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Introduction to Computer Graphics
1. What are the applications of Computer Graphics? List them with brief note.
The application of Computer Graphics is enormous and is growing rapidly as computers with graphics capability became commodity products. Some of the sample applications are:
i) Design Simulation & User Interface.
ii) Display of information in Industry & Business.
Entertainment: Animation & Gaming represent major use in creation and manipulation of pictures with the aid of a computer. Here, we can classify interactive and non-interactive applications say titles shown in TV and other forms of computer art are examples of non-interactive or passive computer graphics. The user has no control over the Text/Image/Animation. We can give the user same control over the Image/Animation by providing him with an input device such as, joystick etc…, so that he can signal his requests to the computer and can get immediate response for that, i.e it involves two-way communication between computer and user. Gaming is one of the familiar examples.
User Interface: Most applications of computer have user interface that rely on desktop window systems to manage multiple simulation activities and on point & click facilities to allow users to select menu items, icons, dialogue boxes and objects on screen. Typing is necessary only to input the text to be stored and manipulated. Word processing, spreadsheet, desktop-publishing programs are typical applications that take advantage of such user-interface techniques.
Display of Information in Industry & Business: Creating 2D & 3D graphics of mathematical, physical & economic functions, histograms bar and pie-charts, task scheduling charts, inventory & production charts are used to present meaningfully and consistently the trends and patterns gleaned from data. So, to clarify complex phenomena and to facilitate informed decision making in industry and business it is been used.
Design: In Computer Aided Design(CAD), interactive graphics is used to design components and systems of mechanical, electrical, electro-mechanics and electronic devices including structures such as building automobile bodies, airplane & ship hulls, VLSI chips etc.,
Sometimes, the user only wants to precise drawings of components and assemblies, as for online drafting or architectural blue prints more frequently. However, the emphasis is on interacting with a computer-based model of the component or system being designed in order to test. For example, its structural, electrical or thermal properties in this model is often interpreted by a simulator that feeds back the behavior of the system to the user for further interactive design and fast cycles.
Simulation: Interactive computer graphics affects our lives in a number of indirect ways. For example, it helps train the pilots of our airplanes. These pilots spend much of their training in virtual environment (VE) (rather than in a real aircraft) on the ground at the controls of a flight simulator. The flight simulator is a mock up of an aircraft flight deck containing all the usual controls and surrounded by screens on, which are projected computer-generated views of the terrain visible on takeoff and landing. As the trainee pilot maneuvers his “Aircraft” these views change; as to maintain an accurate impression of the planes motion.
Other than these few examples there are some other applications:
· Office automation & electronic publishing(DTP)
· Art & Commerce
· Process Control
· Cartography………etc.
· Virtual Reality – Augmented reality-Immersive & Non-Immersive environment, Virtual Surgery, Virtual meeting etc
2. Explain the Interactive Graphics System with the help of an appropriate block diagram.
Fig: 1.1 Interactive Graphics System
The High-level conceptual framework can be used to describe almost any Graphics System. At the hardware level, a computer receives input from interaction devices, and outputs images to a display device. The software has three components. The first is the application program, which creates, stores into, and retrieves from the second components, the application model, which represents the data or objects to be pictured on the screen. The application program also handles user input. It produces views by sending to the graphics system, the third component, a series of graphics output commands that contain both the detailed geometric description of the thing to be viewed and the attributes describing the way the objects should appear. It is responsible for actually producing the picture from the picture from the detailed descriptions and for passing the user’s input to the application program for processing.
The graphics system is thus an intermediary between the application program and the display hardware that effects an output transformation from objects in the application model to a view of the model. Symmetrically, it effects an input transformation from user actions to inputs to the application program that will cause the application to make changes in the model and/or picture. The fundamental task of the designer of an interactive graphics application program is to specify the classes of data items or objects that are to be generated and represented pictorially, and how the user and the application program are to interact in order to create and modify the model and its visual representation. Most of the programmer’s task concerns creation and editing of the model and handling user interaction, not actually creating views, since the Graphics System handles it.
3. Explain the working principles of CRT with the help of a neat diagram.
Fig 1.2: An Architectural View of a CRT.
An Interactive Computer Graphics demands display devices whose images can be changed quickly. The figure above shows an architectural view of a CRT. The electron gun emits a stream of electrons, which accelerate towards the phosphor-coated screen by a high positive voltage applied near the face of the tube. On the way to the screen, the electrons are forced into a narrow beam by the forcing mechanism and are directed toward a specific point on the screen by the magnetic field produced by the deflection coils. When the electrons hit the screen, the phosphor emits a visible light. Since the phosphor’s light output decays exponentially with time, the entire picture must be refreshed cyclically, so that the viewer sees a still, un-flickering picture.
The refresh rate of a CRT is the number of times per second the image is redrawn. As the refresh rate decreases, flicker develops because the eye can no longer integrate the individual light impulses coming from a pixel. The refresh rate above, which a picture stops to flicker, and fuses into a steady image is called the critical fusion frequency (CFF). The process of fusion is familiar to us, as we know it occurs when we watch a television or any motion pictures. A flicker-free picture appears still or steady to the viewer, irrespective of any given point being “OFF” much longer than “ON”. The refresh rate of a Raster-Scan Display is usually at least 60 frames per second, which is independent of picture complexity. The refresh rate for vector systems depends directly on picture complexity.
Any given phosphor has various quantum levels to which electrons can be excited, each corresponding to a color associated with the return to an unexcited state. Further, electrons on some levels are less stable and return to the unexcited state more rapidly than others. A phosphor’s fluorescence is the light emitted as these very unstable electrons lose their excess energy while electrons are striking the phosphor. A phosphor’s persistence is defined as the time from the removal of excitation to the movement when phosphorescence has decayed to 10% of the initial light output. The range of persistence of different phosphors can reach many seconds, but it is usually 10 to 60 microseconds for most phosphors used in graphics equipments. This light output decays exponentially with time.
4. Explain the working principles of shadow-Mask CRT with the help of a neat diagram.
Here, just behind the phosphorus coated face of the CRT, there is a metal plate. The shadow-mask is pierced with small round holes in a triangular pattern. In place of the usual electron gun the shadow-mask tube uses three guns, grouped in a triangle or delta. These three guns are responsible for red, green and blue components of the light output of the CRT.
The deflection system of the CRT operates on all three electron beams simultaneously, bringing all three to the same point of focus on the shadow-mask. Where the three beams encounter holes in the mask, they pass through and strike the phosphor. Since they originate at three different points, however, they strike the phosphor in three slightly different spots. The phosphor of the shadow-mask tube is therefore laid down very carefully in groups of three spots- one red, one green and one blue- under each hole in the mask, in such a way that each spot is stuck only by electrons from the appropriate gun. The effect of the mask is thus to “shadow” the spots of red phosphor from all but the red beam, and likewise for the green and blue phosphor spots. We can therefore control the light output in each of the three component colors by modulating the beam current of the corresponding gun.
Fig 1.3: Shadow-mask CRT
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