Procedural Urban Modeling in Practice

Procedural Methods for Urban Modeling Tutorial Procedural Urban Modeling in Practice Benjamin Watson ■ North Carolina State University Pascal Müller ■ Procedural Inc. Peter Wonka ■ Arizona State University Chris Sexton ■ Johns Hopkins University Oleg Veryovka and Andy Fuller ■ Electronic Arts rocedural modeling is finally going main- such as fire and smoke2—most famously the “Gen- stream. Until recently it was a boutique esis effect” in Star Trek. Perlin’s noise3 simulates solution, used only when no other options clouds and natural textures, while Reynolds’ Pexisted. Now procedural modeling is often the boids4 apply agent-based methods to animate cost-effective solution, used because the alterna- flocks, schools, and herds, including stampeding tives are too expensive. In film, wildebeests in The Lion King. Genetic techniques games, and other applications, similar to those described by Sims5 will soon see Film and game studios can no consumers expect richer, higher- use in Spore, a game from Electronic Arts (EA). longer meet audience demand quality digital content for their Synthesizing natural landscapes has a similarly for visual content by increasing dollar. Because budgets won’t al- long research history, but since cities sit on landscapes production budgets. Instead low content producers to increase this approach applies more directly to urban problems they are turning to procedural cost significantly, they have only than the techniques discussed previously. Fournier modeling, particularly for one choice: they must improve and colleagues use stochastic, fractal techniques in- their tools. Procedural model- spired by Mandelbrot.6 Musgrave and colleagues ex- modeling cities. The authors ing is the primary ingredient in tend these techniques to model erosion effects.7 More review procedural modeling, these tools. recently, Zhou and colleagues applied texture syn- examine the CityEngine One of the main drivers of thesis—used to make large, unique textures from tool, and study the use of these trends is urban content. small source patches—to create natural terrain that procedural urban modeling Cities are huge, richly detailed respects artist constraints.8 Planetside’s Terragen, a in Electronic Arts’ Need for artifacts often required in digital commercial software package widely used in film Speed games. productions. Modeling them with and games, implements many of these techniques. existing, nonprocedural tools can take hundreds of man-years. Several researchers are Synthesizing urban terrain creating procedural techniques specifically for auto- Procedural techniques dedicated to urban synthe- mating city modeling. sis have only begun to appear more recently. Par- ish and Müller use L-systems to model extensive A brief history street layouts and buildings.9 Given input maps Computer graphics practitioners have long used of geography, population density, and layout con- procedural modeling to generate nonurban constraints, their system generates streets, subdivides tent. L-system grammars generate plants,1 while land, and creates skyscrapers. Figure 1 shows an agent-based particle systems model fuzzy objects example of road generation. 18 May/June 2008 Published by the IEEE Computer Society 0272-1716/08/$25.00 © 2008 IEEE Figure 1. Examples of Lechner and colleagues use agent-based technol- procedural ogy to build transportation networks, subdivide road synthesis.9 land, allocate use, and manage population density.10 Their agents adapt to their environment, letting artists steer development interactively to meet specific application constraints. A recent ex- tension to their work by Sexton and Watson vec- torizes the gridded simulation output, producing realistic, smoothed, urban terrains that you can import into various geographic information system (GIS) tools for further analysis.11 Esch and colleagues support interactive modifica- © 2001 ACM tion of street layouts through 2D tensor fields (ten- sors are a generalization of vectors).12 Artists can change layouts indirectly by manipulating the tensor supporting beams and infrastructure. Pottmann field or directly by reshaping the roads themselves. and colleagues synthesize the quadrilateral, pen- In gaming and other interactive applications, tagonal, and hexagonal meshes that characterize the ability to synthesize urban content in real many beam layouts.20 They can find layouts for time would be especially useful. Greuter and col- almost any curved surface. Smith and colleagues leagues describe a first pass at this problem, laying automate the design of the truss structures that out cities using a simple grid, and creating simple support buildings, bridges, and many other struc- skyscrapers from input footprints on the fly.13 To tures.21 Their method accommodates the real-life meet real-time constraints, they pay particular at- engineering constraints that relate geometry to tention to caching and limiting computation out- mass and stress. side the view frustum. Floor plans describe building interiors and often dictate the appearance of building exteriors, Synthesizing buildings and other structures especially in homes. Harada and colleagues apply In 1971, the architect Stiny introduced shape optimization algorithms to the design of 2D ar- grammars to bring a new formalism and rigor chitectural floor plans,22 while Martin devises a to designing and analyzing architecture.14 Many grammar that constructs graphs in which nodes designers adopted shape grammars, but they re- represent rooms and links connections between mained largely conceptual tools, synthesizing only rooms.23 He then translates these graphs into in- conceptual mass models of buildings and other terior and exterior building geometry. Hahn and structures. Wonka as well as Müller and colleagues colleagues generate building floor plans and cor- took the next step, introducing split grammars for responding 3D interiors in real time by randomly synthesizing detailed building facades,15 building dividing rectangular floors into rectangular rooms a shape grammar called CGA Shape for creating and hallways.24 The division respects several basic entire building exteriors,16 and creating the City- architectural constraints such as connecting ad- Engine integrated modeling environment. More jacent hallways and ensuring that small private recently, they have used computer vision to auto- rooms are immediately accessible from public matically generate grammars describing facades.17 spaces. Like many researchers, Hahn and col- Aliaga and colleagues describe a similar system leagues store and reuse random seeds during real- that facilitates authoring grammars for texturing time generation, ensuring that a given floor has building exteriors.18 the same floor plan at each viewing. Merrell generates buildings (and many other shapes) using a texture-synthesis-inspired tech- Synthesizing other urban content nique.19 The artist partitions a small example Urban content is more than building placement model into parts using a 3D grid. This partition and shape. Legakis and colleagues synthesize “cel- then generates a set of constraints: two parts might lular” textures of brick, tile, and masonry for only be adjacent in the output model if they were buildings and other structures.25 Textures respect adjacent in the example model. Merrell uses opti- component (such as brick) shape, commonly mized search in this constrained space to generate used tiling patterns, and structure geometry. For output models example, brick textures on each side of a corner Many structures such as bridges and train sta- reflect the fact that the same 3D bricks occupy tions are best characterized by the layout of their both textures. IEEE Computer Graphics and Applications 19 Procedural Methods for Urban Modeling Tutorial Figure 2. The workflow Design idea/concept for a typical How difficult is using the CityEngine? In the architectural Analyze design and parameters three example projects we describe, we collaborated procedural- with archeologists, urban planners, and artists to modeling produce their models and found that shape gram- project. Create elements and textures mars are as easy to learn as scripting languages. Users unfamiliar with scripting generated grammars without difficulty using the CityEngine’s vi- Encode design rules sual interface or its image-based methods. Soon the market will render its own judgment: Proce- Add stochastic behavior dural Inc. plans to release the CityEngine in the second quarter of 2008. Generate models Workflow Workflow in the CityEngine typically begins with a specific idea stemming from a photograph, a drawing, an architectural figure, or a new de- Weathering and wear are important elements sign concept (see Figure 2).9,31 The next step is to of urban realism. Dorsey and colleagues model analyze the design and find the most important the weathering of stone, including oxidation and parameters. erosion.26 Chen and colleagues simulate weather’s For example, consider Le Corbusier’s Cruciform effects on additional materials and model other Skyscraper. Le Corbusier designed several detailed types of weathering, including moss, rust, and dirt variations of the skyscraper between 1920 and accumulation.27 1930, with the most famous incarnations appear- Without people, cities are ghost towns. Thomas ing in the master plans for his Contemporary City and Donikian populate cities with cars and pedes- (1922) or the Plan Voisin (1925). The enormous trians, each with matching behaviors and anima- (for that time) 60-story

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