OpenCSG is a library that does image-based CSG rendering using OpenGL.
What is CSG, anyway? CSG is short for Constructive Solid Geometry
and denotes an approach to model complex 3D-shapes using simpler
ones. I.e., two shapes can be combined by taking the union of them,
by intersecting them, or by subtracting one shape of the other.
The most basic shapes, which are not result of such a CSG operation,
are called primitives. Primitives must be solid, i.e.,
they must have a clearly defined interior and exterior.
By construction, a CSG shape is also solid then.