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Note for Computer Graphics - CG by sapan das

  • Computer Graphics - CG
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Sapan Das
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What is computer Graphics? Computer graphics is an art of drawing pictures, lines, charts, etc. using computers with the help of programming. Computer graphics image is made up of number of pixels. Pixel is the smallest addressable graphical unit represented on the computer screen. Introduction • • • • • • • Computer is information processing machine. User needs to communicate with computer and the computer graphics is one of the most effective and commonly used ways of communication with the user. It displays the information in the form of graphical objects such as pictures, charts, diagram and graphs. Graphical objects convey more information in less time and easily understandable formats for example statically graph shown in stock exchange. In computer graphics picture or graphics objects are presented as a collection of discrete pixels. We can control intensity and color of pixel which decide how picture look like. The special procedure determines which pixel will provide the best approximation to the desired picture or graphics object this process is known as Rasterization. The process of representing continuous picture or graphics object as a collection of discrete pixels is called Scan Conversion. Advantages of computer graphics • • • • • • • Computer graphics is one of the most effective and commonly used ways of communication with computer. It provides tools for producing picture of “real-world” as well as synthetic objects such as mathematical surfaces in 4D and of data that have no inherent geometry such as survey result. It has ability to show moving pictures thus possible to produce animations with computer graphics. With the use of computer graphics we can control the animation by adjusting the speed, portion of picture in view the amount of detail shown and so on. It provides tools called motion dynamics. In which user can move objects as well as observes as per requirement for example walk throw made by builder to show flat interior and surrounding. It provides facility called update dynamics. With this we can change the shape color and other properties of object. Now in recent development of digital signal processing and audio synthesis chip the interactive graphics can now provide audio feedback along with the graphical feed backs. Application of computer graphics • • • • User interface: - Visual object which we observe on screen which communicates with user is one of the most useful applications of the computer graphics. Plotting of graphics and chart in industry, business, government and educational organizations drawing like bars, pie-charts, histogram’s are very useful for quick and good decision making. Office automation and desktop publishing: - It is used for creation and dissemination of information. It is used in in-house creation and printing of documents which contains text, tables, graphs and other forms of drawn or scanned images or picture. Computer aided drafting and design: - It uses graphics to design components and system such as automobile bodies structures of building etc.

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• • • • • • Simulation and animation: - Use of graphics in simulation makes mathematic models and mechanical systems more realistic and easy to study. Art and commerce: - There are many tools provided by graphics which allows used to make their picture animated and attracted which are used in advertising. Process control: - Now a day’s automation is used which is graphically displayed on the screen. Cartography: - Computer graphics is also used to represent geographic maps, weather maps, oceanographic charts etc. Education and training: - Computer graphics can be used to generate models of physical, financial and economic systems. These models can be used as educational aids. Image processing: - It is used to process image by changing property of the image. Display devices • • Display devices are also known as output devices. Most commonly used output device in a graphics system is a video monitor. Cathode-ray-tubes Fig. 1.1: - Cathode ray tube. • • • • • • • • • • • It is an evacuated glass tube. An electron gun at the rear of the tube produce a beam of electrons which is directed towards the screen of the tube by a high voltage typically 15000 to 20000 volts Inner side screen is coated with phosphor substance which gives light when it is stroked bye electrons. Control grid controls velocity of electrons before they hit the phosphor. The control grid voltage determines how many electrons are actually in the electron beam. The negative the control voltage is the fewer the electrons that pass through the grid. Thus control grid controls Intensity of the spot where beam strikes the screen. The focusing system concentrates the electron beam so it converges to small point when hits the phosphor coating. Deflection system directs beam which decides the point where beam strikes the screen. Deflection system of the CRT consists of two pairs of parallel plates which are vertical and horizontal deflection plates. Voltage applied to vertical and horizontal deflection plates is control vertical and horizontal deflection respectively. There are two techniques used for producing images on the CRT screen:

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1. Vector scan/Random scan display. 2. Raster scan display. Vector scan/Random scan display In this technique, the electron beam is directed only to the part of the screen where the picture is to be drawn rather than scanning from left to right and top to bottom as in raster scan. It is also called vector display, stroke-writing display, or calligraphic display. Picture definition is stored as a set of line-drawing commands in an area of memory referred to as the refresh display file. To display a specified picture, the system cycles through the set of commands in the display file, drawing each component line in turn. After all the line-drawing commands are processed, the system cycles back to the first line command in the list. Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second. Architecture of a vector display CPU I/O Port Display buffer memory (Interaction data) (Display command) Display controller Keyboard CRT Mouse Fig. 1.2: - Architecture of a vector display. • • Vector scan display directly traces out only the desired lines on CRT. If we want line between point p1 & p2 then we directly drive the beam deflection circuitry which focus beam directly from point p1 to p2.

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• • • • • • • • • • • If we do not want to display line from p1 to p2 and just move then we can blank the beam as we move it. To move the beam across the CRT, the information about both magnitude and direction is required. This information is generated with the help of vector graphics generator. Fig. 1.2 shows architecture of vector display. It consists of display controller, CPU, display buffer memory and CRT. Display controller is connected as an I/O peripheral to the CPU. Display buffer stores computer produced display list or display program. The Program contains point & line plotting commands with end point co-ordinates as well as character plotting commands. Display controller interprets command and sends digital and point co-ordinates to a vector generator. Vector generator then converts the digital co-ordinate value to analog voltages for beam deflection circuits that displace an electron beam which points on the CRT’s screen. In this technique beam is deflected from end point to end point hence this techniques is also called random scan. We know as beam strikes phosphors coated screen it emits light but that light decays after few milliseconds and therefore it is necessary to repeat through the display list to refresh the screen at least 30 times per second to avoid flicker. As display buffer is used to store display list and used to refreshing, it is also called refresh buffer. Raster scan display In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in memory area called the Refresh Buffer or Frame Buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and “painted” on the screen one row scanline at a time as shown in the following illustration. Each screen point is referred to as a pixel pictureelement or pel. 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.

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