Implementation of Flight Simulator using 3-Dimensional Terrain Modeling

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1 Implementation of Flight Simulator using 3-Dimensional Terrain Modeling 1 1, First Author School of Computer Engineering, Hanshin University, Osan City, S. Korea, stryoo@hs.ac.kr Abstract During the last three decades, flight simulators have played an important role in the training of both military and commercial pilots because it enables us to cut down on the enormous cost that would be needed for a real flight. This paper presents the development of the flight simulator which consists of a terrain modeler, an object modeler, a terrain renderer and a flight path generator. A terrain modeler transforms the terrain information into a digital terrain model. The object modeler constructs and locates the virtual object, and the renderer represents the object using pseudo shading and image mapping. Finally, the flight path generator extracts the virtual path using the cubic spline curve Keywords: Digital Terrain Model(DTM), Triangulated Irregular Network(TIN), Flight Simulation 1. Introduction Digital elevation model (DEM) is often used as a generic term for Digital Surface Model (DSM) and Digital Terrain Models (DTM) [1, 2]. In most cases a Digital Terrain Model represents the bare ground without any objects like buildings and plants on it. Digital Surface Model on the other hand represents the earth's surface and includes all objects on it. Both of them have their uses in numerous applications: extracting the shape and elevation differences of the earth's surface, modeling the flow of water or movement of masses, engineering and infrastructure design, geomorphology and physical geography, gravimetric measurements, visualizing the earth's surface in traditional maps, 3D maps and models, 3D flight planning and flight simulation, line-of-sight-analysis and other areas. During the last three decades, flight simulators have played an important role in the training of both military and commercial pilots because it enables us to cut down on the enormous cost that would be needed for a real flight. To develop the flight simulation using digital terrain model, we suggest the flight simulator which consists of a terrain modeler, an object modeler, a terrain renderer and a flight path generator. Figure 1. The structure of the flight simulator International Journal of Digital Content Technology and its Applications(JDCTA) Volume 7, Number 12, August

2 2. Suggested Flight Simulator As shown in Figure 1, the flight simulator consists of a terrain modeler, an object modeler, a renderer and a flight-path generator. A terrain modeler makes a Digital Terrain Model using DEM. A DEM is a binary array that saves the height of each terrain and is the same as recording the height of each spot from the indicated terrain into a large binary array. An object modeler constructs virtual objects using a script file or an icon based primitives and locates them on the terrain. A renderer represents a three-dimensional object as a shaded two-dimensional projection on a view surface. A flight path generator extracts the virtual path for the flight simulation Terrain Modeler Figure 2 shows the flow of the terrain modeler that transforms DEM into a digital terrain model(dtm). Such a DTM has two primary forms of representation. The first is the rectangular grid model and the second is the triangulated irregular network(tin) model. To model the terrain, we use both of them Rectangular Grids Figure 2. The flow of the terrain model Rectangular grids are generated by rounding off known data points to the nearest grid point and then iteratively interpolating all the unassigned grid positions. The rectangular grid model is fast and simple and is suitable for flat area. However, it is hard to represent a curved region exactly[3][4] Triangulated irregular networks Triangulated network are created by linking triplets of nodes and maintaining pointers from each triangle to its three neighbors. Such network can be used to describe almost any surfaces. Compared to the rectangular grid model, the TIN produces more accurate surface representations with less data storage. To make such a TIN model, a preprocessing step that extracts surface-specific points and constructs a triangulated network with the extracted points is necessary[5, 6]. In this paper, the 8 neighborhood method for extracting surface-specific point and the delaunay triangulation method[3, 4, 7, 8, 9] for making a triangulated network are used. Figure 3 show the results of the rectangular grid model and TIN model. (a) DEM (b) Rectangular Grid Model (c) Triangulated Irregular Network Figure 3. Terrain modeling 211

3 2.2. Object Modeler Figure 4. The process of an object modeler The object modeler is a module that constructs the object and positions the selected object. It can be divided into a modeler that constructs the virtual object based on the primitives and a mapper that positions the selected object onto the selected region (Figure 4). The object on the region can either consist of a single building or a number of buildings. The form of the building may also vary. Therefore, this object modeler should be able to contain all these different types of buildings.. Two different methods are used for generating objects. One is by using a script file and the other is by using the icon based primitives. A script file is necessary for the object modeling to produce according to the user s demand. This script file consists of the structured form, size and the direction value of each object. In the icon based approach, eight basic primitives, as shown in Figure 4, have been used for the object modeling. The mapper that positions the object onto the actual region is just as important as the object modeler. Objects that exist on the ground are not always located on flat ground. So, the mapper should calculate the shape and steepness of the object to place it in the correct position. Figure 5 shows the procedure of object generation. At first, selects the desired area on the terrain with the mouse. The selected area is flattened for object mapping, as shown in Figure 5.b. 'flattening' means changing the height of the selected region into the average height of the region. Secondly, chooses one from the icon based primitives. Finally, the user inputs the size, height and direction of the object using the dialog box. After these procedures, the object mapper locates the objects to the actual position on the ground as shown in Figure 5.c. (a) DTM (b) flattening (c) object mapping Figure 5. Flatting and object mapping of selected area 212

4 2.3. Terrain Renderer A digital terrain model is a two dimensional digital elevation map accompanied by a satellite image or an aerial photograph. Three dimensional visualization of the terrain represented by the DTM has been used in flight simulation, environmental planning, GIS applications and other areas[10, 11]. There are three approaches to rendering the terrain in this paper. One is a method giving the color according to its height, so-called pseudo shading(figure 6-a). This method is simple but not photorealistic. The other is the image mapping method[12]. This method maps a real image, taken from the satellite, onto the surface of the terrain(figure 6-b). Another method is rendering terrain using heightfield ray tracing(figure 6-c). Satellite image mapping method is a little bit slower than pseudo shading. However, most flight simulations use this mapping method, because it can generate a more realistic image. (a) pseudo shading (b) image mapping (c) height-field RT Figure 6. Terrain rendering 2.4. Flight Path Generator The flight path can be predetermined before the simulation if information such as the flight's route or its destination has been given. If the flight path has been predetermined by the above conditions, the simulation can be done without user control. The flight path can be determined by marking the flight's route with a few check points and connecting them with a smooth curve. In this study, the cubic spline curve algorithm[13] has been used to smoothly connect the random check points existing on the three-dimensional space. It has been made possible to select the flight path using mouse and dialog box. Figure 7 shows the flight path created by selecting 3 basic flight points. When the 3 basic flight points have been put in, a cubic spline curve using these three control points is created. This cubic spline curve is used in the virtual path. When the flight path makes a collision with the terrain, an error message is given and the flight path is adjusted in this study Figure 7. Extraction flight path by cubic spline 3. The Result of Flight Simulation Figure 8 shows the image of the flight simulation system. The main window displays 3 dimensional terrain model along the virtual path at each view point, the right-top window shows 2 dimensional virtual path on the terrain and the right-bottom window shows the location of current 3 dimensional 213

5 virtual path. Figure 8.a shows the image of the wireframe using the rectangular grid method, Figure 8.b the image of the pseudo shading, Figure 8.c the rendered image using triangulated irregular network and Figure 8.d the rendered image using the satellite image mapping method. Figure 9 is the final result of flight simulation using the digital terrain model. (a) wireframe (b) pseudo shading (rectangular grid) (c) pseudo shading (TIN) (d) satellite image mapping Figure 8. Suggested flight simulator system 4. Conclusions and Future Work In this study, a flight simulation has been implemented through terrain modeling, using the rectangular grid method and the TIN method, and rendering, using a wireframe, pseudo shading and the satellite image mapping method. Also, it can change the characteristic of an object using the object browser and can save or load the terrain information using a script file. Future work is necessary on the area of FOV(Field of View) for real time simulations and the research of collision detection & prevention. Also, we can improve on the satellite image mapping to get the high quality of the flight simulation and it is necessary to study the flight-path user input module using other media, not keyboard.. 5. Acknowledgements This research was supported by Hanshin University Research Grant 6. References [1] John P. Wilson, Digital terrain modeling, Geomorphology, Volume 137, Issue 1, pp , [2] F. Schroder and P. Robbach, Managing The Complexity of Digital Terrain Models, Computer & Graphics, Vol. 18, No. 6, pp ,

6 [3] B. K. Choi and H. Y. Shin, Triangulation of scattered data in 3D space, Computer-aided design, Vol. 20, No. 5, pp , [4] Lee J., Comparison of existing methods for building triangulated irregular network models of terrain from grid digital elevation models, Int. J. of GIS 5(3), pp , [5] T. K. Peuker and D. H. Douglas, Detection of surface-specific points by local parallel processing of discrete terrain elevation data, Computer Vision Graphics and Image Processing, Vol. 4, pp , [6] S. Takahashi, Algorithms for Extracting Correct Critical Points and Constructing Topological Graphs from Discrete Geographical Elevation Data, Eurographics '95, Vol. 14, pp , [7] Franz Aurenhammer, Voronoi Diagrams A Survey of a Fundamental Geometric Data Structure, ACM Computing Surveys, Vol. 23, No. 3, [8] B. Joe, Construction of three-dimensional Delaunay triangulations using local transformations, Computer Aided Geometric Design, Vol. 8, pp , [9] A. Mirante and N. Weingarten, The radial sweep algorithm for constructing triangulated irregular networks, IEEE Computer Graphics & Application, Vol. 2, pp , [10] Hou Han-dana and Zhang Jian-feib, Research on Real-Time Visualization of Large-scale 3D Terrain, Procedia Engineering, Volume 29, issue (2012), pp , [11] Che Mat RUZINOOR, Abdul Rashid Mohamed SHARIFF, Biswajeet PRADHAN, Mahmud RODZI AHMAD and Mohd Shafry Mohd RAHIM, A review on 3D terrain visualization of GIS data: techniques and software, Geo-spatial Information Science, Vol.15, Issue 2, pp , [12] B. Geymayer, M. Prantl, H. Muller-Seelich and B. Tabatabai, Animation of Landscapes Using Satellite Imagery, Eurographics '91, Vol. 10, pp , [13] David F. Rogers and J. Alan Adams, Mathematical Elements for Computer Graphics, McGraw- Hill International Editions, USA, Figure 9. The result of flight simulation using DTM 215