Basics of Rule-based Modeling
The CGA shape grammar of the CityEngine is a unique programming language specified to generate architectural 3D content. The term CGA stands for Computer Generated Architecture. The idea of grammar-based modeling is to define rules that iteratively refine a design by creating more and more detail. These rules operate on shapes which consist of a geometry in a locally oriented bounding-box (so-called scope). The following rule derivation illustrates the process: on the left side the initial shape is shown and on the right side the resulting generated model is displayed.
In this section the essential concepts of procedural modeling with the CGA shape grammar are listed: First, it is described in detail what a shape is. Afterwards, the definition of rules is given, the idea of shape operations is explained and the concept of inserting arbitrary geometry, so-called assets, is presented. Finally it is described how rules are applied to generate a model. This results in a data structure called model hierarchy (or shape tree) which plays an important role for the understanding of the modeling process.
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