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In contrast to photorealistic landscape images, the rendering of synthetic plant and landscape sketches has undergone little research so far. However, the already-addressed areas of application in architecture and landscaping require this kind of rendering. Currently, prefabricated images, which are combined via computer, most often must suffice. In addition, there are several collections of images [58] that incorporate lots of plant images in different styles and scales. These images can be copied and added to the drawings.
Aside from the enormous manual effort that is needed to combine the images, this method, of course, is only applicable for single images. New media, however, permit entirely new forms of presentation. The user should be able to analyze planned buildings virtually, to see them in different views, and to modify the corresponding scenes interactively. This is not possible using traditional plant images as part of a series. Digital plant models and drawings are required, which can be turned and moved and thereby reflect a spatial coherence. Since this kind of modeling is also useful in areas outside of architecture and landscaping, such as biology, botany and education, developing the proper methods and tools is a worthwhile undertaking that reaches far beyond the rendering of plants.
In this chapter we will introduce different approaches for the production of plant images in the style of hand-drawn artworks. After a short introduction into the area of nonphotorealistic rendering (NPR), traditional drawing styles used by artists will be examined with regard to their algorithmic implementation. It has become evident that some drawing styles are replicated satisfactorily via the computer, while others cannot be simulated. In a new method, some of the traditional techniques are imitated, and sample plants are produced in various drawing styles. Sample animations can be found on the accompanying website that prove that models generated in this way have the required spatial coherence, i. e., they can be turned and scaled without interfering flickering effects.
An important aspect when drawing trees is the rendering of the trunk and the branches. Here the methods differ from those used for the leaves, since we are
dealing with a totally different form of geometry. In particular, the production of cross-hatchings on the trunk and branches is an interesting problem that is, however, partly solved by existing approaches.
