Extrusion is typically the first step to generate a 3D building from a 2D footprint. This operation increases the dimension, i.e. a two-dimensional building footprint can be extruded into a three-dimensional mass model.

Facade selection a
   extrude(4) Building
The picture in example 1 shows a simple extrusion of a 2D building footprint (left) to a 3D mass model (right). The rule to achieve this is printed on the right. The extrusion direction is normal to the footprint polygon.
If a building lies on hill ground, it might be desired to extrude along a world coordinate axis rather than the face normal.

Extrude along axis a

Extrude along axis b
   extrude(world.up, 30)					
In example 2, the extrusion is along the global y-axis.

Another extrusion variant is to taper the fooprint, see the taper shape operation.

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