Defence Science Journal, Vol. 57, No. 1, January 2007, pp. 149-162 2007, DESIDOC Revised 16 March 2006 Real-time Photorealistic Visualisation of Large-scale Multiresolution Terrain Models Anupam Agrawal 1 , R.C. Joshi 2 , and M. Radhakrishna 1 1 Indian Institute of Information Technology, Allahabad–211 011 2 Indian Institute of Technology, Roorkee–247 667 ABSTRACT Height field terrain rendering is an important aspect of GIS, outdoor virtual reality applications such as flight simulation, 3-D games, etc. A polygonal model of very large terrain data requires a large number of triangles. So, even most high-performance graphics workstations have great difficulty to display even moderately sized height fields at interactive frame rates. To bring photorealism in visualisation, it is required to drape corresponding high-resolution satellite or aerial phototexture over 3-D digital terrain and also to place multiple collections of point-location- based static objects such as buildings, trees, etc and to overlay polyline vector objects such as roads on top of the terrain surface. It further complicates the requirement of interactive frame rates while navigation over the terrain. This paper describes a novel approach for objects and terrain visualisation by combination of two algorithms, one for terrain data and the other for objects. The terrain rendering is accomplished by an efficient dynamic multiresolution view- dependent level-of-detail mesh simplification algorithm. It is augmented with out-of-core visualisation of large-height geometry and phototexture terrain data populated with 3-D/2-D static objects as well as vector overlays without extensive memory load. The proposed methodology provides interactive frame rates on a general-purpose desktop PC with OpenGL- enabled graphics hardware. The software TREND has been successfully tested on different real- world height maps and satellite phototextures of sizes up to 16K*16K coupled with thousands of static objects and polyline vector overlays. Keywords: Digital terrain models, level-of-detail management, multiresolution modelling, real-time rendering, photorealistic visualisation, terrain-rendering algorithm 1. INTRODUCTION Computer-generated perspective imagery of mission-critical terrain has been recognised as a valuable tool for acquiring strategic and tactical insight. Currently available high quality digital representation of earth features is facilitating visualisation with greater detail and informational content. Photorealistic terrain visualisation is achieved by combining terrain topography with terrain image or phototexture. Terrain topography is represented in the form of elevation samples and is also known as height field or digital elevation model (DEM). There are varieties of sources to acquire DEM data of the area of interest, which includes digitised contour data from topographic maps, satellite or aerial optical stereo image pair, microwave synthetic aperture radar (SAR) images (using interferometry or radargrammetry), etc. The image or terrain phototexture is obtained from a satellite or by any other suitable mechanism 149