Splines

A spline is a smooth curve through or near a series of control points. Fig files have three kinds, each open or closed; all six modes work the same way and differ in the curve they make.

• Approximated splines pass near the points (the points pull the curve). Modes Approximated spline and Closed approximated spline.
• Interpolated splines pass through every point. Modes Interpolated spline and Closed interpolated spline.
• X-splines let you choose per point how sharp or smooth the curve is (see below). Modes X-spline and Closed X-spline. This is the most flexible kind.

Buttons in all six modes: left adds points, middle draws freehand or finishes, right cancels. A closed spline joins its last point to the first. A closed spline needs three points (the middle button is dead until the last one is missing); an open spline needs two, otherwise the message panel says "Not enough points for spline" and you keep drawing. Shift+left removes the last point but never the first; Shift+middle finishes at the last point placed without adding the clicked one. Angle Geom can constrain the points.

Approximated spline

Key S.

• Left, “first point”, then each next point.
• Middle, “freehand” to start freehand drawing; middle again, “final point”, finishes.
• Right: cancels.

Closed approximated spline

Key Shift+S. As Approximated spline, but the curve closes on itself.

• Left, “first point”.
• Middle, “freehand”.

Interpolated spline

Key I. The curve passes through each point you click.

• Left, “first point”.
• Middle, “freehand”.

Closed interpolated spline

Key Shift+I.

• Left, “first point”.
• Middle, “freehand”.

X-spline

No key. The curve's shape at each control point is a number between -1 and 1: 0 makes a sharp corner, 1 a smooth curve that approximates the point, -1 a smooth curve through it. New points are smooth approximations; change them later with Edit attributes (right button on a point of the spline).

• Left, “first point”.
• Middle, “freehand”.

Closed X-spline

No key. As X-spline but closed.

• Left, “first point”.
• Middle, “freehand”.

Shape factor on the canvas

In the editing modes (not the drawing modes), hold Ctrl and Shift and click a spline point: the left button makes the point more approximating (shape +0.04 up to 1), the right button more interpolating (-0.04 down to -1), the middle button cycles through interpolating (-1), angular (0) and approximating (1). The spline becomes an X-spline. The end points of an open spline are refused ("Cannot edit end-points"); a click away from spline points says "Only spline points can be edited". The indicator shows "More approx / Cycle shapes / More interp" while both keys are held. Undo restores the old shape.

Changing a spline later

Use Points to move, add and delete control points, and Convert line and spline to turn a polyline into a spline and back.