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Note for Computer Graphics - CG By Narendra Singh

  • Computer Graphics - CG
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  • VEL TECH ARTS college - veltech
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SAZ4C – COMPUTER GRAPHICS UNIT – I Brief Survey of Computer Graphics – Graphics Systems: Video Display Devices – Types – Raster-Scan Systems and Random-Scan Systems – Input Devices – HardCopy Devices – Graphics Software. (I) Brief Survey of Computer Graphics Computer graphics is an art of drawing pictures, lines, charts, excusing computers with the help of programming. Computer graphics is made up of number of pixels. Pixel is the smallest graphical picture or unit represented on the computer screen. Basically there are two types of computer graphics namely 1) Interactive computer graphics: It is the computer graphics in which user can interact with the image on the computer screen. Here exist two-way communication between the user and the image. The image is totally under the control of user. Example: Playing the computer game in the computer. Here user controls the image completely. According to the user wish image makes the movements on the screen. 2) Non-interactive computer graphics: It is the computer graphics in which user does not have any kind of control over the image. Image is merely the product of static stored program and will work according to the instructions given in the program linearly. The image is totally under the control of program instructions not under the user. Example: screen savers. Two Kinds of Computer Graphics There are two kinds of computer graphics   Raster (composed of pixels) Vector (composed of paths).  Raster images are more commonly called bitmap images.A bitmap image uses a grid of individual pixels where each pixel can be a different color or shade. Bitmaps are composed of pixels.  Vector graphics use mathematical relationships between points and the paths connecting them to describe an image. Vector graphics are composed of paths.  The image to the left below is representative of a bitmap and the image to the right is representative of a vector graphic. They are shown at four times actual size to exaggerate the fact that the edges of a bitmap become jagged as it is scaled up. 1

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(II) Applications of Computer Graphics 1. Computer Aided Design (CAD): Used for designing buildings, automobiles, aircraft, water craft, space craft, computers etc… the software package for CAD application includes multiple window. 2. Presentation graphics: Used to summaries mathematical, financial, scientific and economic data for research reports, managerial reports, consumer information, and bulleting. Ex: bar charts, line graphs, surface graphs pi charts etc… 3. Computer arts: CG methods are used in fine arts & commercial art applications. Fine arts: to create pictures the artist uses a combination of 3D modeling packages, texture mappings, drawing programs & CAD. Commercial arts: Different methods are applied in commercial art for logos, page layouts, combining text & graphics. 4. Entertainment: Creating pictures, music videos, motion pictures & television shows. 5. Education & training: using digital materials. 6. Visualization: The data are converted to visual form, the trends and patterns. 7. Image processing: It is a technique is combined with CG to produce many of the data visualizations. Applying techniques to modify or interpret existing pictures such as photographs & TV scans. Principle applications: To improve picture quality and Machine perceptions of visual information as used in robotics. 8. Graphical user interface (GUI): The window manager allows a user to display multiple window areas. To activate a window, simply click that window. 2

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(III) Video Display Devices 1. Cathode Ray Tube working principle The Primary output device in Computer Graphics is a Monitor which operates on the standard cathode-ray tube(CRT) design and a few more technological hardware have also come into the concept. Computer graphics is a complex and diversified technology. Refresh Cathode-Ray Tubes The following figure illustrates the basic operation of how does a CRT work.  An electron beam comes from the electron gun, passes through focus and deflection systems that send the beam towards directed positions on the phosphor-coated screen.  The phosphor in return emits a small spot of light at each position where ever the electron beam makes contact. As the light which is emitted by the phosphor fades very easily, some mechanism is required for managing the picture on the screen.  One method to make the phosphor glowing is to keep on redrawing the picture in a repeated manner by quickly projecting the electron beam over the same points again and again.  The heated metal cathode and a control grid are the key components of an electron gun in a CRT. Through the coil of wire, called the filament, inside the cylindrical cathode structure, heat is supplied to the cathode by directing a current which makes electrons to be 'boiled off" the hot cathode surface.  In the electron beam, electrons spread all over the screen as a result of repulsion among them. To make the electron beam strike at one point, focusing anode is present in the CRT. Hence our focusing mechanism makes the electron beam to strike the phosphor screen at a small spot and focusing is following by usage of magnetic and electric field. 3

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 Horizontal deflection of electron bean is accomplished by one pair of coils and vertical deflection is carried out by the others.  Electric deflection is carried out by using two pairs of deflecting plates inside CRT; the two pairs are mounted vertically and horizontally.  Horizontal deflecting plates provide vertical deflection to the electron beam and vertical deflecting plates provide horizontal deflection to the electron beam. Important terminologies in CRT are as follows: Dot Pitch: It denotes the distance which marks separation between two phosphor dots of the same color. A dot pitch equal to or less than .25mm is of use comfortably, whereas we should avoid monitors with a dot pitch equal to or greater than 0.28 mm. Refresh rate: It denotes the number of images which are displayed every second, or we can say that it is the number of times the images is remapped per second. And It is also known as vertical scan rate and is expressed in Hertz (Hz). Resolution : It denotes the number of pixels per surface unit and can be abbreviated as DPI or dots per inches and is calculated both vertically and horizontally. A resolution of 200dpi means that 200 columns and 200 rows of pixels per square Size : It is calculated by taking the dimension of the diagonal of the screen and is expressed Aspect Ratio : It is termed as the ratio of vertical points to horizontal points. 2. Refresh and Raster scan Display System.  Our home television sets use Raster scan technologies.  In this sort of Display Mechanism, an electron beam scans every row of the screen display row by row starting from top to the bottom.  Each screen point represents the intensity value either 0 or 1 and the intensity value is kept in refresh buffer or frame buffer.  Thus, each pixel value or screen point keeps on changing from 0 to 1 or from 1 to 0 depending on its intensity value in refresh buffer. And this is the way the screen is painted one row at a time. And this is shown in the Figure. 4

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