recursIV Modeling and Rendering of Natural Phenomena for Outdoor Virtual Environments Project duration: 1996-1997 Contact: Dieter Schmalstieg
Description	Computer generated images are expected to look realistic. For virtual outdoor environments, this involves presenting models of trees, plants, mountains and other natural phenomena that are usually very complex objects.
Application	Outdoor scenes are required for flight simulation, urban reconstruction, landscape development, architectural simulation, geographical information systems and any kind of virtual environment that involves a real-world surrounding.
Problems	Natural phenomena are very complex objects (just think of the number of leaves on a tree). They are hard to create, consume a lot of storage, and take a long time to transmit over a network.
Approach	recursIV is a software capable of direct rendering of natural phenomena such as trees, plants or mountains from a procedural representation at interactive frame rates. Our approach is based on directed cyclic graphs. It allows to represent extremely complex scenes with little memory and modeling effort. Very large scale virtual environments are supported by a bandwidth-preserving networking approach that makes use of the compact representation and on-the-fly database amplification. The implementation was done on top of Silicon Graphics' Open Inventor software toolkit. The resulting software is called recursIV.
Publication	Modeling and Rendering of Outdoor Scenes for Distributed Virtual Environments : D. Schmalstieg, M. Gervautz

Download SGI (IRIX) demo executable of recursIV (ZIP file, 0.7MB)

Linear fractals such as the Menger sponge and the Sierpinsky tetrahedron can be represented with directed cyclic graphs.

With a simple recursion, sympodial branching structure can be created. Recursion depth controls the development of the structure and is not a very good measure to control level of detail. Instead, whole subgraphs should be replaced by appropriate simpler structures, such as exercised for the conifer tree.

Unlike texture-mapped trees usually used for flight simulators and similar applications, a recursIV tree exhibits detail also at close-ups. This can be done excessively (such as modeling the veins of individual leaves), if the image generator is powerful enough.

Natural phenomena such as terrain and trees can be combined for realistic outdoor scenes.

Combination of procedural and conventionally modeled geometry is easily possible. For example, the christmas tree twig is a procedural model, while Santa's head was modeled in 3D Studio.