 It provides instead a number of operators for creating 3D models (polygons, b-reps, subdivision surfaces).As a "shape programming language," it is a true generalization of "flat" 3D file formats like OBJ, DXF, or VRML that contain just lists of geometric primitives.Together with its OpenGL-based runtime engine the GML can also be seen as a viewer with an integrated modeller, to overcome the usual separation of 3D modelling from interactive visualization.