OpenSceneGraph is an open-source 3D graphics application programming interface, used by application developers in fields such as visual simulation, computer games, virtual reality, scientific visualization and modeling.
Overview of the openscenegraph architecture.
Irrlicht is an open-source game engine written in C++. It is cross-platform, officially running on Windows, macOS, Linux and Windows CE and due to its open nature ports to other systems are available, including FreeBSD, Xbox, PlayStation Portable, Symbian, iPhone, AmigaOS 4, Sailfish OS via a QT/Qml wrapper, and Google Native Client.
Per-pixel lighting in Irrlicht