Unit 1 notes
Download as DOC, PDF0 likes127 views
The document discusses various topics in computer graphics including: 1. Computer Aided Design (CAD) is used to design products like buildings, vehicles, and aircraft using computer modeling and visualization. 2. Presentation graphics are used to illustrate reports using charts, graphs, and diagrams to summarize data. 3. Computer art uses 3D modeling, texture mapping, and other techniques in both fine art and commercial design applications like logos. 4. Entertainment uses computer graphics for movies, TV, videos, and animation through techniques like morphing objects.
Unit 1 notes
- 1. Computer Aided Design (CAD) 1. Used to design the buildings, automobiles, aircraft, watercraft, spacecraft, computers, textiles and many other products. 2. CG is a useful tool for generating the architects, drawing and visualizing structures 3. A computer takes the data about building and makes various images of the building from different angles 4. Animations are used in CAD applications 5. Real time animations using wire frame displays on a video monitor are useful for testing performance of vehicles and also to see the interior of the vehicle and to watch the behavior of inner components during motion. Presentation Graphics 1. Used to produce illustrations for reports 2. Commonly used to summarize financial, statistical, mathematical, scientific and economic data for research reports, managerial reports, consumer information bulletin and other reports. 3. Typical examples are bar charts, line graphs, surface graphs, pie charts and other display showing relationship between multiple parameters. 4. Time charts and task networks layouts are used in project management to schedule and monitor the progress of projects. Computer Art 1. CG is used in both fine arts and commercial arts applications 2. Artists use various computer methods such as special-purpose hardware, artist’s paintbrush program(Lumena), other paint packages(Pixelpaint, SuperPaint), symbolic mathematics packages(Mathematica), CAD packages, desktop publishing software’s, animation packages that provide facilities for designing object shapes and specifying object motions. 3. Fine artists use other computer technologies to produce images. He uses a combination of three-dimensional modeling packages, texture mapping, drawing programs and CAD software. 4. These methods are also applied in commercial art for logos and other designs, page layouts combining text and graphics, TV advertising spots and other areas. 5. A common graphics method employed in many commercials is morphing, where one object is transformed into another. This method is used in TV commercials to turn oil can into automobile engine, etc. Entertainment Computer Graphics methods are commonly used in making motion pictures, music videos, television shows and cartoon animation films. 2. Many TV series regularly employ computer graphics methods. 3. Music videos use graphics in several ways. Graphics objects can be combined with live action, or graphics and image processing techniques can be used to produce a transformation of one person or object into another(morphing) Education and Training 1. Computer generated models like physical systems, financial systems and economic systems used as education aids 2. Models of physical system, physiological system, population trends or equipment, can help trainees to understand the operation of the system. 3. Examples of some specialized systems are the simulators for practice sessions or training of ship captains, aircraft pilots, heavy-equipment operators, and air traffic-control personnel. 4. Various educational pictures with animations are used to present better understanding for learning with animations Image processing 1. Image processing applies techniques to modify or interpret existing pictures, such as photographs and TV scans. 2. Two principle applications (1) Improving picture quality (2) Machine perception of visual information used in robotics
- 2. 3. To digitize the shading and color, interesting, sharpen, improve the contrasting scanning image and to transfer them to monitor or screen or visual display unit. 4. These techniques are used in commercial art applications and to analyze satellite photos of the earth and photos of galaxies. 5. Medical applications also make extensive use of image-processing techniques (1) Tomography is a technique of X-ray photograph that allows cross-sectional views of physiological system to be displayed. (2) Computerized Axial Tomography (CAT) is used to compose the 3D model of the brain by taking x-ray of it, which can be used to detect problems like brain tumor etc (3) Ultrasonic are used to generate digital data. (4) Nuclear medicine scanners collect digital data from radiation emitted from ingested radionuclide and plot color-coded images. (5) Computer-aided surgery Graphical User Interface 1. A major component of a graphical interface is a window manager that allows a user a display multiple-window area. 2. Each window can contain a different process that can contain graphical or nongraphical displays. 3. Interfaces also display menus and icons for selection of processing options or parameter values. (1) An icon is a graphical symbol that is designed to look like the processing option it represents. (2) Menus contain lists of textual descriptions and icons. The challenge to computer graphics is to make that virtual world look real, sound real, move and respond to interaction in real time, and even feels real. Video-Display devices 1. Cathode Ray Tubes 2. Raster scan Display 3. Vector scan/Random scan Display 4. Color CRT monitors 5. Direct View Storage Tubes 6. Flat panel Display 7. Three-Dimensional Viewing Devices Cathode Ray Tube Typically the primary output device in a graphics system is a video monitor. The operation of most video monitors is based on the standard Cathode-ray-tube design. Basic operation is as shown on Cathode Ray Tubes
- 3. · A beam of electrons emitted by an electron gun, pass through focusing and deflection systems that direct the beam towards specified positions on the phosphor coated screen The phosphor then emits a small spot of light at each position contacted by the electron Beam · The light emitted by phosphorous fades very rapidly to keep glowing is done by redraw the picture repeatedly by quickly directing the electron beam back over the same points is called as “refresh CRT” · Heated metal cathode and Control grid are the main components of electron gun (figure 1) · The heat is supplied to the cathode through current passing in coil of wire called filament · This makes electrons to be “boiled off” the hot cathode surface · The free negatively charged electrons inside CRT are accelerated towards the Phosphorous coated by high positive voltage generated by positively charged metal coating on the inside of CRT · Intensity of electron beam is controlled by setting voltage levels in the control grid, which is fit over the cathode · A high negative voltage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of the control grid structure. A smaller negative voltage on the control grid simply decreases the number of electrons passing through. · Since the amount of light emitted by the phosphor coating depends on the number of electrons striking the screen, the brightness of a display is controlled by varying the voltage on the control grid. The intensity level is specified for individual screen positions with graphics software commands. · A beam of electrons emitted by an electron gun, pass through focusing and deflection systems that direct the beam towards specified positions on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. The light emitted by phosphorous fades very rapidly to keep glowing is done by redraw the picture repeatedly by quickly directing the electron beam back over the same points is called as “refresh CRT”. · The focusing system in a CRT is needed to force the electron beam to converge into a small spot as it strikes the phosphor. Otherwise, the electrons would repel each other, and the
- 4. beam would spread out as it approaches the screen. Focusing is performed with either electric or magnetic fields. Focusing Meaning Electro static With electro static focusing, the beam pass through positively charged metal cylinder focusing that forms electrostatic and it focus the beam at the centre of the screen. (as shown in figure 2) Magnetic With magnetic field set up by a coil mounted around the outside of the CRT envelope. focusing (as shown in figure 3) Different kinds of phosphors are available for use in a CRT. The properties with which the phosphors vary are: Term Description Persistence Resolution Aspect ratio Persistence is defined as the time it takes the emitted light from the screen to decay to one tenth of its original intensity. The maximum number of points displayed on the CRT screen or the number of points/centimeter that can be plotted horizontally and vertically. The ratio of vertical points to horizontal points necessary to produce equal length line in both directions on the screen.
- 5. Raster-Scan Display o The most common type of graphics monitor employing a CRT is the Raster-scan displays, based on television technology o JPG images are raster based o Light occurs when an electron beam stimulates a phosphor. In Raster scan, the electron beam from electron gun is swept horizontally across the phosphor one row at time from top to bottom.(figure 4) o After each horizontal sweep the beam is moved. o After the bottom line is swept, the beam returns to the top and the sweep process begins again. o As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots o Picture definition is stored in a memory area called the refresh buffer or frame buffer o Each screen point is called as “pixel” o This memory area holds the set of intensity values for all the screen points o This is part of the system memory o The stored intensity values are then retrieved from frame buffer and painted on the screen one row at a time o Intensity range for pixel position depends on capability of the raster system o In black and white system, the point on screen is either on or off o Only one bit is needed to control the intensity of the screen o In case of color systems, 2 bits are required o One to represent ON (1), another one is OFF (0). o Refreshing on raster scan is carried out at the rate of 60 to 80 frames per seconds PixMap The frame buffer with multiple bits per pixel (for color display). BitMap The frame buffer with One per pixel (for Black and White display). Horizontal Retrace Horizontal retrace of the electron beam means the return to the left of the screen. After refreshing each Sean line. Vertical Retrace
- 6. If it returns to the top left corner of the screen to begin in the next frame called “vertical retrace”. o On some raster-scan systems (and in TV sets), each frame is displayed in two passes using an interlaced refresh procedure. In the first pass, the beam sweeps across every other scan line from top to bottom. Then after the vertical retrace, the beam sweeps out the remaining scan lines (Figure 5). o Interlacing of the scan lines in this way allows us to see the entire screen displayed in one- half the time it would have taken to sweep across all the lines at once from top to bottom. o Interlacing is primarily used with slower refreshing rates. On an older, 30 frame per-second, noninterlaced display, for instance, some flicker is noticeable. But with interlacing, each of the two passes can be accomplished in 1/60th of a second, which brings the refresh rate nearer to 60 frames per second. This is an effective technique for avoiding flicker, providing that adjacent scan lines contain similar display information. o Frame buffer size = to store bits per pixel* resolution Advantages ¾ High degree realism is achieved in picture with the aid of advanced shading and hidden surface technique. ¾ Decreasing memory costs have made raster systems popular. ¾ Computer monitors and TVs use this method Disadvantages ¾ Raster displays have less resolution. ¾ The lines produced are ziz-zag as the plotted values are discrete. Random-Scan Display In a Random scan system, also called vector, stoke writing, or calligraphic the electron beam directly draws the picture. o A pen plotter operates in a similar way and is an example of a random-scan, hard-copy device. o When operated as a random-scan display unit, a CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn.
- 7. o 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). o Here the electron gun of a CRT illuminates points and / or straight lines in any order. o Refresh rate on a random-scan system depends on the number of lines to be displayed. o Picture definition stored as a set of line drawing commands in an area of memory called “refresh display file” or also called as display list or display program or refresh buffer o This displays to draw all the component lines of picture 30 to 60 frames/second o This system is designed for line drawing applications o Vector displays produces smooth line drawings but raster produces jagged lines that are plotted points To display a given picture, the system cycles through the set of commands in the display file, drawing each component line in turn o After all line drawing commands have been processed, the system cycles back to the first line drawing command in the list o Random scan suitable for applications like engineering and scientific drawings o Graphics patterns are displayed by directing the electron beam along the component lines of the picture o A scene is then drawn one line at a time by positioning the beam to fill in the line between specified end points Advantages • Very high resolution, limited only by monitor. • Easy animation, just draw at different position. • Requires little memory (just enough to hold the display program). Disadvantages • Requires intelligent electron beam, i.e., processor controlled. • Limited screen density before have flicker, can’t draw a complex image. • Limited color capability (very expensive). Color CRT Monitors o Colored pictures can be displayed by using a combination of phosphorous that emit different colored light o By combining the emitted light from the different phosphors, a range of colors can be generated.
- 8. o The two basic techniques for producing color displays with a CRT are the beam-penetration method and the shadow-mask method. Beam-penetration method o The beam-penetration method for displaying color pictures has been used with random-scan monitors. o Two layers of phosphor, usually red and green, are coated onto the inside of the CRT screen, and the displayed color depends on how far the electron beam penetrates into the phosphor layers. o A beam of slow electrons excites only the outer red layer. A beam of very fast electrons penetrates through the red layer and excites the inner green layer. At intermediate beam speeds, combinations of red and green light are emitted to show two additional colors, orange and yellow. o The speed of the electrons, and hence the screen color at any point, is controlled by the beam-acceleration voltage. o Beam penetration has been an inexpensive way to produce color in random-scan monitors, but only four colors are possible, and the quality of pictures is not as good as with other methods. Shadow-mask methods o Shadow-mask methods are commonly used in raster scan systems (including color TV) because they produce a much wider range of colors than the beam penetration method. o A shadow-mask CRT has three phosphor color dots at each pixel position. o One phosphor dot emits a red light, another emits a green light, and the third emits a blue light. o This type of CRT has three electron guns, one for each color dot, and a shadow-mask grid just behind the phosphor-coated screen. o Figure 7 illustrates the delta-delta shadow-mask method, commonly used in color CRT systems. o The three electron beams are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns. o When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small color spot on the screen. o The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask. o Another configuration for the three electron guns is an in-line arrangement in which the three electron guns, and the corresponding red-green-blue color dots on the screen, are aligned along one scan line instead of in a triangular pattern. o This in-line arrangement of electron guns is easier to keep in alignment and is commonly used in high-resolution color CRTs. o Color variations are obtained in a shadow-mask CRT by varying the intensity levels of the three electron beams. By turning off the red and green guns, we get only the color coming from the blue phosphor. o Other combinations of beam intensities produce a small light spot for each pixel position, since our eyes tend to merge the three colors into one composite. The color we see depends on the amount of excitation of the red, green, and blue phosphors. A white (or gray) area is the result of activating all three dots with equal intensity. Yellow is produced with the green and red dots only, magenta is produced with the blue and red dots, and cyan shows up when blue and green are activated equally. o In some low-cost systems, the electron beam can only be set to on or off, limiting displays to eight colors. More sophisticated systems can set intermediate intensity levels for the electron beams, allowing several million different colors to be generated. Color graphics systems can be designed to be used with several types of CRT display devices. Some inexpensive home-computer systems and video games are designed for use with a color TV set and an RF (radio-frequency) modulator. Composite monitors are adaptations of TV sets that allow bypass of the broadcast circuitry. These display devices still require that the picture information be combined, but no carrier signal is needed.
- 9. Picture information is combined into a composite signal and then separated by the monitor, so the resulting Video Display Devices picture quality is still not the best attainable. Color CRTs in graphics systems are designed as RGB monitors. These monitors use shadow-mask methods and take the intensity level for each electron gun (red, green, and blue) directly from the computer system without any intermediate processing. High-quality raster-graphics systems have 24 bits per pixel in the frame buffer, allowing 256 voltage settings for each electron gun and nearly 17 million color choices for each pixel. An RGB color system with 24 bits of storage per pixel is generally referred to as a full-color system or a true-color system. Direct-View Storage Tubes o A direct-view Storage Tube (DVST) stores the picture information as a charge distribution just behind the phosphor-coated screen. o Two electron guns are used in a DVST. o One, the primary gun, is used to store the picture pattern; o the second, the flood gun, maintains the picture display. o An alternative method for maintaining a screen image is to store the picture information inside the CRT instead of refreshing the screen. A DVST monitor has both disadvantages and advantages compared to the refresh CRT. Advantages · It has a flat screen. · Refreshing of screen is not required. · Because no refreshing is needed, very complex pictures can be displayed at very high resolutions without flicker. Disadvantages · This has poor contrast. · Performance is inferior to the refresh CRT. · Selective or part erasing of screen is not possible. · They ordinarily do not display color and that selected parts of a picture cannot he 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. Flat-Panel Displays
- 11. o The term Flat-panel display refers to a class of video devices that have reduced volume, weight, and power requirements compared to a CRT. o A significant feature of flat-panel displays is that they are thinner than CRTs, and we can hang them on walls or wear them on our wrists. Current uses for flat-panel displays include small TV monitors, calculators, pocket video games, laptop computers, armrest viewing of movies on airlines, as advertisement boards in elevators, and as graphics displays in applications requiring rugged, portable monitors. o Flat-panel displays are of two categories: o Emissive displays e.g. Plasma panel, LEDs etc o Nonemmissive displays. e.g. LCD The emissive displays (or emitters) are devices that convert electrical energy into light. Plasma panels, thin-film electroluminescent displays, and Light-emitting diodes are examples of emissive displays. Flat CRTs have also been devised, in which electron beams arts accelerated parallel to the screen, then deflected 90' to the screen. But flat CRTs have not proved to be as successful as other emissive devices. Nonemmissive displays (or nonemitters) use optical effects to convert sunlight or light from some other source into graphics patterns. The most important example of a Nonemmissive flat-panel display is a liquid-crystal device. o Plasma panels, also called gas-discharge displays, are constructed by filling the region between two glass plates with a mixture of gases that usually includes neon. o A series of vertical conducting ribbons is placed on one glass panel, and a set of horizontal ribbons is built into the other glass panel (Figure 8). o Firing voltages applied to a pair of horizontal and vertical conductors cause the gas at the intersection of the two conductors to break down into glowing plasma of electrons and ions.
- 12. o Picture definition is stored in a refresh buffer, and the firing voltages are applied to refresh the pixel positions (at the intersections of the conductors) 60 times per second. o Alternating-current methods are used to provide faster application of the firing voltages, and thus brighter displays. o Separation between pixels is provided by the electric field of the conductors. o One disadvantage of plasma panels has been that they were strictly monochromatic devices, but systems have been developed that are now capable of displaying color and grayscale. o Thin-film electroluminescent displays are similar in construction to a plasma panel. o The difference is that the region between the glass plates is filled with a phosphor, such as zinc sulfide doped with manganese, instead of a gas (Figure 9). o When a sufficiently high voltage is applied to a pair of crossing electrodes, the phosphor becomes a conductor in the area of the intersection of the two electrodes. o Electrical energy is then absorbed by the manganese atoms, which then release the energy as a spot of light similar to the glowing plasma effect in a plasma panel. o Electroluminescent displays require more power than plasma panels, and good color and gray scale displays are hard to achieve. o A third type of emissive device is the light-emitting diode (LED). A matrix of diodes is arranged to form the pixel positions in the display, and picture definition is stored in a refresh buffer. o As in scan-line refreshing of a CRT, information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display. o LCDs are commonly used in small systems, such as calculators and portable, laptop computers. o These Nonemmissive devices produce a picture by passing polarized light from the surroundings or from an internal light source through a liquid-crystal material that can be aligned to either block or transmit the light. Three-Dimensional Viewing Devices o Graphics monitors for the display of three-dimensional scenes have been devised using a technique that reflects a CRT image from a vibrating, flexible mirror.
- 13. o As the varifocal mirror vibrates, it changes focal length. These vibrations are synchronized with the display of an object on a CRT so that each point on the object is reflected from the mirror into a spatial position corresponding to the distance of that point from a specified viewing position. This allows us to walk around an object or scene and view it from different sides. o Such systems have been used in medical applications to analyze data from ultrasonography and CAT scan devices, in geological applications to analyze topological and seismic data, in design applications involving solid objects, and in three-dimensional simulations of systems, such as molecules and terrain. o Another technique for representing three-dimensional objects is displaying stereoscopic views. This method does not produce true three-dimensional images, but it does provide a three-dimensional effect by presenting a different view to each eye of an observer so that scenes do appear to have depth. o To obtain a stereoscopic projection, we first need to obtain two views of a scene generated from a viewing direction corresponding to each eye (left and right). We can construct the two views as computer-generated scenes with different viewing positions, or we can use a stem camera pair to photograph some object or scene. When we simultaneous look at the left view with the left eye and the right view with the right eye, the two views merge into a single image and we perceive a scene with depth. o Stereoscopic viewing is also a component in virtual-reality systems, where users can step into a scene and interact with the environment. A headset containing an optical system to generate the stereoscopic views is commonly used in conjunction with interactive input devices to locate and manipulate objects in the scene. A sensing system in the headset keeps track of the viewer's position, so that the front and back of objects can be seen as the viewer "walks through" and interacts with the display. o An interactive virtual-reality environment can also be viewed with stereoscopic glasses and a video monitor, instead of a headset. This provides a means for obtaining a lower cost virtual- reality system. Input devices An input device is that is used to interact with or provide data to the computer. Various devices are available for data input on graphics workstations. The most common input devices are the mouse and keyboard. However, additional devices like trackball, spaceball, joysticks, digitizers, button boxes are specially designed for interactive input. Some other input devices used in particular applications are data gloves, touch panels, image scanners and voice systems. KEYBOARD 1. An alphanumeric keyboard on a graphics system is used primarily as a device for entering text strings 2. Keyboard is an efficient device for inputting such as non graphic data as picture labels associated with a graphic display Cursor keys and function keys are common features on general purpose keyboards 4. Function keys allow users to enter frequently used operations in a single keystroke 5. Cursor control keys can be used to select displayed objects or coordinate positions by positioning the screen cursor 6. Other types of cursor pointing devices such as track ball or joystick are included on some keyboards 7. A numeric keypad is often included on the keyboard for fast entry of numeric data 8. 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 9. Buttons and switches are often used to input predefined functions and dials are common devices for entering scalar values 10. Real numbers within some defined range are selected for input with dial rotations 11. Potentiometers are used to measure dial rotations, which are then converted to deflection voltages for cursor movement MOUSE 1. 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
- 14. 2. Another method for detecting mouse motion is with an optical sensor 3. For these systems, the mouse is moved over a special mouse pad that has a grid of horizontal and vertical lines 4. The optical sensor detects movement across the lines in the grid 5. 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 6. One, two or three buttons are usually included on the top of the mouse for signaling the execution of some operation such as recording cursor position or invoking a function 7. Additional devices can be included in the basic mouse design to increase the number of allowable input parameters 8. The z mouse includes 3 buttons, a thumb wheel on the side, a track ball on the top and a standard mouse ball underneath 9. With 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 3-D scene 10. Applications of z-mouse include virtual reality, CAD and animation Track ball and Space ball 1. A track ball is a ball that can be rotated with fingers or palm of the hand to produce screen cursor movement 2. Potentiometers, attached to the ball measure the amount and direction of rotation 3. Track balls are often mounted on keyboards or other devices such as the z-mouse 4. While a track ball is a 2-D positioning device, a space ball provides six degrees of freedom 5. Unlike the track ball, 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 6. Space balls are used for 3-D positioning and selection operations in virtual reality systems, modeling, animation, CAD and other applications. Joystick 1. A Joystick has a small, vertical lever (called the stick) mounted on the base and used to steer the screen cursor around 2. It consists of two potentiometers attached to a single lever 3. Moving the lever changes the settings on the potentiometers 4. The left or right movement is indicated by one potentiometer and forward or back movement is indicated by other potentiometer Data Glove The data glove is used to grasp a virtual object 2. It is constructed with a series of sensors that detect hand and finger motions 3. Each sensor is a short length of f fiber optic cable, with a light-emitting diode at one end and a phototransistor at the other end 4. The surface of a cable is roughened in the area where it is to be sensitive to bending 5. When the cable is fixed, some of the LED’s light is lost, so less light is received by the phototransistor Digitizers 1. A common device for drawing, painting, or interactively selecting coordinate positions on an object is a digitizer. 2. These devices can be used to input coordinate values in either a two-dimensional or a three- dimensional space. 3. 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. 4. One type of digitizer 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. 5. The artist's digitizing system uses electromagnetic resonance to detect the three-dimensional position of the stylus. This allows an artist to produce different brush strokes with different pressures on the tablet surface. Tablet size varies from 12 by 12 inches for desktop models to 44 by 60 inches or larger for floor models.
- 15. 6. Graphics tablets provide a highly accurate method for selecting coordinate positions, with an accuracy that varies from about 0.2 mm on desktop models to about 0.05 mm or less on larger models. 7. Many graphics tablets are constructed with a rectangular grid of wires embedded in the tablet surface. 8. 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. 9. Depending on the technology, signal strength, coded pulses, or phase shifts can be used to determine the position on the tablet. 10. Acoustic (or sonic) tablets 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. Image Scanners 1. 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. 2. The gradations of gray scale or color are then recorded and stored in an array. 3. 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. 4. Some scanners are able to scan either graphical representations or text, and they come in a variety of sizes and capabilities. Touch Panels 1. Touch panels allow displayed objects or screen positions to be selected with the touch of a finger. 2. A typical application of touch panels is for the selection of processing options that are represented with graphical icons. 3. Some systems, such as the plasma panels, are designed with touch screens. 4. Other systems can be adapted for touch input by fitting a transparent device with a touch sensing mechanism over the video monitor screen. 5. Touch input can be recorded using optical, electrical, or acoustical methods. 6. Optical touch panels employ a line of infrared light-emitting diodes (LEDs) along one vertical edge and along one horizontal edge of the frame. 7. The opposite vertical and horizontal edges contain light detectors. These detectors are used to record which beams are interrupted when the panel is touched. 8. The two crossing beams that are interrupted identify the horizontal and vertical coordinates of the screen position selected. Positions can be selected with an accuracy of about ¼ inch 9. With closely spaced LEDs, it is possible to break two horizontal or two vertical beams simultaneously. In this case, an average position between the two interrupted beams is recorded. 10. The LEDs operate at infrared frequencies, so that the light is not visible to a user. 11. An electrical touch panel is constructed with two transparent plates separated by a small distance. One of the plates is coated with a conducting material, and the other plate is coated with a resistive material. When the outer plate is touched, it is forced into contact with the inner plate. This contact creates a voltage drop across the resistive plate that is converted to the coordinate values of the selected screen position. 12. In acoustical touch panels, high-frequency sound waves are generated in the horizontal and vertical directions across a glass plate. 13. Touching the screen causes part of each wave to be reflected from the finger to the emitters. 14. The screen position at the point of contact is calculated from a measurement of the time interval between the transmission of each wave and its reflection to the emitter. Light Pens 1. Such pencil-shaped devices are used to select screen positions by detecting the light coming from points on the CRT screen. 2. They are sensitive to the short burst of light emitted from the phosphor coating at the instant the electron beam strikes a particular point.
- 16. 3. Other Light sources, such as the background light in the room, are usually not detected by a light pen. 4. 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. 5. As with cursor-positioning devices, recorded Light-pen coordinates can be used to position an object or to select a processing option. 6. Disadvantages: · When a light pen is pointed at the screen, part of the screen image is obscured by the hand and pen. · Prolonged use of the light pen can cause arm fatigue. · Light pens require special implementations for some applications because they cannot detect positions within black areas. · To be able to select positions in any screen area with a light pen, we must have some nonzero intensity assigned to each screen pixel. · Light pens sometimes give false readings due to background lighting in a room. Voice Systems 1. Speech recognizers are used in some graphics workstations as input devices to accept voice commands 2. The voice-system input can be used to initiate graphics operations or to enter data. 3. These systems operate by matching an input against a predefined dictionary of words and phrases. 4. A dictionary is set up for a particular operator by having, the operator speak the command words to be used into the system. 5. 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. 6. Later, when a voice command is given, the system searches the dictionary for a frequency- pattern match.7. 8. 7. Voice input is typically spoken into a microphone mounted on a headset9. 10. 8. The microphone is designed to minimize input of other background sounds. 11. 9. If a different operator is to use the system, the dictionary must be reestablished with that operator's voice patterns.12. 13. 10. Voice systems have some advantage 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. 14.