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OpenSG Scene Graph Scripting
Using the GML

Volker Settgast, Dr. Sven Havemann, Prof. Dr. Dieter Fellner

Computer Graphics and Knowledge Visualization, Graz University of Technology, Austria, www.cgv.tugraz.at

Motivation

The Generative Modeling Language (GML) is a suitable tool for creating various sorts of complex 3D models on a per object basis. A wide range of parametric objects has already been generated, including buildings and car rims. Combining GML with a scene graph like OpenSG benefits in

• having a hierarchical organization for multiple generative objects
• saving memory by instantiating the same shape multiple times
• using transformation nodes for animations etc.
• using the build-in OpenSG funtionalities like clustering
• scripting of user interactions in OpenSG applications

Creation of Geometry

Creation of Geometry nodes, which can be added by GML:

• Combined BReps generated by Euler operators
• Parameterized primitves like boxes, cylinders, etc.
• Static meshes supported by OpenSG (e.g. VRML)

The creation of geometry also includes proper material definitions. Nodes can be added at any place in an existing scene graph. Additionally GML can create new branches.

Scripted User Interaction

GML’s event handling uses a call back mechanism to receive mouse and keyboard inputs from the user. Those call backs are registered in the GML code. Event Handling is used to manipulate the generative creation of objects in an interactive way. The same event handling can also be used to manipulate the rest of the OpenSG scene graph. Simple interaction may only modify transformations as more advanced ones also trigger the generation of extra geometry.

Creation of Complex Scenes

A combination of OpenSG and GML allows to generate more complex scenes containing multiple objects, either generated by GML or modeled in a conventional way. Parametric parts of the scene, e.g. ornaments of a building, can be generated once and then instantiated as an OpenSG node core. Transformation nodes are also created by GML code to translate, rotate and scale generated parts.

References

• Sven Havemann, Generative Mesh Modeling,
  Phd Thesis at Braunschweig Technical University, November 2005
• Björn Harmen Gerth, Generative Scene Manipulation in OpenSG,
  Master Thesis at Braunschweig Technical University, Juni 2005