640008: Computer Graphics

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1 Q. What is interactive computer graphics? List the applications of computer graphics. What are components of ICG graphic package? Interactive computer graphics (ICG): It is study of techniques to improve communication between man and computer. ICG provide facility of pictorial representation of data. Components: It should provide user with a variety of functions for creating and manipulating pictures. These functions are used for input, output, viewing transformation. The basic building blocks are called as output primitives. These are line, point, curve, polygon, circle etc. GKS and PHIGS are graphics software packages. Using package viewing transformation are be possible. Using package clipping of objects is possible. We can rotate object in clockwise and anticlockwise direction using packages. We can translate, scale objects. Q. List applications of computer graphics 1. Computer Aided Design (CAD) 2. Presentation graphics 3. Computer Art 4. Entertainment 5. Education & Training 6. Visualization 7. Image processing. 8. Medical science. Q. What is the application of presentation graphics (Graphs and Charts)? It is used to produce illustrations for reports or to generate slides or transparencies for use with projectors. It is commonly used to summarize financial, statistical, mathematical, scientific, and economic data for research reports, managerial reports, customer information bulletins, and other types of reports. Typical examples of computer graphics are bar charts, line graph, pie charts, surface graphs, and other displays showing relationship between multiple parameters. Q. Write short note on Computer Aided Design. CAD A major use of computer graphics is in design process, particularly for engineering and architectural system, but almost all products are now computer designed. Generally referred to as CAD. Computer-Aided Design methods are now routinely used in the design of buildings, automobiles, aircraft, watercraft, spacecraft, computers, textiles, and many other products. Circuits and networks for communication, water supply, or other utilities are contracted with repeated placement of a few graphical shapes. The shapes used in a design represent the different network or circuit components. Standard shapes for electrical, electronic and logic circuits are often supplied by the design package. For other applications, a designer can create personalized symbols that are to be used to construct the network or circuit the system is then designed by successively placing components into the layout, with the graphics package automatically providing the connections between components. Animations are often used in CAD applications. Real-time animations using wire-frame displays on a video monitor are useful for testing performance of a vehicle or system, when we do not display objects with rendered surfaces, the calculations for each segment of the animation can be performed quickly to produce a smooth real-time motion on the screen. Also, wire frame displays allow the designer to see into the interior of the vehicle and to watch the behavior of inner components during motion. CAD packages also provide facilities for experimenting with three-dimensional interior layouts and lighting. 1

2 Many other kinds of system and products are designed using either general CAD package or specially developed CAD software. Q. What is virtual reality environment? A more recent application of computer graphics is in the creation of virtual reality environment in which a user can interact with the objects in a three dimensional scene. Specialized hardware devices provide three dimensional viewing effects and allow the user to pick up object in a scene. Animations in virtual reality environments are often used to train heavy equipment operators or to analyze the effectiveness of various cabin configurations and control placements. Virtual reality environment allows the user to move the object in the environment. And user can see more that one view of the same. Q. What is visualization? Scientists, engineers, medical personnel, business analyst, and others often need to analyze large amount of information or to study the behavior of certain processes. Scanning these large sets of numbers to determine trends and relationship is a tedious and ineffective process. If the data are converted to a visual form, the trends and patterns are often immediately generated. Once we have plotted the values in this way, we can see easily the overall pattern of the data. Producing graphical representation for scientific, engineering, and medical data sets and process is generally referred to as Scientific Visualization. Producing graphical representation for commerce, industry and other non-scientific date and process is generally referred to as Business Visualization. Q. Explain application of graphics in Education and training Computer-generated models of physical, financial, and economic systems are often used as educational aids. For some training applications, special systems are designed. Examples of such specialized systems are the simulators for practice sessions or training of ship captains, aircraft pilots, heavyequipment operators, and air traffic control personnel. Some simulators have no video screens; for example, a flight simulator with only a control panel for instrument flying. But most simulators provide graphics screens for visual operation. Viewing systems are used in simulators for training aircraft control-tower personnel. The keyboard is used to input parameters affecting the airplane performance or the environment, and the pen plotter is used to chart the path of the aircraft during a training session. This simulator is used to investigate the behavior of drivers in critical situations. The drivers' reactions are then used as a basis for optimizing vehicle design to maximize traffic safety. Q. Explain use of computer graphics in computer art Computer graphics methods are widely used in both fine art and commercial art applications. Artists use a variety of computer methods, including special purpose hardware, Photoshop, Corel, specially developed software, symbolic mathematics packages (such as Mathematics), CAD packages, desktop publishing software, and animation packages that provide facilities for designing object shapes and specifying object motions. Fine artists use a variety of other computer technologies to produce images. To create pictures such as, the artist uses a combination of three-dimensional modeling packages, texture mapping, drawing programs, and CAD software. 2

3 Q. Write short note on Image Processing. Image processing: It is technique to modify or interpret existing picture, such as photographs and TV scans. Application: Two principal application of image processing are 1. Improving picture quality and, 2. Machine perception of visual information as used in robotics. To apply image-processing methods, we first digitized a photograph or other picture into an image file. Then digital methods can be applied to rearrange picture parts, to enhance color separations, or to improve the quality of shading. It is extensively used in commercial art applications that involve the retouching and rearranging of sections of photograph and other artwork. Medical applications also make extensive use of image-processing technique for picture enhancement, in tomography, and in simulations of operation Q. Write short note on graphical user interface It is common now for software packages to provide a graphical interface. A major component of a graphical interface is a window manager that allows a user to display multiple-window areas. Each window can contain a different process that can contain graphical or non-graphical displays. To make a particular window active, we simply click in that window using an interactive pointing device. Interfaces also display menus and icons for fast selection of processing options or parameter values. An icon is a graphical symbol that is designed to look like the processing option it represents. The advantages of icons are that they take up less screen space than corresponding textual descriptions and they can be understood more quickly if well designed. Menus contain lists of textual descriptions and icons. Q. What are the various displays available? Explain with example. 1. Raster scan displays: It is based on television technology. In this system an 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 & off to create a pattern of illuminated spots. Picture definition is stored in a memory area called frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the buffer & plotted on the screen one row at a time. e.g. printers 2. Random Scan displays: Here a CRT has the electron beam directed only to the parts of the screen where a picture are to be drawn Random scan monitors draw a picture one line at a time. e.g.: pen plotter. 3. Color CRT monitors: They display color pictures by using a combination of phosphors that emit different colored light. By combining the emitted light form the different phosphors, a range of colors can be generated. 4. A direct view storage tube (DVST) stores picture information as a charge distribution just behind the phosphor created 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. 5. Flat-panel displays: It refers to a class of video devices that have reduced volume, weighty & power requirements compared to a CRT. They are divided into two categories: 3

4 a) Emissive displays: are devices that convert electrical energy into light. e.g. Plasma panels. b) Non-emissive displays: are devices that convert sunlight or light from some other source into graphics patterns. E.g.: LCD. Q. What are the various input devices? Explain the working principle of each of them. 1. Keyboards: is used primarily as a device for entering text strings. Functions keys allow users to enter frequently used operations & cursor control keys can be used to select displayed objects or coordinate positions by positioning the screen cursor. 2. Mouse: is a 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. 3. Trackball: is a ball that can be rotated with fingers or palm. or the hand, to produce screen-cursor movement. Potentiometers, attached to the ball, measure the amount & direction of rotation. 4. Joystick: It consists of a small, vertical lever attached to a base that is used to steer the screen cursor around. Most joysticks select screen positions with actual stick movement, others respond to pressure on the stick. 5. Data glove: is used to grasp a virtual object. The glove is constructed with or series of sensors that detect hand & finger motions. 6. Digitizers: is a device for drawing, painting, or selecting coordinate positions of an object. It is used to scan over a drawing or object & to input a set of discrete coordinate positions, which can be joined with straight line segments to approximate the curve or surface shapes. 7. Image Scanners: Drawings, graphs, color & text can be stored for computer processing with an image scanner by passing an optical scanning mechanism over the information to be stored. 8. Touch panels: allow displayed objects or screen positions to be selected with the touch of a finger. Touch input can be reordered using optical, electrical or acoustical methods. 9. Light pens: These pencil-shaped devices are used to select screen positions by detecting the light coming from points on the CRT screen. 10. 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 & phrases. Q. Explain CRT Construction The figure of CRT is given below. Focusing System Heating Filament Electron Beam Cathode Control Grid Accelerating System Vertical Deflection Plate Phosphor- Coated Screen A beam of electrons, emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor coated 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, the picture is redrawn repeatedly by 4

5 quickly directing the electron beam back over the same points. This type of display is called a Refresh CRT. Electron Gun: It generates electrons whenever metal cathode is heated by filament by passing current in it. Control Grid: Setting voltage levels on the control grid controls intensity of the electron beam. A high negative voltage applied to the control grid will shut off the electron beam by repelling electrons. A smaller negative voltage on the control grid decreases the number of electrons passing through the grid. Since the amount of light emitted by the phosphor coating depends on the number of electrons striking the screen, we control the brightness of a display by varying the voltage on the control grid. Focusing System: The focusing system in a CRT is needed to force the electron beam to join into a small spot as it strikes the phosphor screen. Deflection Plates: Similarly, deflection of the electron beam can be controlled by electric or magnetic fields. Horizontal deflection is done by a pair of deflection coils and vertical deflection by another pair of coils. Phosphorous coating: When the electrons in the beam collide with the phosphor coating, the phosphor coating glows. Different kinds of phosphors are available for use in a CRT. After the CRT beam is removed, the amount of time phosphor continues to emit light is known as the Persistence. The maximum number of points that can be displayed without overlap on a CRT is referred to as the Resolution. The ratio of vertical points to horizontal points necessary to produce equal-length lines in both directions on the screen is called Aspect Ratio. Q. Write detailed note on Raster Scan Display In a raster- scan system, the electron beam moves 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 screen points. Stored intensity values are then taken back from the refresh buffer and put on the screen one row at a time. Each screen point is referred to as a Pixel or Picture Element. In a simple black and white system each pixel is either turned on or off, so only one bit per pixel is needed to control the intensity value per pixel position. On a black and white system, frame buffer store one bit per pixel that is known as bitmap. In colour system frame buffer stores multiple bits per pixel then it is referred to as pixmap. Refreshing on raster-scan displays is carried out at the rate of 60 to 80 times per second. At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line. The return to the left of the screen, after refreshing each scan line, is called the Horizontal Retrace of the electron beam. And at the end of each frame, the electron beam returns to the top left corner of the screen from bottom right corner to begin the next frame. Scanning of the screen from top left to bottom right once is referred to as one Frame. This is known as the Vertical Retrace. Figure showing scan lines, horizontal retrace and vertical retrace in a Raster-Scan Display 5

6 On some raster-scan systems, each frame is displayed in two passes using an Interlacing procedure. Figure showing interlacing scan lines on raster-scan display. First all solid scan lines are displayed then dashed scan lines are displayed. Interlacing allows us to see the entire screen displayed in half the time it would have taken to sweep across all the lines at once from top to bottom. Interlacing is mainly used with slower refreshing rates as it can reduce flicker caused due to slow refresh rates. On non-interlaced display having 30 Hz refresh rate, some flickers developed on the screen. But with interlacing each of the two pass can be accomplished in 1/60 th of a second. Q. Explain Random Scan / Stroke Writing / Calligraphic Display In random-scan display systems, a CRT have 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. So they are also known as Vector Displays or Stroke-Writing Displays or Calligraphic Displays. The lines of a picture can be drawn and refreshed in any order randomly. Picture definition is stored as a set of line-drawing commands in a memory referred to as the Refresh Display File or Display List or Display Program or Refresh Buffer. To display the specified picture, the system draws each component line from the display file. Refresh rate depends on the number of lines to be displayed. Random-scan displays are designed to draw all the component lines of picture 30 to 60 frames per second. Random-scan systems are designed for line-drawing applications and cannot display shaded scenes, but have higher resolution than raster system. Also, random-scan displays produce smooth line drawings. Q. Write detailed note on Colour CRT Monitor A CRT monitor displays colour picture by using combination of phosphor that emit different coloured light. The two basic techniques for producing colour displays with a CRT are: 1. Beam Penetration Method: The beam penetration method for displaying colour pictures is generally used with random-scan monitors. 6

7 Two layers of phosphor, generally red and green, are coated onto the inside of the CRT screen, and the displayed colour depends on how far the electron beam passes into the phosphor layers. A beam of slow electrons strikes only the outer red layer while a very fast beam will pass through the red layer and strike the inner green layer. At intermediate beam speeds, combination of red and green generate two additional colours, orange and yellow. Beam Penetration method is an economical way to produce colour in random-scan monitors. The disadvantage of beam penetration method is that it produces only four colours, and the quality of pictures is not as good as other methods. 2. Shadow Mask Method: Shadow-Mask method is commonly used in raster-scan systems because they produce a much wider range of colours than the beam penetration method. A Shadow-mask CRT has three phosphor colour dots at each pixel position. One phosphor dot emits a red light, another emits a green light, and the third emits a blue light. This type of CRT has three electron guns, one for each colour dot, and a shadow-mask grid just behind the phosphor-coated screen. 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. When the three beams pass through a hole in the shadow mask, they activate a dot triangle, which appears as a small colour spot on the screen. This arrangement is known as delta-delta method If three dots are in-line (OOO) this arrangement is known as in-line arrangement and is commonly used in high-resolution colour CRTs. We obtain colour variations 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 colour coming from the blue phosphor. Other combinations of beam intensities produce a small light spot for each pixel position, since our eyes merges the three colours into one. The colour we see depends on the amount of electrons that strike red, green, and blue phosphors. Colour systems in graphics systems are known as RGB Monitors or True Color System. Shadow Mask Electron Gun Screen Q. What is Direct View Storage Device? Write advantage and Disadvantages of it. A Direct View Storage Tube (DVST) stores the picture information inside the CRT just behind the phosphor-coated screen. Two electron guns are used in DVST. One, the primary gun, is used to store the picture pattern and the second, the flood gun, maintain the picture display. Advantage: As no refreshing is needed, very complex pictures can be displayed at very high resolutions without flicker. Disadvantage: DVST systems do not display colour and that selected parts of the screen or picture cannot be erased. 7

8 Flood Gun Screen Storage Grid Primary Electron Gun Collector Grid Q. Write note on Hard copy device We can obtain hard-copy output i.e. output on paper or as photograph using either printer or plotter. Printers: Printers produce output by either impact or non-impact methods. Impact Printers press characters press character faces against an inked ribbon onto the paper. Examples of impact printers are Dot-Matrix Printer, Line Printer, and Chain Printer. A dot-matrix printer consists of a head containing a rectangular array of protruding wire pins, with the number of pins depending on the quality of the printer. Individual characters or graphics pattern are obtained by retracting certain pins so that the remaining pins form the pattern to be printed. The head moves across the paper and the pins do the printing. Non-Impact Printers does not strike against the paper. They use laser techniques, ink sprays, photocopier process, electrostatic methods and electro thermal process to get images onto paper. Examples of non-impact printers are Laser Printer, Ink-Jet Printer, etc. Ink-jet methods produce output by spraying ink in horizontal rows across a roll of paper wrapped on a drum. The electrically charged ink is deflected by an electric field to produce patterns. An electrostatic device places a negative charge on the paper, one row at a time in the form of pattern to be printed. Then the paper is exposed to a positively charged toner and it gets attracted to the negatively charged areas on paper and produces the specified output. Electro thermal methods use heat in a print head to output patterns on heat-sensitive paper. We can get some colors on an impact printer by using different color ribbons. Non-impact printers use various techniques to produce a range of color patterns. Plotters: A Pen Plotter has one or more pens mounted on a carriage or a crossbar that crosses a sheet of paper. Pens with different colors and widths are used to produce a variety of shading and line styles. Different types of pen such as ball-point, ink pens are also used with a pen plotter. Plotter paper can be flat or be rolled onto a drum. Crossbars can be either moveable or stationary, with the pen moving sidewise along the bar. Q. Write detailed note on plotter. A plotter is a vector graphics printing device that connects to a computer. Drafting layouts and other drawings are typically generated with inkjet or pen plotters Plotter is a output device used to produce hard copies of graphs and design. A Pen Plotter has one or more pen mounted on the carriage, or cross bar, that spans a sheet of paper Plotters print their output by moving a pen across the surface of a piece of paper. They can draw complex line art, including text, but do so very slowly because of the mechanical movement of the pens. Pens with varying colors and widths are used to produce a variety of shadings and line styles. Wet ink, ball point, and felt-tip pens are all possible choices for use with a pen plotter. 8

9 A Plotter is best for reproducing drawings. The technology is electro-mechanical. The image components are converted to signals that drive two motors, each moving the pen along one of two perpendicular directions. Appropriate combinations of the two motions can produce straight lines in any direction and curves as well. The motion of the pen is in such small increments that create straight lines curves, text etc. is quite smooth and continuous. Area shading is produced by repeatedly moving the pen across the surface within the defined boundaries, with increments not more than the pen line width, after each step. Plotter paper can lie flat or rolled on to a drum or belt. Types of Plotters: There are two types of plotter 1. Flat-Bed Plotter As the name implies, a flat bed plotter plots on paper that are spread and fixed over a rectangular flatbed table. Normally the paper doesn't move and the pen holding mechanism is designed to provide all the motion. One motor moves the pen along a bar and another moves the bar along parallel Normally pens with ink of different colors are used for multicolored plotting. The plot size is restricted by the area of bed. 2. Drum Plotter In this type, the paper is carried around a drum that rotates, while the pen moves transversely along the topmost edge of the drum, again achieving a combination of two perpendicular components of pen motion. This is more suited to large drawings as for civil, architectural and industrial drawings. Demerits of Plotters: Plotters are normally very slow in motion because of excessive mechanical movement required during plotting. Hence there is a great mismatch between the speed of the CPU and the speed of a plotter. Plotters are restricted to line art, rather than raster graphics as with other printers. Q. What are the various methods through which input is recorded in touch panels? 1. Optical Touch Panel: Optical touch panels employ a line of infrared light-emitting diodes (LEDs) along one vertical edge and along one horizontal edge of the frame. The opposite vertical and horizontal edges contain light detectors. These detectors are used to record which beams re interrupted when the panel is touched. 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. 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. 2. Electrical Touch Panel: 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. 3. Acoustical Touch Panel: In acoustical touch panels, high-frequency sound waves are generated in the horizontal and vertical directions across a glass plate. Touching the screen causes part of each wave to be reflected from the finger to the emitters. 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. 9

10 Q. Explain the working of LCDs. LCDs are commonly used in small systems, such as calculators and portable, laptop computers. These non-emissive devices produce a picture by passing polarized light from the surrounding or from an internal light source through a liquid crystal material that can be aligned to either block or transmit the light. The term liquid crystal refers to the fact that these compounds have a crystalline arrangement of molecules, yet they flow like a liquid. It commonly use liquid crystal compounds that tend to keep the long axes of the rod shaped molecules aligned. In a flat panel display, two glass plates, each containing a light polarizer at right angles to the other plate, sandwich the liquid crystal material. Rows of horizontal transparent conductors are built into one glass plate and columns of vertical conductors are put into the other plate. The intersection of two conductors defines a pixel position. Polarized light passing through the material is twisted so that it will pass through the opposite polarizer. The light is then reflected back to the viewer. To turn off the pixel, we apply a voltage to two intersecting conductors to align the molecules so that the light is not twisted. Q. How is plasma panels constructed? Explain. Plasma panels also called gas discharge displays are constructed by filling the region between two glass plates with a mixture of gases that usually contain neon. 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. 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. Picture definition is stored in a refresh buffer and the firing voltages are applied to refresh the pixel positions 6o times per second. Alternating current methods are used to provide faster applications of the firing voltages and thus brighter displays. Separation between pixels is provided by the electric field of the conductors. One disadvantage of plasma panels have been that they were strictly monochromatic devices, but systems have been developed that are now capable of displaying color and grayscale. Q. What are the different techniques used for representing three-dimensional objects? The various techniques used are: 1) Graphics monitors: o It is using a technique that reflects a CRT image from a vibrating, flexible mirror. o In this system, as the mirror vibrates, it changes focal length. o These vibrations are synchronized with the display of an object on a CRT so that each point on the object is reflecting from the mirror into a spatial position corresponding to the distance of that point from a specified viewing position. o This allows us to walk around an object or scene and view it from different sides. 2) Stereoscopic views: o 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. We can construct the two views as computer-generated scenes with different viewing positions, or we can use a stereo camera pair to photograph some object or scene. 10

11 o When we simultaneously look at the left view with the left eye and the right view with the right eye, the two images 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. o A headset containing an optical system to generate the stereoscopic view is commonly used in conjunction with interactive input devices to locate and manipulate objects in the scene. o 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. Q. Write short note on Software Standards. The primary goal of standardized graphics software is portability. When packages are designed with standard graphics functions, software can be moved easily from one hardware system to another and used in different implementation and applications. GKS (Graphical Kernel System) This is first graphic software standard developed by International and national standard planning organization, and was adopted by ISO (international standard organization) and others including ANSI. It is two dimensional graphics package. PHIGS (Programmer s Hierarchical Interactive Graphics Standard) It is a three dimensional GKS extension. It has increased capability for object modeling, color specifications, surface rendering, and picture manipulation. PHIGS+ It was developed to provide three-dimensional surface rendering capabilities not available in PHIGS. Q. Write short note on Graphics Functions A general-purpose graphics package provides user with variety of functions for creating and manipulating pictures. These functions can be categorized according to whether they deal with output, input, attributes, transformations, viewing or general control. The basic building blocks for pictures are referred to as output primitives. They include character strings, points, straight line, curved line, filled area, and shape defined with arrays of color points. Functions for generating output primitives provide the basic tools for constructing pictures. Attributes are the properties of the output primitive; that is, an attribute describes how a particular primitive is to be displayed. They include intensity and color specification, line styles, text styles, and area filling patterns. Functions within this category can be used to set attributes for an individual primitive. We can change the size, position, or orientation of an object within a scene using transformation functions. Viewing transformation functions are used to specify the view that is to be presented and the portion of the output display area that is to be used. Input Functions are used to control and process the data flow from the input device. Q. What is a pixel? A pixel or picture element is a point on the screen that holds a set of intensity values. It is also called a picture element. It is also defined as smallest addressable unit of screen. More will be the number of pixels, better will be the resolution. 11

12 Q. What is a frame buffer? Picture definition is stored in a memory area called frame buffer. It is a temporary memory that holds the set of intensity values for all the screen points. In frame buffer, a used pixel is represented by bit value 1 and unused pixel is represented by bit value 0.It is also called refresh buffer. Q. Define persistence. It is defined as the time it takes the emitted light from the screen to decay to one-tenth of its original intensity. For interlaced systems beam has to scan whole screen. So to avoid flicker such persistence was used. This is average persistence. The range of persistence is between 10 to 60 microseconds. High persistence is used for static and complex pictures. Low persistence is used for animation. Q. Define resolution. It is defined as the number of points per centimeter that can be plotted horizontally & vertically. More will be pixels; more will be the resolution and better will be the picture. Q. What is aspect ratio? What is the benefit of aspect ratio in drawing polygons? It is defined as the ratio of vertical points to horizontal points necessary to produce equal length lines in both directions on the screen. An aspect ration of ¾ means that a vertical line plotted with 3 points has the same length as a horizontal line plotted with 4 points. By using proper aspect ratio we can draw images and polygons showing realistic appearance. Q. What are the disadvantages of a direct view storage tube? 1. They do not display color. 2. Selected parts of a picture cannot be erased. Q. What are emissive & non-emissive flat panel displays? Emissive flat panel displays: are devices that convert electrical energy into light e.g.: Plasma panels, light-emitting diodes. There is use of emitters. Non-emissive flat panel displays: are devices that use optical effects to convert sunlight or light from some other source into graphic patterns. e.g. LCD (Liquid Crystal display). No emitters are used in non emissive display. Q. What is a workstation? A workstation is not just a computer. It contains multiple input & other devices. For example, an artist s workstation may include a color monitor, keyboard, graphics tablet in addition to data storage & telecommunication services. Q. What is meant by horizontal & vertical retrace? At the end of each scan line, the electron beam returns to the left side of the screen to begin scanning the next line. It returns to the left corner of screen, after refreshing each scan line, is called the horizontal retrace of the electron beam. At the end of each frame, the electron beam returns to the top left corner of the screen to begin the next frame. This is called vertical retrace. Q. What are pointing and positioning devices? For some applications keyboard is inconvenient or inadequate. If we have a number of symbols on screen and each symbol is given a name, then we can select symbol by simply pointing, so for faster 12

13 selection pointing devices are used. Pointing device is used for selecting already existing symbols whereas positioning devices are used for new items. Mostly all input devices can be used for positioning and pointing. Light pen is best example of pointing device whereas trackball, spaceball, joystick are positioning devices. Q. Explain the merits and demerits of Penetration techniques. The merits and demerits of the Penetration techniques are as follows; [1] It is an inexpensive technique [2] It has only four colors [3] The quality of the picture is not good when it is compared to other techniques [4] It can display color scans in monitors Q. Explain the merits and demerits of DVST. The merits and demerits of direct view storage tubes [DVST] are as follows; [1] It has a flat screen [2] Refreshing of screen is not required [3] Selective or part erasing of screen is not possible [4] It has poor contrast [5] Performance is inferior to the refresh CRT. Q. Explain the merits and demerits of Plasma panel display. Advantages: [1] Refreshing is not required [2] Produce a very steady image free of Flicker [3] Less bulky than a CRT. Disadvantages: [1] Poor resolution of up to 60 dpi [2] It requires complex addressing and wiring [3] It is costlier than CRT. Q. What is the difference between impact and non-impact printers? Impact printer s press formed character faces against an inked ribbon on to the paper. A line printer and dot-matrix printer are examples. Non-impact printer and plotters use Laser techniques, inkjet sprays, Xerographic process, electrostatic methods and electro thermal methods to get images onto the papers. Examples are: Inkjet/Laser printers. Q. What are the features of Inkjet printers? [a] They can print 2 to 4 pages/minutes. [b] Resolution is about 360d.p.i. Therefore better print quality is achieved. [c] The operating cost is very low. The only part that requires replacement is ink cartridge. [d] 4 colors cyan, yellow, magenta, black are available. Q. What are the advantages of laser printers? [1] High speed, precision and economy. [2] Cheap to maintain. [3] Quality printers. 13

14 [4] Lasts for longer time. [5] Toner power is very cheap. Q. What are the advantages of electrostatic plotters? [1] They are faster than pen plotters and very high quality printers. [2] Recent electrostatic plotters include a scan-conversion capability. [3] Color electrostatic plotters are available. They make multiple passes over the paper to plot color pictures. Q. What is the difference between interlacing and non-interlacing? In interlaced systems scanning each horizontal line traced from top to bottom. Due to which fading of display of object may occur. This problem can be solved by non interlaced scanning. In this first of all odd numbered lines are traced by electron beam, then in next cycle even number of lines are traced. In non-interlaced system refresh rate of 30 frames per second is used. For interlaced system refresh rate of 60 frames per second is used. Q. What is flickering? If the persistence of phosphor do not match with refresh rate, then flicker said to occur.due to flicker some part of picture appear dull as compared to another part. Q. What are the advantages of LCD over CRT? 1. It is light in weight. 2. Power requirements are very less. 3. It is used for manufacturing of portable devices. 4. No refreshing is required 5. It has long life as compared to CRT. Q. Compare and contrast shadow mask and beam penetration method? Beam Penetration Use one electron gun Generates only four color Color is displayed based on the penetration of electron beam into phosphorus layer It has use two layer of phosphorus namely red and green It is not suitable to draw realistic display Shadow Mask Use three electron gun More than one million color is possible Color is displayed based on intensity of electron beam from three electron gun It has three layers of phosphorus namely red, green and blue. It is suitable to draw realistic display. Q. Explain the role & working of a video controller in Raster Scan systems. Interactive raster graphics systems typically employ several processing units. In addition to the CPU, a special-purpose processor, called the video controller is used to control the operation of the display device. A fixed area of memory is reserved for the frame buffer & the video controller is given direct access to the frame buffer memory. Frame buffer locations & the corresponding screen positions are referenced in Cartesian coordinates for many graphics monitors, the coordinate origin is defined at the lower left corner. The screen surface is represented as the first quadrant of the 2-D system. Scan lines are then labeled from ymax at the top of the screen to 0 at the bottom along each scan line, screen 14

15 pixel positions are labeled from o to xmax. The refresh operations of the video controller are programmed as below. Two registers are used to store the coordinates of the screen pixels. Initially the x register is set to zero & the y register is set to ymax. The value stored in the frame buffer for this pixel position is then retrieved & used to set the intensity of the CRT beam. Then the x register is incremented by 1, & the process repeated for the next pixel on the top scan line. This procedure is the last pixel on the top scan line has been processed, the x register is reset to zero & the y register is decremented by 1. Q. Explain why double buffering is needed to maintain display files? Two buffers are needed in high quality systems. One buffer is used for refreshing, while other is used to store intensity value. The role of two buffers can be interchanged. This method provides fast mechanism for generating real time animation. Fast changing of intensity values is also possible. Q. Differentiate between raster scan and random scan displays. Raster scan system It paints picture from left to right and top to bottom, pixel by pixel It is suitable to draw picture with color shading It stores picture definition using pix-map or bit-map It produces jagged lines because lines are plotted as discrete point sets. Random scan system It draws picture by drawing lines which construct picture It is not suitable to draw picture with color shading It stores picture definition as set of line drawing command. It produces smooth line drawing because CRT beam directly follows the line path. Q. Write short note on line attributes. Also explain line cap and line joins The followings are line attributes Line type: Solid Line, Dashed Line, Dotted Line etc. Line Width: specifies width of the line Pen and Brush Option: Shape, Size and pattern of pen Line Color: specifies drawing color Line caps: To display thick line end smoothly line caps are used there are three types of line caps are used Butt cap Round cap Square cap Line Joins: The joints are required to smoothly join two line segments. A miter join (figure a) is accomplished by extending the outer boundaries of each of the two lines until they meet. A round join (figure b) is produced by capping the connection between the two segments with a circular boundary whose diameter is equal to the line width, and 15

16 A bevel join (figure c) is generated by displaying the line segments with butt caps and filling in the triangular gap where the segments meet. The three joins are shown in figure given below (a) (b) (c) Q. Explain area fill attributes The followings are area fill attributes 1) Fill Style: There are three basic fill style 1) Hollow 2) Solid and 3) Pattern a. Hollow fill: Hollow areas are displayed using only the boundary outline; with the interior color the same as the background color. b. Solid fill: Soft fill is displayed in a single color up to and including the borders of the region. c. Pattern fill: Hatch Pattern, such as diagonal hatch fill and diagonal cross hatch fill. Hatch pattern is used to fill an area with selected hatch patterns- parallel lines or crossed lines as shown in figure given below. d. Soft fill: In soft fill the fill color is combined with the background color. Q. Explain character attributes The followings are character attributes: 1. Font: It is set of characters with particular design style such as Times new roman. It also displayed with bold face, italics, and outline, underline or shadow style. 2. Size: We can display character in different size. 3. Color: The color of text to be displayed. 4. Orientation: orientation specifies directions of the text to be displayed such as Horizontal, Vertical or Inclined. Q. Write short note on Character Generation: Letters, numbers, and other characters can be displayed in verity of sizes and styles. The overall design style for set of characters is called a typeface. Typefaces (or fonts) can be divided into two broad groups: serif and sans serif. Serif type has small lines or accents at the ends of the main character strokes, while sans-serif type does not have accent. Serif type is generally more readable; that is easier to read in longer blocks of text. On the other hand, the individual characters in sans-serif type are easier to recognize. Two different representations are used for storing computer fonts. o A simple method for representing the character shapes in a particular typeface is to use rectangular grid patterns. The set of character are then referred to as a bitmap font. o Another more flexible, scheme is to describe character shapes using straight line and curve sections, in this case, the set of character is called an outline font. 16

17 Figure shows two methods of representing character. Q. What is Gray scale? With monitors that have no color capabilities, color functions can be used in an application program to set the shades of gray, or grayscale, for displayed primitives. Numeric values over the range from 0 to 1 can be used to specify grayscale levels, which are then converted to appropriate binary codes for storage in the raster. This allows the intensity settings to be easily adapted to systems with differing grayscale capabilities. Intensity codes for a four-level grayscale system Intensity Code Stored Intensity values in the Displayed Grayscale Frame Buffer (Binary Code) (00) Black (01) Dark gray (10) Light gray (11) White Q. Explain types of anti-aliasing methods. There are three types of anti-aliasing methods. 1) Super-sampling / Post-Filtering: This technique of sampling object characteristics at a high resolution and displaying the results at a lower resolution is called super-sampling or post-filtering, since the general method involves computing intensities at sub-pixel grid position, then combining the result to obtain the pixel intensities. 2) Area-sampling / Post-Filtering: An alternative to super-sampling is to determine pixel intensity by calculating the areas of overlap of each pixel with the objects to be displayed. Anti-aliasing by computing overlap areas is referred to as area sampling or pre-filtering since the intensity of the pixel as a whole is determined without calculating sub-pixel intensities. Pixel overlap areas are obtained by determining where object boundaries interest individual pixel boundaries. 3) Pixel Phasing: Raster objects can be also be anti-aliased by shifting the display location of pixel areas. This technique, called pixel phasing, is applied by micro-positioning the electron beam in relation to object geometry. Q. What is geometric transformation? Changes in orientation, size and shape are accomplished with geometric a transformation that alters the coordinate descriptions of objects. The basic geometric transformations are translation, rotation and scaling. 17

18 Q. What is translation? Translation is a rigid body transformation that moves object without deformation. That is, every point on object is translated by the same amount. The matrix form of translation is: X1 X1 T X P = P = T = Y1 Y1 T Y P = P + T Q. What is rotation? In a rotation about pivot point the rotation is applied to object by repositioning it along a circular path in the xy plan. To generate rotation we specify a rotation angle and position of the pivot point (xr, yr). The positive value of indicates counter clockwise rotation and negative value indicates clockwise rotation. (x, y ) (x, y) xr yr Q. What is scaling? Explain types of scaling. A scaling transformation alters the size of an object. This operation can be carried out for polygons by multiplying the coordinate values (x,y) for each vertex by scaling factors Sx and Sy to produce the transformed coordinates (x,y ): X` = x Sx, Y` = y Sy, Scaling factors Sx, scales objects in the x direction, while Sy scales in the y direction. The Transformation equations 5-10 can also be written in the matrix form: Or X` Sx 0 x P = = Y` 0 Sy y P` = S P Uniform Scaling: a uniform scaling is produced that maintains object proportions. It is produced with equal values os Sx and Sy. Differential Scaling: Unequal values for Sx and Sy result in a differential scaling that is often used in design application, where pictures are constructed from a few basic shapes that can adjusted by scaling and positioning transformation. 18