2012

Spline
Uses the vertices of the selected polyline as the control points, or frame, of a
curve approximating a B-spline. This curve, called a spline-fit polyline, passes
through the first and last control points unless the original polyline was closed.
The curve is pulled toward the other points but does not necessarily pass
through them. The more control points you specify in a particular part of the
frame, the more pull they exert on the curve. Quadratic and cubic spline-fit
polylines can be generated.
Spline-fit polylines are very different from the curves produced by the Fit
option. Fit constructs pairs of arcs that pass through every control point. Both
of these curves are different from true B-splines produced with the SPLINE
command.
If the original polyline included arc segments, they are straightened when the
spline's frame is formed. If the frame has width, the resulting spline tapers
smoothly from the width of the first vertex to the width of the last vertex. All
intermediate width information is ignored. Once spline-fit, the frame, if
displayed, is shown with zero width and CONTINUOUS linetype. Tangent
specifications on control point vertices have no effect on spline-fitting.
When a spline-fit curve is fit to a polyline, the spline-fit curve's frame is stored
so that it can be recalled by a subsequent decurving. You can turn a spline-fit
curve back into its frame polyline by using the PEDIT Decurve option. This
option works on fit curves in the same manner as it does on splines.
Most editing commands act the same when applied to spline-fit polylines or
fit curves.
■ MOVE, ERASE, COPY, MIRROR, ROTATE, and SCALE operate on both the
spline curve and its frame, whether the frame is visible or not.
■ EXTEND changes the frame by adding a new vertex where the initial or
final line of the frame intersects the boundary geometry.
■ BREAK and TRIM generate a polyline with only the fit spline, which is
consistent with fit curves, where the curve fitting is permanent.
1454 | Chapter 16 P Commands