Clipping is explained in detail here and also here. Basically points are clipped if they are outside of the frustum defined by the projection matrix.
I do not see what you mean with "primitives", but OpenGL generally uses homogeneous coordinates, thus a 4D vector (x,y,z,w)
. w
is 1
initially but may be different after projection. See also this article for more info about homogeneous coordinates.