3. Geometry module

Gmsh's geometry module provides a simple CAD engine, using a bottom-up (boundary representation) approach: you need to first define points (using the Point command: see below), then lines (using Line, Circle, Spline, …, commands or by extruding points), then surfaces (using for example the Plane Surface or Ruled Surface commands, or by extruding lines), and finally volumes (using the Volume command or by extruding surfaces).

These geometrical entities are called “elementary” in Gmsh's jargon, and are assigned identification numbers when they are created:

1. each elementary point must possess a unique identification number;
2. each elementary line must possess a unique identification number;
3. each elementary surface must possess a unique identification number;
4. each elementary volume must possess a unique identification number.

Elementary geometrical entities can then be manipulated in various ways, for example using the Translate, Rotate, Scale or Symmetry commands.

Compound groups of elementary geometrical entities can also be defined and are called “physical” entities. These physical entities cannot be modified by geometry commands: their only purpose is to assemble elementary entities into larger groups, possibly modifying their orientation, so that they can be referred to by the mesh module as single entities. As is the case with elementary entities, each physical point, physical line, physical surface or physical volume must be assigned a unique identification number. See Mesh module, for more information about how physical entities affect the way meshes are saved.