Home > Splines

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A freehand curve made of anchors and handles, drawn with the Pen and edited directly on the canvas.

A spline is a chain of anchors with handles. The anchors sit on the curve, always. The handles sit off it and shape the curve between them.

There are no off-curve control points to chase. Every point you can grab is either on the curve or a handle belonging to an anchor that is.

The curve between two anchors is a span. One connected run of anchors is a piece, and a spline can have several.

Every anchor is one kind or the other, and the shape on screen says which:

• A square is a corner. The curve may turn however it likes there.
• A circle is smooth. The two handles stay in line through the anchor, so the curve runs through without a crease.
• A filled arrowhead marks the spline's start, its point sitting on the first anchor. See What each end tells you.

Double-click an anchor to change one into the other.

Shape carries the kind, deliberately. No color is used for it, because color on this overlay means something else: a filled anchor is selected and a hollow one is not.

The two ends of a spline are not interchangeable, and each one says which it is.

• The start carries a filled arrowhead, its point on the first anchor and its body behind, aimed the way the spline runs. An open spline has one and so does a closed one, so a finished outline still reports where it begins and which way round it goes.
• A loose end carries a short bar straight through the anchor, across the direction of travel. Only an open spline has one, and only at the far end: the start says start, the other end says loose.

Closing a spline takes the bar away and leaves the arrowhead. Which anchor the arrowhead sits on is what an export writes first, and on a closed outline you can move it with Make Anchor First from the path menu.

Both marks are drawn in the interface slate, never in the selection color. Neither of them means selected.

A handle is either derived or explicit, and you can tell at a glance:

• Hollow is derived. The app computes it from the anchor's two neighbors, so it re-curves whenever a neighbor moves.
• Filled is explicit. It stays exactly where it was put.

Both handles of an anchor share one state. Drag either one and the pair becomes explicit together.

Gold is no part of this. Gold means selected, for an anchor and for a handle alike.

Dragging a handle is what makes it explicit. Until then the app is deciding.

The rule it decides by is worth knowing, because it is why a spline here behaves like compass work rather than like a tension slider: through any three consecutive anchors, the curve follows the circle through those three points. A circle is the same object the rest of Locus is built out of, and the only number involved is a handle length.

Toggling an anchor between corner and smooth drops both of its handles. That surprises people, so it is worth saying plainly: it is not a side effect, it is how you ask for the handles to be computed again.

One more case where the app corrects itself. Move both handles of a smooth anchor so they are no longer in line through it, and the anchor becomes a corner. The label never sits over a visible kink.

Anchors appear on every selected spline. Select one and its anchors are there to grab. You do not have to open it for editing first to see them.

A selection can hold anchors from more than one spline. Shift-click adds across splines, a rubber band collects across all of them, and dragging, nudging and Delete act on every member wherever it sits.

The moment an anchor or a handle is selected, no spline is drawn selected. Gold moves down with the selection, onto the anchors, handles and pieces you picked, and both splines go back to their ordinary stroke. Select a spline and gold still means that spline; it is the first anchor click that spends it. With nothing picked at the anchor level, a selected spline looks selected as it always did.

Two things sit outside this. An imported path has no anchors and cannot take part in any of it, so it keeps its selection color throughout. A locked spline shows no anchors at all, selected or not.

A spline has no construction points of its own. It adds nothing to Select Free Points, and nothing else in the drawing can attach to it: no crossing with a spline, no point riding along one, no tangent at a point on one.

Align and Distribute refuse a selection that contains a spline, and a spline is not a direct boolean input.

• The Pen: how a spline gets drawn.
• The path menu: the commands that edit one, and where to find them.
• Convert to Spline: making an editable spline from an import, a circle, a region or an arc.
• Selecting objects: selecting anchors and handles, across one spline or several.
• The Layers panel: how a spline and a path are told apart in the list.

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