Arc-edges can now be specified with a sector angle (in degrees) and an
axis of the circle of which the arc forms a part. The new syntax is as
follows:
edges
(
arc <vertex-0> <vertex-1> <angle> (<axis-x> <axis-y> <axis-z>)
);
This is often more convenient than the alternative specification where a
third third point somewhere in the arc is given; it usually does not
require any additional calculation on the part of the user, and multiple
entries are very likely to be identical.
Which specification is used depends on the form of the entry that comes
after the two vertices. If the entry is a vector then it is assumed to
be a point in the arc; if it is scalar then is is taken to be the angle
and the axis is assumed to follow.
For example, to put a 90 degree arc between the vertices 12 and 13, at
(1 0 0) and (0 1 0) respectively, the following specification can now be
used:
edges
(
arc 12 13 90.0 (0 0 1)
);
This is equivalent to the existing point-in-arc speficiation below:
edges
(
arc 12 13 (0.707107 0.707107 0)
);
An edge's points are ordered on the perimeter of the circle according to
a right-hand screw rule on the given axis. Changing the the side of the
edge on which the arc is defined can therefore be achieved by reversing
either the edge or the direction of the axis.
If the given axis is not perpendicular to the line between the vertices,
then the arc gains some axial length and becomes a helix.
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