Modeling a city poses a number of problems to computer graphics. Every urban area has a transportation network that follows population and environmental influences, and often a superimposed pattern plan. The buildings appearances follow historical, aesthetic and statutory rules. To create a virtual city, a roadmap has to be designed and a large number of buildings need to be generated. We propose a system using a procedural approach based on L-systems to model cities. From various image maps given as input, such as land-water boundaries and population density, our system generates a system of highways and streets, divides the land into lots, and creates the appropriate geometry for the buildings on the respective allotments. For the creation of a city street map, L-systems have been extended with methods that allow the consideration of global goals and local constraints and reduce the complexity of the production rules. An L-system that generates geometry and a texturing system based on texture elements and procedural methods compose the buildings.

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Procedural modeling of cities

From the Publisher:
Level of detail (LOD) techniques are increasingly used by professional real-time developers to strike the balance between breathtaking virtual worlds and smooth, flowing animation. Level of Detail for 3D Graphics brings together, for the first time, the mechanisms, principles, practices, and theory needed by every graphics developer seeking to apply LOD methods.
Continuing advances in level of detail management have brought this powerful technology to the forefront of 3D graphics optimization research. This book, written by the very researchers and developers who have built LOD technology, is both a state-of-the-art chronicle of LOD advances and a practical sourcebook, which will enable graphics developers from all disciplines to apply these formidable techniques to their own work. 
Features
Is a complete, practical resource for programmers wishing to incorporate LOD technology into their own systems. Is an important reference for professionals in game development, computer animation, information visualization, real-time graphics and simulation, data capture and preview, CAD display, and virtual worlds. Is accessible to anyone familiar with the essentials of computer science and interactive computer graphics. Covers the full range of LOD methods from mesh simplification to error metrics, as well as advanced issues of human perception, temporal detail, and visual fidelity measurement. Includes an accompanying Web site rich in supplementary material including source code, tools, 3D models, public domain software, documentation, LOD updates, and more. 

Author Biography:David Luebke
David is an Assistant Professor in the Department of Computer Science at the University of Virginia. His principal research interest is the problem of rendering very complex scenes at interactive rates. His research focuses on software techniques such as polygonal simplification and occlusion culling to reduce the complexity of such scenes to manageable levels. Luebke's dissertation research, summarized in a SIGGRAPH '97 paper, introduced a dynamic, view-dependent approach to polygonal simplification for interactive rendering of extremely complex CAD models. He earned his Ph.D. at the University of North Carolina, and his Bachelors degree at the Colorado College. 
Martin Reddy
Martin is a Senior Computer Scientist at SRI International where he works in the area of terrain visualization. This work involves the real-time display of massive terrain databases that are distributed over wide-area networks. His research interests include level of detail, visual perception, and computer graphics. His doctoral research involved the application of models of visual perception to real-time computer graphics systems, enabling the selection of level of detail based upon measures of human perception. He received his B.Sc. from the University of Strathclyde and his Ph.D. from the University of Edinburgh, UK. He is on the Board of Directors of the Web3D Consortium and chair of the GeoVRML Working Group. 
Jonathan D. Cohen
Jon is an Assistant Professor in the Department of Computer Science at The Johns Hopkins University. He earned his Doctoral and Masters degrees from The University of North Carolina at Chapel Hill and earned his Bachelors degree from Duke University. His interests include polygonal simplification and other software acceleration techniques, parallel rendering architectures, collision detection, and high-quality interactive computer graphics. 
Amitabh Varshney
Amitabh is an Associate Professor in the Department of Computer Science at the University of Maryland. His research interests lie in interactive computer graphics, scientific visualization, molecular graphics, and CAD. Varshney has worked on several aspects of level-of-detail simplifications including topology-preserving and topology-reducing simplifications, view-dependent simplifications, parallelization of simplification computation, as well as using triangle strips in multiresolution rendering. Varshney received his PhD and MS from the University of North Carolina at Chapel Hill in 1994 and 1991 respectively. He received his B. Tech. in Computer Science from the Indian Institute of Technology at Delhi in 1989. 
Benjamin Watson
Ben is an Assistant Professor in Computer Science at Northwestern University. He earned his doctoral and Masters degrees at Georgia Tech's GVU Center, and his Bachelors degree at the University of California, Irvine. His dissertation focused on user performance effects of dynamic level of detail management. His other research interests include object simplification, medical applications of virtual reality, and 3D user interfaces. 
Robert Huebner
Robert is the Director of Technology at Nihilistic Software, an independent development studio located in Marin County, California. Prior to co-founding Nihilistic, Robert has worked on a number of successful game titles including "Jedi Knight: Dark Forces 2" for LucasArts Entertainment, "Descent" for Parallax Software, and "Starcraft" for Blizzard Entertainment. Nihilistic's first title, "Vampire The Masquerade: Redemption" was released for the PC in 2000 and sold over 500,000 copies worldwide. Nihilistic's second project will be released in the Winter of 2002 on next-generation game consoles. Robert has spoken on game technology topics at SIGGRAPH, the Game Developer's Conference (GDC), and Electronic Entertainment Expo (E3). He also serves on the advisory board for the Game Developer's Conference and the International Game Developer's Association (IGDA). Robert's e-mail address is .

Level of Detail for 3D Graphics

Hundreds of millions of people play computer games every day For them, game content—from 3D objects to abstract puzzles—plays a major entertainment role Manual labor has so far ensured that the quality and quantity of game content matched the demands of the playing community, but is facing new scalability challenges due to the exponential growth over the last decade of both the gamer population and the production costs Procedural Content Generation for Games (PCG-G) may address these challenges by automating, or aiding in, game content generation PCG-G is difficult, since the generator has to create the content, satisfy constraints imposed by the artist, and return interesting instances for gamers Despite a large body of research focusing on PCG-G, particularly over the past decade, ours is the first comprehensive survey of the field of PCG-G We first introduce a comprehensive, six-layered taxonomy of game content: bits, space, systems, scenarios, design, and derived Second, we survey the methods used across the whole field of PCG-G from a large research body Third, we map PCG-G methods to game content layers; it turns out that many of the methods used to generate game content from one layer can be used to generate content from another We also survey the use of methods in practice, that is, in commercial or prototype games Fourth and last, we discuss several directions for future research in PCG-G, which we believe deserve close attention in the near future

Procedural content generation for games: A survey

We describe a model for simulating crowds of humans in real time. We deal with a hierarchy composed of virtual crowds, groups, and individuals. The groups are the most complex structure that can be controlled in different degrees of autonomy. This autonomy refers to the extent to which the virtual agents are independent of user intervention and also the amount of information needed to simulate crowds. Thus, depending on the complexity of the simulation, simple behaviors can be sufficient to simulate crowds. Otherwise, more complicated behavioral rules can be necessary and, in this case, it can be included in the simulation data in order to improve the realism of the animation. We present three different ways for controlling crowd behaviors: by using innate and scripted behaviors; by defining behavioral rules, using events and reactions; and by providing an external control to guide crowd behaviors in real time. The two main contributions of our approach are: the possibility of increasing the complexity of group/agent behaviors according to the problem to be simulated and the hierarchical structure based on groups to compose a crowd.

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Hierarchical model for real time simulation of virtual human crowds

Modeling and rendering of natural scenes with thousands of plants poses a number of problems. The terrain must be modeled and plants must be distributed throughout it in a realistic manner, reflecting the interactions of plants with each other and with their environment. Geometric models of individual plants, consistent with their positions within the ecosystem, must be synthesized to populate the scene. The scene, which may consist of billions of primitives, must be rendered efficiently while incorporating the subtleties of lighting in a natural environment. We have developed a system built around a pipeline of tools that address these tasks. The terrain is designed using an interactive graphical editor. Plant distribution is determined by hand (as one would do when designing a garden), by ecosystem simulation, or by a combination of both techniques. Given parametrized procedural models of individual plants, the geometric complexity of the scene is reduced by approximate instancing, in which similar plants, groups of plants, or plant organs are replaced by instances of representative objects before the scene is rendered. The paper includes examples of visually rich scenes synthesized using the system.

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Realistic modeling and rendering of plant ecosystems

We present a rule-based approach combined with traditional geometric modelling techniques that allows easy generation of many branching objects including flowers, bushes, trees, and even nonbotanical objects. A set of components describing structural and geometrical elements of plants maps to a graph that forms the description of a specific plant and generates the geometry. Users get immediate feedback on what they've created-geometrical parameters, tropisms, and free-form deformations can control the overall shape of a plant. We demonstrate that our method handles the complexity of most real plants.

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Interactive modeling of plants

The reproduction of nature via computer has fascinated scientists in computer graphics and artists, ever since synthetic imaging was thought to be possible. This book illustrates and exemplifies methods for the creation of artificial plant models, and the application of these methods within areas such as simulation, virtual reality, botany, landscaping, and architecture. The models are combined to create gardens, parks, and even entire landscapes. The range of creating representational forms reaches from deceptively authentic looking pictures to abstract presentations. In addition, with similar methods organic objects can be produced, changed, and animated.

Digital Design of Nature: Computer Generated Plants and Organics

We describe the HUMANOID environment dedicated to human modeling and animation for general multimedia, VR, and CAD applications integrating virtual humans. We present the design of the system and the integration of the various features: generic modeling of a large class of entities with the BODY data structure, realistic skin deformation for body and hands, facial animation, collision detection, integrated motion control and parallelization of computation intensive tasks.

The HUMANOID Environment for Interactive Animation of Multiple Deformable Human Characters

We present a modelling method and graphical user interface for the creation of natural branching structures such as plants. Structural and geometric information is encapsulated in objects that are combined to form a description of the model. The model is represented graphically as a structure graph and can be edited interactively. Global and partial constraint techniques are integrated on the basis of tropisms, free-form deformations and pruning operations to allow the modelling of specific shapes. We show examples to illustrate the design process and evaluate the user interface.

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Bernd Lintermann

Papers

Realistic modeling and rendering of plant ecosystems

Interactive modeling of plants

Digital Design of Nature: Computer Generated Plants and Organics

The HUMANOID Environment for Interactive Animation of Multiple Deformable Human Characters

A modelling method and user interface for creating plants