B-Spline Curves
B-spline curves follow the general shape of a control polygon,
the vertices of which are known as control points. These control points
act like magnets which pull the curve towards them. With a little practice, you
will find this a very natural way of controlling the shape of a curve. To
interactively create a B-spline curve, set the input mode to
B-spline, and use the command
CURVEÞ
Design:Input to enter the control polygon.

To move, add, or delete control points, enter the command
CURVEÞDesign:Edit.
You could also produce sharp
angles, straight-line segments and circular arcs etc. In other words, practically
any curve shape may be defined using the available curve editing functions.

It is good to know the following facts about the B-spline curve properties:
- The curve follows the form of the control polygon smoothly, but it does not
usually pass through the control points.
- Generally, a curve will be smooth if it does not have too many control points,
and if the points are distributed in an even manner along the curve. Adding too
many points will not only make DeskArtes work harder but is likely to produce a
curve with small fluctuations as you have to be more precise in the positioning
of the points.
- Moving a single control point only affects the part of the curve that is close
to the point. The rest of the curve remains unchanged.
- You can create a double control point (two control points at the
same location). This has the effect of pulling the curve much closer to the point.
You have effectively doubled the strength of the ‘magnet’. This produces a tight,
smooth corner at the control point.
- You can create a triple control point (three control points at
the same location). This has the effect of pulling the curve through the point
producing a sharp corner at the control point.
- The endpoints of an open curve are automatically made triple.
The last three control points of a closed curve (one which forms a closed loop)
are the same as the first three. These facts are usually hidden from the user,
but useful to know.
- The curve always leaves a triple point (such as an endpoint) in the direction
of the next control point.