Abstract
The effort of generating large city scenes is greatly reduced when using procedural modelling techniques. Nonetheless. users are limited to defining rules and tuning parameters in a define-generation reanalyse cycle, reducing direct control. Describing buildings with split-based operations may disrupt the creative process by forcing the user to focus on how to describe a building instead of how a building should look. We use planar shapes and two-dimensional boo lean operations allowing procedural descriptions that are More aligned with humans' perceptions of buildings while letting users directly manipulate shapes (e.g., by dragging) in an interactive application.

Abstract
The effort of creating virtual city environments is reduced using procedural modelling techniques. However, these typically use split-based approaches which can impose limitations on the final geometry, usually enforcing a grid-like structure and require complex geometry to be imported. Layered shape grammars can increase the variability of procedural buildings, while the vectorial definition of shapes introduces the possibility of creating complex shapes that seamlessly blend into the model. We evaluate the contributions with a modelling example and a comparison with split-based procedural modelling techniques. Results show that layers allow more flexibility than split-based techniques in creating variations. Vectorially defined shapes are a step forward in shape grammar expressiveness.