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.