Splines and polylines: SPLINE, SPLINEDIT, PEDIT

What you will make

A curve through five points, then the same curve edited three ways, and a polyline built from three lines.

Draw a spline through points

1. Type SPL and press ReturnEnter.
2. Click or type the points the curve should pass through: 0,0, 100,80, 200,-30, 350,50, 500,0. Press ReturnEnter after the last one.
3. Start tangent: click a point the curve should leave towards, or press ReturnEnter to let it decide. Do the same for the end tangent.

The curve passes through every point. Type C instead of ReturnEnter to close it (the curve comes back to the first point). UU (Undo) takes the last point back while you are drawing.

Draw one from a control polygon

Type M at the first prompt, then C (Control vertices) and a degree from 1 to 5 (3 is the usual). Now the points you pick are the corners of a polygon that pulls the curve towards them; the curve starts on the first and ends on the last. A closed one wraps round, with no corner at the join.

Turn a polyline into a spline

At the first prompt type O (Object) and select polylines or lines. They become splines through their vertices; the originals go.

Edit a spline

Select it and type SPE (or type SPE and click it). Grips show the fit points (or the control points of a spline that has none): drag one to reshape the curve. At the prompt:

• Fit data (a spline drawn through points): A adds a fit point (click where), D deletes one (click near it), M moves one (click near it, then click its new place), T sets the end tangents, P purges the fit points, so the spline is edited by its control points from then on.
• Edit vertex: A adds a control point without changing the curve, M moves one, W gives one a weight (above 1 pulls the curve closer to it, below 1 lets it go).
• Open or Close, and Reverse (the curve runs the other way).
• convert to Polyline (P): the spline becomes a polyline; the precision is how many segments each span gets (10 is the usual).
• Undo takes the last change back; eXit ends.

Properties shows a spline's degree, how many fit and control points it has, its length, and lets you open or close it.

Edit a polyline

Select a polyline and type PE. A line, an arc or a spline you pick is first offered as a polyline to turn into (a spline asks for the precision).

• Close / Open joins or separates the ends.
• Join adds lines, arcs and polylines whose ends meet the polyline's end. Fuzz distance is how far apart ends may be and still join (0 means they must meet).
• Width gives the whole polyline one width (0 removes it).
• Fit replaces the polyline by a smooth curve that passes through its vertices; Spline by a cubic B-spline that follows them (it starts and ends at the first and last vertex and stays near the others). Both become a polyline of short segments, and the original corners are kept, so Decurve brings them back.
• Ltype gen says whether a dashed linetype runs on along the whole polyline or starts again at each vertex.
• Reverse changes the order of its vertices. Multiple (M at the first prompt) edits several polylines with the same options.

Editing the vertices of a polyline in Properties or with grips stays as it was: PEDIT has no vertex editor.

In files

DWG and DXF 2000 keep a spline as a spline: degree, control points, knots, weights, fit points and tangents. A spline you open and don't change is written back exactly as it came. DXF R12 has no spline: it is saved as a polyline along the curve, and the history says how many.

Offset, trim, extend, fillet, break and join do not work on splines. EXPLODE a spline, or PEDIT it, to get a polyline those commands take.

Common mistakes

• The curve overshoots between close points. A curve through points bends where the points do. Use fewer points, or a control polygon.
• "That is not a spline." SPLINEDIT only takes splines; PEDIT edits polylines.
• A closed fit spline has a slightly different shape in another program. CadCraft closes it with one smooth join at the first point; another program may close it with a periodic curve. A closed control-point spline is the same in both.