepts covered in other units. The objective of this unit is mainly to provide description of CAD/CAM hardware and its related terminology. The types of architectures of CAD/CAM systems, input devices and output devices is mainly covered under this unit. The images generated during Computer Aided Designing (CAD) are to be put on an output device. Almost all personal computers and workstations use huge monitors and are based on either Cathode Ray Tube (CRT) technology or Liquid Crystal Display (LCD) technology, while notebook or laptop computers have relatively small sized display device and are based only on LCD technology. Objectives After studying this unit, you should be able to understand the various display devices used for computer graphics applications, know about different input devices, and identify the range of output devices to obtain hard copies. 1.2 DISPLAY DEVICES In this section of the unit, different types of display devices are discussed along with the principles on which these work. The main display devices used with CAD systems can be broadly classified as : CRT devices, and LCD devices Other technologies used are laser, flat panel displays or plasma panel displays. 5

2 Introduction to Computer and Computer G hi Cathode Ray Tube Technology (CRT) CRT is an analog device that is used by a digital device computer. In this section, the interface between these two devices is described. The major components of CRT are given below : Electron Gun This generates a narrow beam of electrons (cathode rays) which passes through the focusing and deflection systems. Focusing System It produces a non-blurred picture by focusing the electrons of the cathode ray into a narrow beam. Deflection System Deflection directs the beam towards specified positions on the phosphor-coated screen. Phosphor-coated Screen This screen emits a small spot of light at each position contacted by the cathode ray. Figure 1.1 CRT is considered to be one of the most important component because the quality of displayed image influences the perception of generated design on the CAD/CAM systems. This enables the user to communicate by manipulating different entities on the screen. Various display technologies are now available. Among the available technologies CRT is the most dominating. Working The electron beam produces a tiny, bright visible spot when it strikes the phosphor-coated screen. A colour CRT monitor has three different coloured phosphors arranged in triangular groups called triads. Each triad consists of three phosphors of colours red, green and blue. So there are three electron guns one for the primary colour red, one for the primary colour green, and one for the primary colour blue, each of which generates an electron beam to excite one of the phosphor dots. The CRT, thus, produces three overlapping images : one in red (R), one in green (G) and one in blue (B). This is the so-called RGB colour model. In order to ensure that an electron beam activates only phosphors of proper colour, a metal grid called shadow mask is used as shown in Figure

3 Different coloured light is generated at the screen position hit by the electron beam according to the phosphor type, which fades quickly in 10 to 60 microseconds. Fading time depends on the persistence (the time from the removal of the excitation to the moment when phospherescence has decayed to 10% of the initial light output) of phosphor coated inside the screen. In order to avoid flickering picture should be kept on the screen for a while, the picture should be redrawn before it disappears off the screen. This is called refreshing of the screen. Introduction to Computer Figure 1.2 One can imagine the CRT screen as a two-dimensional matrix which has m rows and n columns and this is usually referred as raster. Each row of the m rows (or the points on a horizontal line of the screen) is called scan-line. The number of points that can be displayed on the screen (n m) is defined as the resolution of the screen. The resolution is related to the size of the point that emits light after the contact of the electron beam. Displays can be divided into random and raster scan displays. They are discussed in details below. SAQ 1 (a) (b) (c) How the flickering of the image on the CRT monitor can be avoided? Why do you think resolution is related to the size of the point that emits light? Explain the principle of colour CRT monitor. (d) Define the following terms related to CRT monitors : (i) (ii) (iii) Refresh, Deflection system, and Focusing system Random Scan Display This device using CRT directs the electron beam only to the parts of the screen where a picture is to be drawn. This kind of device draws only a line on the screen given by its two end points as shown in Figure 1.3. That is why these devices are referred as vector displays (or calligraphic displays). In case of display device, it is not only an object which has to be drawn but it is also important to refresh it before it disappears in a given period of time depending on the persistence. No frame buffer exists for this kind of systems. When the object consists of too many lines, refreshing time might be longer than the persistence of the phosphor. This may lead to flickering of the picture on screen. In such systems, there may be a display processor that reads a display file. Display file consists of a sequence of graphics commands generated by the application program to refresh the screen. Advantages and disadvantages of the random scan display are as follows : 7

4 Introduction to Computer and Computer G hi Advantages Figure 1.3 : Random Scan Display Early computer displays: basically an oscilloscope. Control X, Y with vertical/horizontal plate voltage. Often used intensity as Z. Disadvantages Just by wire-frame, it is almost impossible to create images with shaded objects or areas filled with a given colour. In case of complex scenes flickering is visible Raster Scan Display A raster scan display device using CRT on the other hand directs the electron beam across the screen, one row at a time from top to bottom. In a raster scan display device, the screen is assumed to consists of a two-dimensional matrix. Remember that the term raster was used for a 2-D matrix. Each point of the screen corresponding to a matrix element is called pixel (picture element) as shown in Figure Figure 1.4 The refresh process must also be performed for raster scan monitors. Most television monitors are raster scan display devices : one scan-line at a time. Raster scan systems

5 uses a memory buffer called frame buffer (or refresh buffer) in which the intensities of the pixels are stored. Refreshing the screen is performed using the information (intensity of each pixel) stored in the frame buffer. Frame buffer can be treated as a two-dimensional array. Raster display devices read the frame buffer and activate the three phosphor dots in the triad according to the value read from the frame buffer. The entire contents of the frame buffer should be read and the pixels should be activated appropriately at a sufficiently high rate to avoid flickering of the picture. Depending on the persistence of the phosphor coated inside the screen, the monitors require different refresh rates. In some raster display devices, a frame buffer is displayed in two passes. This kind of refreshing is called interlaced. In the first pass the odd rows are refreshed, while the even rows are refreshed in the second pass. Thus, the screen is refreshed in half the time. Though this approach does not really increase the rate of refreshing the screen, but is quite effective in reducing the flicker. Introduction to Computer In the interlaced scan, the refresh cycle of 1/30 seconds is divided into two subcycles each lasting 1/60 seconds. The first subcycle displays the even-numbered scan lines and the second displays the odd-numbered scan lines. This technique produces an image with almost a refresh rate of 60 Hz. In non-interlaced scheme, entire scan lines are refreshed 60 times in a second or once every 1/60 seconds. Figure 1.5 The other property of raster display devices is the number of pixels that can be shown on the screen. This is called the resolution. On a display device with a resolution of , there are 768 rows (scan-lines) and in each scan line there are 1024 pixels. Aspect ratio, on the other hand, is the ratio of horizontal to vertical pixels. In the case of raster scan displays, rasterisation (scan conversion) which is the process of converting the geometric entities to pixels in frame buffer, finds the intensity values (black or white for monochrome displays) of the pixels and sets the corresponding elements (memory locations). Therefore, once the rasterisation is performed, the refreshing process reads the frame buffer and turns the electron beam on or off depending on the intensity stored in the frame buffer. The video controller controls the operation of the display device. Here, the frame buffer can be anywhere in the system memory. The picture to be displayed is put into the frame buffer a fixed area of the system memory. Figure 1.6 Video controller then retrieves the values stored in the frame buffer to set the intensity of the CRT beam for the corresponding pixels (Figure 1.6). 9

6 Introduction to Computer and Computer G hi Figure 1.7 Most of the present raster-scan systems contain a separate processor called as display processor. This processor performs graphics functions such as rasterisation and other raster operations. Frame buffer is not a fixed area in the system memory anymore. By a separate display processor, the disadvantages of a simple raster-scan system are avoided. Its advantages are : (i) Rasterisation is now performed faster since a special purpose processor is used. (ii) The CPU is not slowed down by the graphics tasks. (iii) The number of memory cycles available to the CPU is more now since video controller does not access the system memory for refreshing the screen. Raster CRT Advantages SAQ 2 (a) (b) (c) Allows solids, not just wireframes Leverages low-cost CRT technology (i.e., TVs) Bright, i.e. display emits light Disadvantages (d) (e) Requires screen-size memory array Discreet sampling (pixels), i.e. the lines are not straight anymore as in random scan display device. The lines now consist of pixels that may lead to jig jag Practical limit on size (call it 40 inches) Bulky Finicky (convergence, warp, etc.). Differentiate between random and raster scan display. What is the function of frame buffer? How is the scanning carried out in interlaced and non-interlaced displays? Which is more efficient? What is the aspect ratio of an image? Compute the frame buffer size for a CRT display terminal of a resolution with 96 pixels per inch. However, portable computers such as notebooks employ liquid crystal displays (LCDs) which are also raster devices. The screen must be refreshed to maintain the displayed picture. Now, we are going to explain it in detail. 10

7 1.2.4 Liquid Crystal Display LCDs are organic molecules, naturally in crystalline state, and they get liquified when excited by heat or E field. Crystalline state twists polarized light by 90º. Like lightemitting diode (LED) and gas-plasma technologies, LCDs allow displays to be much thinner than cathode ray tube (CRT) technology. LCDs consume much less power than LED and gas-displays because they work on the principle of blocking light rather than emitting it. An LCD is made with either a passive matrix or an active matrix (a polysilicate layer provides thin film transistors at each pixel, allowing direct pixel access and constant illumination) display grid. The active matrix LCD is also known as a thin film transistor (TFT) display. The passive matrix LCD has a grid of conductors with pixels located at each intersection in the grid. A current is sent across two conductors on the grid to control the light for any pixel. An active matrix has a transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more frequently, improving the screen refresh time. For example, mouse will appear to move more smoothly across the screen. Some passive matrix (Pixels are illuminated in scanline order like a raster display but the lack of phospherescence causes flicker) LCDs have dual scanning, meaning that they scan the grid twice with current in the same time that it took for one scan in the original technology. However, active matrix is still a superior technology. As stated before, display devices using CRT and LCD technology need to refresh the screen before the picture disappears. Introduction to Computer Figure 1.8 In the next sections, we will discuss the input and output devices. 1.3 INPUT DEVICE Various devices are available for data input on graphics workstations. Most systems have a keyboard and one or more additional devices specially designed for interactive input. There are different types of input devices Logical Devices Locator, to indicate a position and/or orientation Pick, to select a displayed entity To input a single value in the space of real numbers Keyboard, to input a character string Choice, to select from a set of possible actions or choices. Locator Devices Mouse 11

8 Introduction to Computer and Computer G hi 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. An optical sensor can also be used to detect the mouse motion. For using it, the mouse is moved over a special mouse pad that has a grid of horizontal and vertical lines. Movements across the lines in the grid are detected by the optical sensor. Since a mouse can be picked up and put down at another position without change in cursor movement, it is used for making relative changes in the position of the screen cursor. For signaling the execution of some operations, different types of buttons are included on the top of the mouse. Most general-purpose graphics systems now include a mouse and a keyboard as the major input devices. Additional devices can be included in the basic mouse design to increase the number of allowable input parameters. The Z mouse includes three buttons, a thumbwheel on the side, a trackball on the top, and a standard mouse ball underneath. This design provides six degrees of freedom to select spatial positions, rotations and other parameters. With the Z mouse, we can pick up an object, rotate it, and move it in any direction, or we can navigate our viewing position and orientation through a three-dimensional scene. Applications of the Z mouse included virtual reality, CAD and animation. 12 Trackball and Thumbwheel Joysticks Figure 1.9 : The Z Mouse (Courtesy : A joystick consists of a small, vertical lever (called the stick) mounted on a base that is used to steer the screen cursor around. Some joysticks are mounted on a keyboard. Most joysticks select screen positions with actual stick movement; while a few respond to the pressure on the stick. Figure 1.10 shows a movable joystick. The distance moved by the stick in any direction from its center position and the distance of the screen cursor movement in that direction are proportionate. Potentiometers mounted at the base of the joystick are used to measure the amount of movement, and the stick returns to the centre position when it is released due to the spring. One or more buttons can be programmed to act as input switches to signal certain actions once a screen position has been selected. In another type of movable joystick, the stick is used to activate switches that cause the screen cursor to move at a constant rate in the selected direction. Eight switches, arranged in a circle, are sometimes provided, so that the stick can select any one of eight directions for cursor movement.

9 Pressure-sensitive joysticks have a non-movable stick (isometric joysticks). In case of pressure sensitive joystick, strain gauges are used to measure the pressure and convert it into movement of the cursor in the specified direction. Introduction to Computer Figure 1.10 : A Movable Joystick (Courtesy : Two thumbwheels are usually required to control the screen cursor in its horizontal and vertical position respectively. As the name implies, a trackball is a ball that can be rotated with the fingers or palm of the hand (Figure 1.11) to produce screen-cursor movement. Potentiometers, attached to the ball, are used to measure the direction and amount of rotation. Trackballs are often mounted on keyboards or other devices such as the Z mouse as explained earlier. Figure 1.11 : Sketch of a Trackball (Courtesy : While a trackball is a two-dimensional positioning device, a spaceball provides six degrees of freedom. Unlike the trackball, a spaceball does not actually move. The amount of pressure applied to the spaceball is measured by the strain gauges and it is to provide input for spatial positioning and orientation as the ball is pushed or pulled in various directions. Spaceballs are used for three-dimensional positioning and selection operations in virtual-reality systems, modeling, animation, CAD and other applications. Touch Panels In case of touch panels, displayed objects or screen positions is allowed to be selected with the touch of a finger. A typical application of touch panels is for the selection of processing options that are represented with graphical icons. Some systems, such as the plasma panels, are designed with touch 13

10 Introduction to Computer and Computer G hi Pick Device Light Pens screens. Other systems can be adapted for touch input by fitting a transparent device with a touch-sensing mechanism over the video monitor screen. Touch input can be recorded using optical, electrical, or acoustical methods. Figure 1.12 shows the light pen used to select screen positions by detecting the light coming from points on the CRT screen. These are sensitive to the short burst of light emitted from the phosphor coating at the instant the electron beam strikes a particular point. Other light sources, such as the background light in the room, are usually not detected by a light pen. An activated light pen, pointed at a spot on the screen as the electron beam lights up that spot, generates an electrical pulse that causes the coordinate position of the electron beam to be recorded. As with cursor-positioning devices, recorded light-pen coordinates can be used to position an object or to select a processing option. Keyboards Figure 1.12 : A Light Pen Activated with a Button Switch (Courtesy : Although light pens still exist, but due to several disadvantages they are not as popular as other input devices that have been developed. For example, when a light pen is pointed at the screen, part of the screen image is obscured by the hand and pen. And prolonged use of the light pen can cause arm fatigue. Also, light pens require special implementations for some applications because they cannot detect positions within back areas. To be able to select positions in any screen area with a light pen, we must have some non-zero intensity assigned to each screen pixel. In addition, light pens sometimes give false readings due to background lighting in a room. An alphanumeric keyboard for a graphics system is used primarily as a device for entering text strings. Keyboards can also be provided with features to facilitate entry of screen coordinates, menu selections, or graphics functions. The keyboard is an efficient device for inputing such monographic data such as picture labels associated with a graphics display. Common features on general-purpose keyboards are cursor-control keys. Functions keys are used 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. Additionally, a numeric keypad is often included on the keyboards for fast entry of numeric 14

11 data. Some keyboards have other types of cursor-positioning devices, such as a trackball or joystick. Introduction to Computer For specialized applications, input to a graphics application may come from a set of buttons, dials, or switches that select data values or customized graphics operations. Buttons and switches are often used to input predefined functions, and dials are common devices for entering scalar values. Figure 1.13 : Ergonomically Designed Keyboard with Removable Palm Rests (Courtesy : Other Input Devices Data Glove Digitizers 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. The transmitting and receiving antennas can each be structured as a set of three mutually perpendicular coils, forming a three-dimensional cartesian coordinate system. Input from the glove can be used to position or manipulate objects in a virtual scene. A two-dimensional projection of the scene can be viewed on a video monitor, or a three-dimensional projection can be viewed with a handset. A common device for drawing, painting, or interactively selecting coordinate positions on an object is a digitizer. These devices can be used to input coordinating values in either a two-dimensional or a three-dimensional space. Typically, a digitizer is used to scan over a drawing or object and to input a set of discrete coordinate positions, which can be joined with straight-line segments to approximate the curve or surface shapes. One type of digitizers is the graphics tablet (also referred to as a data tablet) which is used to input two-dimensional coordinates by activating a hand cursor or stylus at selected positions on a flat surface. A hand cursor contains cross hairs for sighting positions, while a stylus is a pencil-shaped device that is pointed at positions on the tablet. Tablet size varies from 12 by 12 inches for desktop models to 44 by 60 inches or larger for floor models. Graphics table provide a highly accurate method for selecting coordinate positions, with an accuracy that varies from about 0.2 mm on desktop models to about 0.5 mm or less on longer models. Many graphics tablet are constructed with a rectangular grid of wires embedded in the tablet surface. Electromagnetic pulses are generated in sequence along the wires, and an electric signal is induced in a wire coil in an activated stylus or hand cursor to record a tablet position. Depending on the technology, either signal strength, coded pulses, or phase shifts can be used to determine the position on the tablet. 15

12 Introduction to Computer and Computer G hi Acoustic (or sonic) tablet use sound waves to detect a stylus position. Either strip microphones or point microphones can be used to detect the sound emitted by an electrical spark from a stylus tip. The position of the stylus is calculated by timing the arrival of the generated sound at the different microphone positions. An advantage of two-dimensional acoustic tablets is that the microphones can be placed on any surface to form the tablet work area. This can be convenient for various applications such as digitizing drawings in a book. Three-dimensional digitizers use sonic or electromagnetic transmissions to record positions. One electromagnet transmission method is similar to that used in the data glove. A coupling between the transmitter and receiver is used to compute the location of a stylus as it moves over the surface of an object. As the points are selected on a non-metallic object, a wireframe outline of the surface is displayed on the compute screen. Once the surface outline is constructed, it can be shaded with lighting effects to produce a realistic display of the object. Resolution of this system is from 0.8 mm to 0.08 mm, depending on the model. Image Scanners Drawings, graphs, colour 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 colour are then recorded and stored in an array. Once we have the internal representation of a 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. Some scanners are able to scan either graphical representations or text, and they come in a variety of sizes and capabilities. Voice Systems Speech recognizers are used in some graphics workstations as input devices to accept voice commands. The voice-system input can be used to initiate graphics operations or to enter data. These systems operate by matching an input against a predefined dictionary of words and phrases. A dictionary is set up for a particular operator by having the operator speak the command words to be used into the system. Each word is spoken several times, and the system analyzes the word and establishes a frequency pattern for that word in the dictionary along with the corresponding function to be performed. Later, when a voice command is given, the system searches the dictionary for a frequency-pattern match. Voice input is typically spoken into a microphone mounted on a headset (Figure 1.14). The microphone is designed to minimize input of other background sounds. If a different operator is to use the system, the dictionary must be reestablished with that operator s voice patterns. Voice systems have some advantages over other input devices, since the attention of the operator does not have to be switched from one device to another to enter a command. A passive graphics device simply draws pictures under computer control; i.e. it allows the computer to communicate graphically with the user. Examples are dot matrix printers, an electrostatic plotter, pen and ink plotters, film recorders and storage tube refresh and raster CRT displays. An active graphics device allows the user to communicate with the computer graphically. Generally, this implies that the user is supplying coordinate information in some indirect manner, i.e. by means other than typing the appropriate numbers. Usually an active graphics device has the ability to reposition the cursor and read its new position. 16

13 Introduction to Computer Figure 1.14 : A Speech-recognition System (Courtesy : OUTPUT DEVICES Hardcopy images are images that can be taken away from the computer and communicated to your audience without any computer mediation. This can be done in several ways, but the basic idea is that any kind of medium that can carry an image is a candidate for hardcopy. Each of these media has its own issues in terms of its capability and how you must prepare your images for the medium. Now, you are aware about the input devices as studied in the previous section. In this section, we will discuss various hard copy technologies. Before discussing the actual hard copy devices it is important to understand the various factors which will effect the image quality achievable by these devices. Some of the factors are explained below. (i) Dot/Spot Size : Diameter of a single dot. (ii) Addressability : Number of individual dots/inch that can be created. It differs in horizontal and vertical directions. It can be calculated in x direction as the reciprocal of the distance between centres of dots at addresses (x, y) and (x + 1, y). Similarly, it can be defined for y, i.e. (x, y) and (x, y + 1). Inter-dot distance is the reciprocal of addressability. (iii) Resolution : As discussed earlier, it is the numbers of distinguishable lines/inch that a device can create and white line can be distinguished by observers. Once we have understood the terminology defined above we can further see their effect in the image quality. For example, if dot size is greater than the interdot distance, then overlapping smooth shapes can be created. However, at the same time it is to be remembered that smaller dot size allows the viewing of finer detail. We can also see that the resolution is related to the dot size and cannot be greater than addressability. In this case, it is clear that resolution is also dependent on the cross-sectionals intensity distribution of a spot. Spot having sharply delineated edges yields higher resolution than the spot whose edges trail off. After going through the above terminology it will easy to understand now the working of various output devices and how it is going to affect the quality Type of Devices Dot Matrix Printers These printers are raster output devices requiring scan conversion of vector images prior to printing. It has 7 to 24 thin pins of stiff pieces of wires which can be individually fired to strike a ribbon against the paper. Thin wires are arranged in the form of matrix on a printer head that moves horizontally across the paper. Printing mechanism involves the electromagnetic release of one or more of spring steel mounted wires. Therefore, the addressability of a dot-matrix printer need not be limited by the physical distance between pins on the print head. There can be two columns of pins, offset vertically by one-half the interpin spacing. Alternatively, two passes over the paper can be used to achieve the same effect, by 17

14 Introduction to Computer and Computer G hi advancing the paper by one-half the interpin spacing between the first and second passes. Coloured ribbons can be used to produce colour hardcopy. Two approaches are possible. The first is using multiple print heads, each head with a different colour ribbon. Alternatively and more commonly, a single print head is used with a multicoloured ribbon. Laser Printers In this, positively charged rotating drum coated with selenium is scanned by a laser beam. The areas where the laser beam hits loose their charge and the remaining portions remain positively charged. These are the areas where the copy is to be black. A negatively charged powdered toner adheres to the positive areas of the drum which is then transferred to black paper to form the copy. In coloured printers, the process is repeated for each primary colour. Inkjet Printers These types of printers spray cyan, magenta, yellow and black ink onto paper. In most cases, the inkjet are mounted on a head in the printer like mechanism. The print head moves across the page to draw once scan line, returns while the paper advances by one interscan line spacing and draws the next scan line. There is one more approach in which the paper is wrap around the drum which rotates rapidly while the printer head moves slowly around the drum. In inkjet printers, all the colours are deposited simultaneously. Thermal Transfer Printers It is a raster hard copy device. Pigments from coloured wax paper are transferred to plain paper by finely spaced heating nibs. Both the papers are drawn together over the strip of heating nibs which are selectively heated to cause the pigment transfer. Nibs can be heated and cooled rapidly and single colour hard copy image can be created in less than one minute. Thermal Sublimation Dye Transfer Printers These are similar to thermal transfer printers in working except the heating and dye transfer process permits 256 intensity each of cyan, magenta and yellow. The process is slower than wax transfer, but the quality is near photographic. Electrostatic Plotters In these, a negative charge is placed on those parts of white papers that are to be black, then flows positively charged black toner over the paper. This leads to the adhering of the toner particles to the paper where the charge has been deposited. In colour electrostatic plotters, the three subtractive primary colours cyan, magenta and yellow and black are applied to the medium in four successive passes. The medium is rebound between applications of each colour. Others use multiple heads to deposit all the colours in the single pass. Electrostatic plotters are often faster than pen plotters. On the other hand, pen plotters create images with higher contrast, since they deposit the toner even in areas where the paper is not negatively charged. Pen Plotters It moves the pen over a piece of paper in random, vector drawing style. To draw a line first the pen is positioned at the start of the line and then lower to the paper, moved in a straight line to the end point of the line, raised, and move to the start of the next line. There are two basic varieties of pen plotters flat bag plotters and drum plotters. In flat bag plotters, a sheet of paper is spread out on a table and held out by the electrostatic charge and pen moves in x-y direction on this sheet, whereas in drum plotters paper moves along one axis and pen moves along the other axis. 18

15 SAQ 3 In pen plotters, plotting speed depends on the acceleration and the velocity with which pen can be moved. Pen acceleration is partially a function of the mass of the plot head. There are some other types of devices like camera and cycolar. (a) (b) (c) (d) (e) (f) (g) What you will see on a TV screen, when the electricity suddenly goes off at night? Explain the principle of LCD display. What are the various technologies used in the LCD devices? List the different types of input devices. Write a brief note about mouse as a locating device. Explain the differences between mouse and track ball. What are the limitations of light pen as a locating device? Write a brief note about digitizer as a locating device. (h) Give brief writeup on any one of the following types of output devices : (i) (ii) (iii) Dot matrix printer, Laser printer, and Electrostatic plotter. Introduction to Computer 1.5 SUMMARY This unit covers the major hardware features of computer graphics systems like video monitors, hard copy devices, keyboard and other devices for graphics input and output. Vector and raster display devices have been elaborated. Raster refresh monitor is based on television technology. It uses a frame buffer to store intensity information for each pixel. Vector displays construct pictures by drawing lines between specified points. Input devices like keyboard, trackballs and joysticks etc., are used to position the screen cursor. In virtual reality, data gloves are used. Other input devices include image scanners, digitizers, touch panels, light pens and voice systems. Hard copy devices for graphics workstations include standard printers and plotters. Printing method includes dot matrix, laser, inkjet, electrostatic and electrothermal. Plotter method includes pen plotting and combination of printer-plotter devices. Exercise 1 (a) (b) (c) (d) What is the resolution of an image? Compute a resolution of a 3 3 inch image that has pixels. What is persistence? What is the function of the control electrode in a CRT? 19