How to calculate the half extents of a cube?

Question

I wish to know what formula should I use to get half extents of cube or rectangular cube.
The library I use to make graphical objects requires this cube half extents (which I don't really know what is).

Answer 1

I've finally got it. Cube half extents is a vector representing half size of the cube along its local axis. Example : a cube having (1,1,1) as size have (0.5,0.5,0.5) as half extents.