Points alone are not enough for making useful drawings. The next
step is to combine them into Paths, which are similar to
Metafont's paths, except that they are three-dimensional.
A Path consists of a number of Points and strings
representing the connectors. The latter are not processed by
3DLDF, but are passed unchanged to the output file. They must be valid
connectors for MetaPost, e.g.:
..
...
--
---
&
curl{2}..
{dir 60}..
{z1 - z2}..
.. tension 1 and 1.5..
..controls z1 and z2..
Usually, it will only make sense to use .. or -, and not
..., --, tension, curl, controls, or any of the
other possibilities, in Paths, unless
you are sure that they will only be viewed with no foreshortening due to
the perspective
projection. This can be the case, when a Path lies in a plane
parallel to one of the major planes, and is projected using parallel
projection onto that plane. Otherwise,
the result of using these connectors is likely to be unsatisfactory, because
MetaPost performs its calculations based purely on the two-dimensional
values of the points in the perspective projection.
While the Points on the Path will be projected correctly,
the course of the Path between these Points is likely to
differ, depending on the values of the Focus
used (see Focuses), so that
different views of the same Path may well be mutually
inconsistent.
This problem doesn't arise with "-", since the perspective
projection does not "unstraighten" straight lines,
but it does with "..", even without tension, curl,
or controls.
The solution is to use enough Points, since a greater number of
Points on a Path tends to reduce the number
of possible courses through the Points.1