Computer Science Honours

Procedural Modelling of Plant Scenes

 

Author:                        Kim Randleff-Rasmussen

Supervisors:                Shaun Bangay

                                   Adele Lobb

 

 Literature Survey

Introduction

This project will model a number of plant scenes using L-Systems and procedural modelling. The modelling of the plants to be rendered will be achieved through the use of L-Systems and procedural modelling is the chosen technique to bring the modelled plants together.

The following literature review summarises the information available on L-Systems and other plant modelling techniques, procedural modelling and computer graphics software, with focus on those which concentrate on translation of L-Systems.

The overview is structured as follows: first, a description of L-Systems is given followed by an investigation into work that has been conducted on, with and concerning L-Systems. Next, this review looks at other ways of modelling plants and organic structures followed by an exploration into procedural modelling. This section will focus on instances in which procedural modelling has been used to model geometrically complex scenes. Lastly, a review is conducted of current graphics software which provides interpretation of L-Systems.

 

Plant Modelling

L-Systems – Definitions and Explanations

In 1968, Hungarian biologist Aristid Lindenmayer developed a formalism for multicellular development [Lindenmayer 1968]. This formalism, subsequently named L-Systems (Lindenmayer Systems), captured the imagination of computer graphics artists who extended and improved the basic string rewriting language to form a model. This model is now widely used in computer graphics for the modelling of plants and other organic structures.

An L-System is a string rewriting formalism which can be interpreted graphically by a turtle graphics program. These rewritings can be applied sequentially or in parallel, to every module or according to contextual information. There has even been the definition of a type of L-System which allows for both – the rewriting of two or more symbols with a single production [Prusinkiewicz 1986]. The rewritten strings are then parsed by a LOGO-style graphics turtle which interprets each symbol in the string as an action – for example, F often means draw a line forward and + represents a rotation around the y-axis by a number of degrees which is defined by a constant or a parameter [Szilard and Quinton 1979].

Since the original definition of L-Systems, many formal explanations and definitions of L-Systems have been published. Among the more prominent writers is Professor Przmyslaw Prusinkiewicz from the University of Calgary who, along with Lindenmayer, wrote what is considered to be the authoritative work on the subject [Prusinkiewicz and Lindenmayer 1990]. He has also written papers describing more specific types of L-Systems.

L-Systems – Extensions

Much has been published regarding different types of L-Systems which allow for certain changes in order to more accurately model reality. Rozenberg, for example, defined table L-Systems [Rozenberg 1973] which allow for the choosing of a set of developmental rules according to certain environmental factors. Environmentally sensitive L-Systems [Prusinkiewicz et al. 1994] were the next step, providing query symbols which allowed L-Systems to change as a result of local environmental conditions. Open L-Systems [Prusinkiewicz and Mĕch 1996] extended this functionality by including bilateral communication with the environment. This extension allowed the environment to be influenced by changes in the tree model, which enabled the realistic modelling of such aspects as nutrient uptake and supply.

Other extensions provide functionality for variation of a single model to be applied to a number of individuals. Parametric L-Systems [Prusinkiewicz et al. 1995] include the facility to involve parameters in a standard L-System which allows many aspects of the model to be changed dynamically rather than statically defined in the model. Such aspects as step length and rotation angle can easily be varied in this manner in response to environmental and local changes. Differential L-Systems (or dL-Systems) [Prusinkiewicz et al. 1993] extend these parametric L-Systems by incorporating continuous time flow. This facility allows smooth animation of plant development rather than the rendering of discrete development steps, as is the standard practice.

Perhaps the most widely used L-Systems in terms of individual variation are stochastic L-Systems [Eichhorst and Savitch 1980]. These use multiple productions for each symbol and assign probabilities to each production. The specific production which is chosen at run-time is then determined by a random number generator.

Many works exist which serve as extensions to Prusinkiewicz and Lindenmayer’s original explanatory work. These compile all of the extensions made to L-Systems since 1990 into formal definitions and explanations including examples. A representative example of this synoptic work can be found in [Prusinkiewicz et al. 1996].

Some L-System extensions which are unlikely to be found in such formal definitions, are ones which are intended to model a specific element of plant development for a specific project. Some of these have been the inclusion of a cut symbol (%) [Hanan 1992] to model self-pruning (see also [Prusinkiewicz et al. 1994]) and the development of erasing productions [Herman and Rozenberg 1975] to enable individual variation in a standard model.

Prusinkiewicz and Lane define a particular type of L-System which allows a set of productions to apply to a set of strings (individual plants) instead of to a single string. These L-Systems, called multi-set L-Systems [Prusinkiewicz and Lane 2002], mean that a dynamic plant population can be maintained and operated on with a single model. This technique was developed in order to manage spatial distribution of a plant population.

Much work has been done on the inclusion of genetic algorithms [McCormack 1993], mutation [Mock 1998] and evolution [Rodkaew et al. 2002] into L-Systems. This amalgamation of techniques allows for such applications as virtual landscaping and adaptation simulations. These simulations can be further aided by a technique known as extrusion in space-time [Hammel and Prusinkiewicz 1996] which extends a two-dimensional structure’s development into a three-dimensional line or curve which represents the progress of time.

L-Systems – Graphical Interpretation

As mentioned, the most common way of translating L-Systems into three- or two-dimensional images is through turtle graphics. A reasonable summary of this technique is to be found in [Prusinkiewicz et al. 1995], an author of which also helped to develop a software program called CPFG [James et al. 1993] which puts these turtle interpretation principles into practice.

[Prusinkiewicz et al. 1999] describes some of the programming principles behind the graphical interpretation of L-Systems. This paper is particularly useful for readers looking for explanations of L-System translation from formal language to programming language.

Other Plant Modelling Techniques

L-Systems are not the only plant modelling technique available. Many other models abound, some of which focus on plant geometry as L-Systems do, and others which model trees based on behavioural aspects.

Some well-known models in the former class include the one developed by de Reffye et al. [de Reffye et al. 1988] which models botanical rules in a mathematical model. Other models focus more on particular aspects of plant geometry. For example, Borchert and Honda [Borchert and Honda 1984] designed a model which illustrates the tendency of a tree to increase the vigour of growth after the loss of a branch. Meyerowitz [Meyerowitz 1994] used the specific plant Aribidopsis thaliana to model genes and mutation as they influence plant growth.

A comprehensive study of plant architectures and models which reflect the range available can be found in [Hallé et al. 1978]. This book asserts that the thousands of plant species in the world can be modelled using a mere twenty-three models. These models take into account individual species architecture and genetic “growth programme” as well as competition for space and nutrients in a single forest. It goes one step further, too, and models the architecture of forests as separate entities. Borchert and Slade [Borchert and Slade 1981] also analysed the growth of a number of tree species in order to create models.

The second class of plant models focuses on how behaviour and the environment influence the manner in which a tree grows. [Prusinkiewicz et al. 2000], for example, models the effect of gravity on the branching behaviour of certain trees. Another paper by one of those authors looks at pruning – forced and self-imposed – and how this affects branch growth [Prusinkiewicz et al. 1994]. Yet another paper [Prusinkiewicz et al. 2001] uses positional information to model the growth and survival of a plant. This model uses such techniques as self-thinning and competition for space to model how tree growth is affected by environmental factors.

A model designed by Lück, Lück and Bakkali [Lück et al. 1990] suggests the use of a growth potential or vigour value to be assigned to branches. This value dictates which of a number of competing branches will triumph. Honda et al. also focussed on branch interaction between trees [Honda et al. 1981]. The value they use to determine the “winning” branch is flow rate which dictates the rate of bifurcation. Sorrensen-Cothern, Ford and Sprugel divide a single plant into a number of modules and model the interaction between these rather than interaction between entire plants [Sorrensen-Cothern et al. 1993].

Another trend to be found in plant modelling is the use of fractals as introduced by Mandelbrot in 1975. [Oppenheimer 1986] contains a useful model of plants through fractals as well as a summary of a number of tree models which have been published through the years. [Smith 1984] also summarises a number of plant models, both fractal and L-System based. His paper also contains a practical summary of formal languages as used in such modelling.

Plant Modelling in Practice

For completeness’ sake, this section contains mention of a number of papers which describe the use of plant models (both L-System and other) in practice. [Mackrill 2003] used L-Systems to model realistic landscapes including trees, grass and terrain. His focus was on the ability to render in real-time. [Prusinkiewicz et al. 1988] describes the use of L-Systems to model not only branches, but leaves and flowers as well. There is focus, here, on the ability to show the animation of plant development, hence the choice of L-Systems as the modelling technique.

[Weber and Penn 1995] designed an entirely new plant modelling language which allowed for realistic depiction of geometry without strict adherence to botanical principles. They also presented a technique for varying detail level which eases the rendering of large, landscape shots of complex forests.

 

Procedural Modelling

Procedural modelling is a well-used computer graphics technique which allows geometrically complex objects to be modelled dynamically at run-time rather than stored statically in an archive [Foley 1996]. This process cuts down dramatically on the computational power needed, as only those objects which are visible need be rendered. As the objects are modelled dynamically, certain aspects of the models can be changed according to the needs of the scene at the time. For example, detail level can be set according to how far from the camera the object is.

A number of papers and books exists which explain the concepts behind procedural modelling. Some of these are [Jeschke et al. 2003] and [Macri and Pallister 2004]. Amburn et al. [Amburn et al. 1986] suggest a number of ways in which traditional procedural models might be improved. These techniques include communication between models and subdivision of labour.

[Smith 1984] also discusses improvements to procedural models. His technique is one called “database amplification” and is a property of formal languages which allows the database of available symbols to be dynamically added to.

Procedural modelling has long been in use in programs which model geometrically complex scenes such as forests, cities and crowds. A number of papers present these programs, describing the use of procedural models to ease the rendering of complex scenes which contain many instances of geometrically similar constructs (such as trees and buildings). Some of the more prominent papers which model large cities are [Birch et al. 2003], [Greuter et al. 2003], [Parish and Müller 2001] and [Wonka et al. 2003]. In terms of forest or landscape generation, some papers to look at include [Guerraz et al. 2003] and [Deussen et al. 2002].

Many of such papers describe not only the use of procedural models, but also the interaction between the procedural models and other aspects of scene modelling. [Deussen et al. 2002] and [Greuter et al. 2003], for example, describe entire systems for scene modelling – drawing all aspects of modelling and rendering together.

 

Software Products

Much research and development has been done into software programs that translate L-Systems into three-dimensional images. Other software products have also been developed which model plants and organic structures without the explicit use of L-Systems.

One of the first L-System translation systems was developed in 1970 [Baker and Herman 1970] in order to simulate organic development for hypothesis testing. Prusinkiewicz and his students and colleagues have developed a number of software products since that time. CPFG [Prusinkiewicz 1993] takes a parameterised procedural model of a plant and outputs a geometric model. This product is used in the modelling system L-Studio [Prusinkiewicz 1998] which provides a graphical user interface for the modelling of plants via L-Systems with comprehensive options for all aspects of L-Systems.

 Laurens Lapré, a student of Professor Prusinkiewicz, developed a system called Lparser [Lapré 1993] which is available in open source and is a good place to start for beginners in L-System translation due to the readable, logical code. Another product developed by a student, though not one taught directly by Prusinkiewicz, is L-Arbor developed by Marco Grubert [Grubert 1999]. This program offers simple animation and L-System modelling facilities and is an easy way to learn basic L-Systems.

 The Institute of Forest Biometry and Informatics at the University of Göttingen is a special interest plant modelling group comprised of experts in a number of fields including forest science, biology and mathematics. Winfried Kurth is the computer graphics expert and he developed a modelling program called GROGRA (GROwth GRAmmar) [Kurth 1994].

 GRAMPS [O’Donnell and Olson 1981] is a graphics language interpreter developed to model molecular structures and to animate their development. It is used in conjunction with an interactive vector display list processor and is focussed on the provision of real-time scene manipulation facilities. A nice feature of this program is the facility to dynamically add new graphics manipulation commands through the use of macros.

 

Conclusion

 L-Systems were chosen over any of the other available modelling techniques due to their pervasiveness and, therefore, the availability of support. Another reason is their effectiveness in accurately modelling plants – which, in conjunction with their pure simplicity, makes them a viable option for the less experienced graphics practitioner.

 Procedural modelling was chosen because, again, it is a widely documented technique. It also allows for objects to be rendered only if they can be seen by the camera, an attribute which makes the creation of complex scenes less computationally intensive.

 Both of these techniques are well-documented and widely used in the modelling and creation of geometrically complex scenes. This literature review is intended to be a representative study of the wide range of literature available on both topics. For completeness’ sake, other plant modelling techniques are explored as well as instances of the use of procedural models in practice to model complex scenes. These scenes include forests, landscapes and cityscapes. A brief look at the software products available for L-System creation and rendering completes this review.