Procedural Methods for the Modelling and
Simulation of Large Environments:
PhD Proposal
Rudolph Neeser October 22, 2007
1
Introduction
Computer graphics is concerned with generating images from computer mod-els. These models have various sources: for instance, they can be obtained through real world measurements — as with diagnostic radiology (CT, MRI, PET, etc.) and laser range scanning — or constructed by artists using com-puter software. Models produced by artists are often seen in the media: movies make extensive use of such content for their special effects, and the content of mosts games is almost exclusively produced by skilled artists. The demand for computer models produced by artists is growing, as can be seen by the increasing reliance of movies on special effects: Peter Jackson’s Lord of the Rings trilogy, the Star Wars movies, and recent movies such as Sin City and 300 all make heavy use of special effects and computer mod-els. The need for such content in games is also growing: World of Warcraft, Everquest and Ultima Online are examples of games in which large worlds are modelled and made available for the players to explore. The production of content on such a large scale is both time consuming and difficult, and the automation of this content production is an important goal not only for the entertainment industry (where the largest commercial interest lies), but also for the heritage, simulation and city planning domains. Such content production deals with how formal systems can be used to specify computer models, how users / artists can interact with such formal systems to produce the desired models, and how capable these formal systems are at producing content at a large enough scale while still remaining useful.
This thesis focuses on the formal computer graphics methods for pro-cedurally (i.e., automatically) producing such content from only a tex-tual description. The current procedural methods of choice are L-systems (Prusinkiewicz and Lindenmayer, 1990) and shape grammars (Stiny, 1975), the former having originally found use in biological modelling and simula-tion, and the latter in architecture. While L-systems have been employed
for content production since 1974 (Hogeweg and Hesper, 1974; Frijters and Lindenmayer, 1974), this thesis examines the use of L-systems for developing extremely large models, models at the very least the size of a city, including the landscape, the flora, human-made structures, furniture, and so on.
2
Title
The proposed title of the thesis is
“Procedural methods for the modelling and simulation of large environments.”
Procedural methods are those that use a well defined method (a procedure or algorithm) for producing content. Being well defined, these methods are suitable for computer automation. L-systems are an example of a procedural method: they take as input a short, textual description of the object that one wishes to produce, and the L-system uses a series of formal rules to determine the structure of the object from the supplied description.
L-systems themselves have not only been used for modelling, but have found application in simulation. In particular, they have been used to simulate the growth of plants (Prusinkiewicz and Lindenmayer, 1990), in-teractions with the environment (Mˇech and Prusinkiewicz, 1996), devel-opment controlled through signaling mechanisms (hormones, for instance) (Prusinkiewicz and Lindenmayer, 1990), and so on.
These and similar methods have also seen use in the production of large environments. Large environments are those that cover large areas and con-tain many separate objects, a city being the standard example of a large en-vironment, with previous work focusing on road network generation (Parish and M¨uller, 2001), and the modelling of building exteriors (M¨uller et al., 2007; M¨uller et al., 2006; Wonka et al., 2003).
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Research Areas
There are a number of areas that this research will be looking at. All of these are focused on the use of procedural methods in the production and simulation of large environments, both human-made and natural.
1. The creation of “high quality” models. Content produced through L-systems are frequently restricted to simple geometric forms: cylin-ders, cubes, and so on, often with prominent features — such as flowers — modelled separately (e.g., Wonka et al., 2003; Parish and M¨uller, 2001; Prusinkiewicz and Lindenmayer, 1990). To produce “high qual-ity” models, free-form surface production methods are required. The
surface modelling (Catmull and Clark, 1978; Doo and Sabin, 1978), which are a family of methods for producing surface models by itera-tively refining a coarse model to produce a finer, smoother, end model. Subdivision surfaces can be applied to the interpretation of the pro-cedural method, although some work has been done on directly rep-resenting subdivision surface techniques with L-system methods (e.g., Prusinkiewicz et al., 2003; Velho, 2003), though this is not yet com-monly applied to procedural modelling applications. The implications of doing so (especially those concerning the usability of such a system) are not yet clear.
2. The production of large, complete models. In recent years work has been carried out on producing large urban environments using L-systems and other procedural methods (see, for example M¨uller et al., 2006; Wonka et al., 2003; Parish and M¨uller, 2001). These methods have so far all been incomplete: they have focused on the exterior qualities of urban environments, such as building fa¸cade (e.g., M¨uller et al., 2006), road networks (e.g., Parish and M¨uller, 2001), and so on. The interiors of these buildings have not been considered. Often, as is the case with M¨uller et al. (2006), the generated building models overlap with one another — while this can create acceptable models of the exterior of a city, the building models are incapable of being used if their interiors are of interest. Currently, the placement of furniture, eating utensils, and so on, have not been considered, although this lies within the domain of procedural content generation.
3. Interfacing with procedural content generation systems. One of the largest flaws of procedural generation systems is that the user has little control over the generated content. This is due to the pro-cess being completely automated: as the amount of content produced from any given description increases, it is no longer clear how the con-tent relates to any given part of the description. If strict requirements over the produced content is required — such as having a particu-lar building that meets specific architectural requirements present at a specific intersection of a procedurally generatated city — it is not always clear how this can be achieved. Some current work that at-tempts to deal with this problem does exist. For instance, there are methods that attempt to determine the procedural description of a building using either photographs (M¨uller et al., 2007) or a combina-tion of modelling and texturing (Aliaga et al., 2007). The development of procedural methods for the modelling of large environments requires the co-development of techniques to combine procedural methods with pre-generated content.
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Related Work
Various papers have dealt with the construction of large environments using procedural methods. Deussen et al. (1998) presents work for construct-ing ecosystems of plants (i.e., large environments of plants). Their work allows the user to either specify a plant distribution (a common interface technique), or to have the distribution determined via an ecosystem simu-lation. A review of ecosystem related procedural methods can be found in Prusinkiewicz (2000).
Another strand in the large environment work is the procedural mod-elling of urban landscapes. Parish and M¨uller (2001) present work for the modelling of a whole city, with a large focus on the development of road networks. This particular paper makes use of L-systems, although later work by M¨uller shifts to shape grammars: M¨uller et al. (2006) use these to construct more detailed building models than those presented in the 2001 paper. However, while the building exteriors are well modelled, the interiors consist of overlapping geometry and are unusable. This work is conceptually related to the similar paper of Wonka et al. (2003), and indeed M¨uller and Wonka pair up to present M¨uller et al. (2007). This 2007 paper attempts to construct procedural models of building fa¸cades using only orthorectified photographs of the fa¸cade. Statistical techniques are used to identify re-peating features (such as windows and floors), and these features are used to populate a procedural model. Windows and other geometric features are also automatically identified in the photograph and extruded in the fa¸cade model, creating real geometry — rather than simple textures — to represent these features. The method allows the user to take an input photograph and build multiple fa¸cade models, all in a similar vein to the original photograph. Aliaga et al. (2007) offer a similar system, although their work constructs whole buildings rather than only fa¸cades. Aliaga et al. make use of Paul De-bevec’s (1997) FACADE photogrammetric modelling method to construct models of buildings from photographs; portions of these models are then semantically tagged by the user, allowing the system to determine how the model may be split up and proceduralised. While M¨uller et al. (2007) pre-sented a method that was automated, it was based only on fa¸cades; the method of Aliaga et al. (2007) has the advantage of being capable of con-structing not only fa¸cades, but whole models that are similar to the input.
These last two works offer an interesting method for combining procedu-ral content with either real world data or with pre-generated models. Indeed, they provide an interesting technique to allow procedurally generated con-tent to conform to pre-generated concon-tent, and are an interesting starting point to some of the research areas discussed in this proposal.
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Anticipated Outcomes
Broadly, this work will attempt to create a method for producing large envi-ronments from a relatively short textual description of the environment. The environment will include the geography of the landscape, flora, and human-made structures. The structures themselves will have both their interior and exterior surfaces modelled, including being furnished. A framework to easily introducing pre-generated content into the environment, and have the environment naturally grow around it, would be an important aspect to the usefullness of such a system.
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Milestones
The milestones for the project can be seen in table 1. My anticipated comple-tion date is the end of 2009. There are plans for two papers to be published while this thesis is in progress (in 2008 and 2009).
References
Daniel G. Aliaga, Paul A. Rosen, and Daniel R. Bekins. Style grammars for interactive visualization of architecture. IEEE Transactions on Visu-alization and Computer Graphics, 13(4):786–797, July / August 2007. E. Catmull and J. Clark. Recursively generated b-spline surfaces on arbitrary
topological meshes. Computer-Aided Design, 10(6):350–355, 1978. Paul E. Debevec. Facade: modeling and rendering architecture from
pho-tographs and the campanile model. In SIGGRAPH ’97: ACM SIG-GRAPH 97 Visual Proceedings: The art and interdisciplinary programs of SIGGRAPH ’97, page 254, New York, NY, USA, 1997. ACM Press. ISBN 0-89791-921-1. doi: http://doi.acm.org/10.1145/259081.259366. Oliver Deussen, Pat Hanrahan, Bernd Lintermann, Radom´ır Mˇech, Matt
Pharr, and Przemyslaw Prusinkiewicz. Realistic modeling and rendering of plant ecosystems. In SIGGRAPH ’98: Proceedings of the 25th annual conference on Computer graphics and interactive techniques, pages 275– 286, New York, NY, USA, 1998. ACM Press. ISBN 0-89791-999-8. doi: http://doi.acm.org/10.1145/280814.280898.
D. Doo and M. Sabin. Behaviour of recursive division surfaces near extraor-dinary points. Computer-Aided Design, 10(6):356–360, 1978.
D. Frijters and Aristid Lindenmayer. A model for the growth and flowering of Aster novae-angliae on the basis of table (1,0)L-systems. In G. Rozenberg
Nov. 2007 A discussion of previous work
Feb. 2008 Interior modelling of buildings complete
Mar. 2008 City modelling (road networks, city footprint) complete
Apr. 2008 Integration with simple procedural generation of flora complete
May. 2008 Paper on city modelling complete Jun. 2008 Background chapter of thesis complete.
Jul. 2008 Integration of subdivision surface rule systems with L-system program.
Jul. 2008 Integration of subdivision surface module with existing city module.
Aug. 2008 Thesis chapter on constructing cities complete. Oct. 2008 Completion of flora subdivision module,
includ-ing the use of subdivision surface modellinclud-ing. Nov. 2008 Thesis chapter on modelling flora complete. Jan. 2008 Development of landscape modelling module
based on subdivision surfaces to be completed. Feb. 2008 Integration of all components complete.
Mar. 2008 Thesis chapter on landscaping complete. Apr. 2009 Paper describing complete system complete. Jul. 2009 Chapter on reverse engineering L-systems
com-plete.
sep. 2009 First draft of thesis complete. nov. 2009 Thesis complete.
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