Digital Design of Nature

Plants

For an adequate evaluation and the ability to generate plant geometry, a com­paratively small range of botanical methods and descriptive formulas is needed. Computer graphics in most cases works with relatively rough visual abstrac­tions of a plant, and therefore it is necessary to identify the shape and the funda­mental geometrical relations in the structure of a plant. An interactive modeling process often even permits us to render a correct shape without explicitly using a functional model of the plant.

Consequently, we concentrate on a purely morphologic aspect of botany. Nat­urally, the shape of a plant and its characteristics are determined by the func­tion of the plant, and within the context of a functional view it is necessary to consider both aspects, although a purely morphologic description allows for the recognition of substantial regularities in the structure. For a more de­tailed overview of the connection between structure and function see [218]. A comprehensive description of plant physiology, transmission and temporal developments is to be found in the standard works on botany, for instance [125, 169,211].

In the following, we are mainly concerned with trees and shrubs. These cor - mophytic growth forms, i. e. plants that consist of roots, bud axes, and leaves are dominant in many landscapes and therefore play a very prominent role in the areas of computer graphics and landscape planning. Within the framework of this book, it is not possible to cover the overwhelming amount of different growth forms found in nature; however, in later chapters we will demonstrate that if synthetic plant production methods provide a carefully selected set of basic growth algorithms, almost all types of plants can be generated.

Let us start with a general description of cormophytical plants. The direction of the bud axis and the form of branching determines to a large extent the charac­teristic shape of a plant. Additionally, the process of bud development, several forms of spatial division, and different tropisms comply with these basic rules and provide additional laws that influence the visual structure of plants. Ac­cording to the pioneering works of Halle, Oldeman and Tomlinson [81], we distinguish between different types of trees according to their structure. The

Chapter 2 authors defined 23 different tree structures that include all the important cor - Plants mophytical branching patterns found in nature.

Aside from the branching structure, also the shape of the leaves and their devel­opment within the growth cycle of the tree are important for the visual appear­ance. Though computer graphics usually works with a rough approximation of the leaf morphology, for computer animations a faithful representation of the leaf shape and its development are important. A more detailed discussion follows in Sect. 9.6 that introduces the rendering methods for single plants. Later in this chapter, we will focus on plant communities. Similar to the intri­cate methods of botanical descriptions of single plants, special attention has to be given when outlining the characteristics of plant communities, and, thus, the complex interactions of a set of single plants. However, for the purpose of this book, only the most important description methods are specified, in particular those that are interesting in computer graphics for the implementation of the various kinds of synthetic construction procedures. We will, therefore, just like in the case of single plants, refrain from presenting a dynamic interpretation of vegetation, and instead of elaborating a causal relationship we will adopt a purely morphologic view. We are not so much concerned with reconstructing the origin and evolution process of a landscape, but rather with the methods that render the overall appearance of the landscape itself.

In geobotany, according to [191], plant populations and communities are ex­amined with regards to their contemporary distribution pattern as well as their dependence on existing environmental conditions. Vegetation geography takes another approach. Here the plant cover of the earth is analyzed with regard to the significance of different regional characteristics. The connections be­tween the actual vegetation and the different environmental factors, such as climate, relief, waters, and soil, are extremely multifaceted, and in this chapter are merely touched upon. More in-depth coverage of the subject is to be found in selected works (see [211, 68]).

Chapter 8 addresses some of these aspects in more detail. Here we discuss the methods used for the specification of terrain and the water regime in the soil, and simulate the development of a plant population according to the lo­cal water conditions. For a more comprehensive treatment of the subject, we suggest selected works on ecology which specifically deal with modeling meth­ods in biology and ecology [22, 79, 102, 160]. The same is true for the view of the temporal, in particular the historical perspective, without which an un­derstanding of the propagation terrain of plants is extremely difficult. Again, viewed from a vegetation-geographical perspective, we are more interested in the static, and not in the temporal bound description of vegetation.

Later in this chapter, we also have to briefly outline description methods of vegetation geography. The plant cover of the earth here has to be described on various abstraction levels, and a number of spatial description forms, ranging from the general vegetation zones to the regional areas, represent habitats of plants in different levels of detail. At the very end of the line, however, there is the single plant, and we will focus on it first.