For each line segment, the angle of inclination to the \(x\)-axis
(in radians) is computed,
and the angles are returned as a numeric vector.
If directed=TRUE, the directed angle of orientation
is computed. The angle respects the
sense of direction from (x0,y0) to (x1,y1).
The values returned are angles in the full range from \(-\pi\)
to \(\pi\). The angle is computed as
atan2(y1-y0,x1-x0). See atan2.
If directed=FALSE, the undirected angle of orientation
is computed. Angles differing by \(\pi\) are
regarded as equivalent. The values returned are angles
in the range from \(0\) to \(\pi\). These angles are
computed by first computing the directed angle,
then adding \(\pi\) to any negative angles.