Curved & Spline Arrows

Molkit draws curved arrows with three geometries: a single-control-point curve (quadratic), an S-curve with two control points (cubic), and a spline built from anchor points with paired Bezier handles. The curved variants in the arrow flyout all use the spline geometry, so most of what you select on canvas is a spline with orange tangent handles at each end.

Drawing a curved arrow

Press A for the arrow tool, then open its flyout. Curved Arrow, Curved Harpoon Up, Curved Harpoon Down, and Spline Curve are spline-based; S-Curve Arrow uses the cubic geometry. The line tool flyout adds headless versions: Curved Line (quadratic), S-Curve Line (cubic), and Spline Line. While the arrow tool is active, 3 switches to Curved Arrow and 9 to S-Curve Arrow.

Drag from tail to head. If your drag path bends, Molkit reads the curvature from the path and shapes the arrow to match; a straight drag or a single click produces a flat arrow you can curve afterward. While drawing, the endpoints snap to lone pairs, radicals, bond midpoints, atom labels, and annotation edges. Hold Shift to skip endpoint snapping. After the arrow is placed, Molkit switches back to the Select tool; hold Alt while releasing to stay in the arrow tool and keep drawing arrows.

Orange control handles

Select a curved arrow to show its handles. A quadratic curve shows one orange circle at the bow, a cubic shows two, and a spline shows a pair of tangent handles at each anchor connected by dashed guide lines. Dragging a handle reshapes the curve around its anchor.

The quadratic control point snaps to a few targets while Smart Guides are on: the perpendicular through either attached bond end, the chord midpoint (flattens the curve to zero curvature), and the perpendicular bisector of the chord (keeps the bow symmetric). Line snaps are sticky, so the point slides along the snap line until you pull away. Hold Alt to disable snapping.

Spline handles use modifiers:

Plain drag snaps the handle angle to 15 degree increments. Endpoint handles pivot around the rendered tip, so this rotates the curve’s approach around the tip in snapped steps. On anchored endpoints the angle is clamped to 90 degrees either side of perpendicular.
Shift pulls the handle toward exact perpendicular of the attached bond or edge, with a sticky band.
Alt drags free with no snapping.
Ctrl keeps the opposite handle collinear for a smooth point.
Alt+Shift locks the two endpoint handle tips coplanar.
Alt+click a waypoint anchor to reset its handles to a smooth tangent.

Shape Properties panel

With a curved arrow selected, the Shape Properties panel shows curve controls. Quadratic curves get a Curvature section with an Amount slider (-100 to 100) and a Flip Curve button. Splines get Beautify (fits a clean arc or S-curve), Straighten (removes all curvature), and a Bow slider. Bow is a percentage of the chord length, from 0 to 100 for arcs and capped at 50 for S-curves; it sets the magnitude only and keeps the curve on its current side. S-curve splines add a Squeeze slider that compresses the two lobes, and a Curviness slider appears after smoothing a waypoint arrow.

Flipping the bow

Press F with a single curved arrow selected to flip the bow to the other side of the chord. This works for spline, cubic, quadratic, and multi-segment arrows. With nothing selected, F sets the fluorine element instead, so select the arrow first.

Spline waypoints

Double-clicking a quadratic or cubic curved arrow near its curve converts it to a spline and adds an anchor at the click point, keeping the curve shape.

Splines start with two anchors but can hold interior waypoints:

Add: with the spline selected, hover near the curve to see a ghost dot, then double-click to insert a waypoint there. The curve shape is preserved by subdividing the segment.
Drag: waypoints move like any anchor, and each carries its own pair of orange handles.
Remove: hover near a waypoint and press Delete. The arrow stays selected and the curve re-fits through the remaining anchors.

Endpoint anchoring

Curved arrow endpoints attach to atoms, bonds, lone pairs, and annotation edges the same way straight arrows do, and the attachment follows the target when it moves. Arrows drawn from a lone pair or radical are upgraded to the spline geometry automatically so the head can curve away from the dot. See Straight Arrows for the attachment system.