It is part of the Virtual Terrain Project and distributed under a completely free open source license.
vtlib contains an abstraction of a Scene Graph API, built on top of OpenGL and other lower-level APIs such as OSG and SGL. Because it is higher-level, vtlib lets you construct 3D geometry and simulations much faster and easier than using the lower-level libraries directly.
In addition to these automatically generated pages, programmer documentation is also available online.
1.4.5