Path editing holds the two settings for the Points tool: whether a click on an outline adds a new point, and whether the whole curve re-smooths itself as you drag. Under them is a crib sheet of the tool’s gestures. Both switches decide how much the app does for you while you reshape.
What it does
The PATH EDITING section appears on the Controls tab while the Points tool is in hand.
‘Add points on click’ is on to begin with. Click an outline between two points, without dragging, and a new point is inserted there. The curve is divided at that spot without changing its shape: the new point sits exactly on the old line, and the handles either side are recalculated so that the two halves trace the original curve. Drag the outline instead of clicking and you bend that stretch of it. Press with Option (Alt) and a point is inserted and dragged in one movement. Turn the switch off and clicks on the outline add nothing, so a stray tap cannot scatter points along a path; dragging the outline still bends it.
‘Smooth while reshaping’ is off to begin with. Turn it on and, each time you move a point, every handle on that outline is recalculated so the line runs through all the points with no kink: its direction, and the rate at which it is bending, both carry on unbroken through each point. The result is a smooth, flowing curve. The price is that corners become curves and any handles you aimed by hand are replaced.
Under the switches is a reminder list of the tool’s gestures: drag a point to reshape; Shift-click to add a point to the selection; drag from empty space to select several; the arrow keys nudge the selected points and Delete removes them; drag a handle to aim it, or Option-drag to break a pair; Option-click a point to switch it between corner and smooth; drag inside the shape to move it.
Reshaping changes the outline you drew; the hole and the piece follow. The shape you are editing is lifted above the others, so paper it now covers is taken from them.
Why it works this way
Adding points is on because the commonest need when reshaping is a point where there is none. The switch exists for careful work on a touch screen, where it is easy to tap an outline by accident.
Smoothing is off because outlines in this app come from very different hands. A Pen path carries handles you aimed one by one. A freehand line from the Draw tool has been fitted with deliberate corners. A rectangle is all corners. A global re-smooth would overwrite all of that at the first nudge. Turn it on when a shape should be organic – a pebble, a cloud, a leaf – and you want to push points about freely while the curve stays fluent.
The metaphor
Before computers, a smooth curve through fixed points was drawn with a spline: a long, thin, springy strip of wood or metal, held in place at chosen points by heavy weights, often called ducks. The strip settles into the smoothest shape it can, bending as little as possible, and the draftsman traces along it. ‘Smooth while reshaping’ is that strip. Move one duck and the whole batten shifts to a new smooth curve through all of them.
The metaphor breaks usefully. A wooden batten can never make a sharp corner; paths here can, and with smoothing off each point keeps whatever handles it had, which no batten allows. Adding a point is like adding a duck exactly where the strip already lies, holding it without moving it.
A history
In art
The art strand here runs through craft more than through the studio. Shipbuilders drew the lines of a hull full size on the floor of a mold loft, bending long battens through plotted points and pinning them with weights, and the word for making those lines smooth and true was fairing. Draftsmen joined points with French curves, sets of templates cut to many different bends. The designer’s question has never changed: which points are essential, and what should the line do between them?
In printing
Type is where curves through points became an everyday craft. For centuries a punchcutter shaped each letter in steel by eye. Digital type turned letters into outlines made of points and curves: PostScript Type 1 fonts are drawn with cubic Bézier curves, the same kind as the paths in this app, while TrueType uses a simpler quadratic curve. A type designer’s working day is spent doing exactly what this tool does – adding points, aiming handles, and deciding which points are corners and which are smooth.
In technology
In 1946 the mathematician Isaac Schoenberg named the mathematical spline after the draftsman’s tool: a curve made of polynomial pieces that join smoothly at the points. When this switch is on, the app uses the smoothing built into Paper.js, the drawing library underneath, which solves for every handle at once so that direction and bending carry smoothly through each point, a spline in Schoenberg’s sense.
The curves themselves came out of French car factories. Paul de Casteljau at Citroën worked out, in 1959, a way to compute and divide such curves, but his work stayed an internal company matter for years. Pierre Bézier at Renault developed and published the approach through UNISURF, his system for designing car bodies, and his name went into the textbooks. De Casteljau’s method is what makes ‘Add points on click’ exact: any such curve can be split at any point into two smaller curves that trace the original precisely. Adobe Illustrator brought Bézier curves and the pen to designers in 1987, and the vocabulary of anchors and handles has been shared ever since.
In professional software
Every vector program edits paths through anchor points and handles, and both switches here have counterparts: an automatic add-and-delete you can turn off, and a way of letting the program keep a curve smooth for you.
- Adobe Illustrator. With a path selected, the Pen tool (
P) adds an anchor when you click a segment and deletes one when you click an anchor; ‘Disable Auto Add/Delete’ in the General preferences turns that off. Holding Option (Alt) with the Pen tool gives the Anchor Point tool, which switches points between corner and smooth. The Curvature tool is the nearest thing to ‘Smooth while reshaping’: drag a point and the curve stays fluent through its neighbors. - Inkscape. The Node tool (
N) adds a node where you double-click a path, and Delete removes a node while trying to keep the shape. Its auto-smooth node type recalculates the handles by itself as nearby nodes move, which is ‘Smooth while reshaping’ applied node by node. - Autodesk AutoCAD. A spline is drawn either through fit points or with control vertices. Hover over a grip and a menu offers to add or remove a fit point or a vertex;
SPLINEDITdoes the same from the command line. - Autodesk Fusion (formerly Fusion 360). Sketches offer a Fit Point Spline, which passes through its points like the curves here, and a Control Point Spline, which is pulled toward its points without touching them. Select a fit point and you can drag its tangent handle to aim the curve.
What carries over. Points, handles, corner and smooth: the vocabulary is the same everywhere, and so is the discipline of using as few points as the shape needs. Illustrator’s ‘Object › Path › Simplify’ and Inkscape’s ‘Path › Simplify’ thin out points you do not need. The pitfall is the automatic behavior. Learn what a single click on a path does in each program, because in one it adds a point and in another it only selects.
Try this
- Draw a rectangle, take the Points tool, turn on ‘Smooth while reshaping’ and nudge one corner. Every corner rounds off. Undo, and turn it off again.
- Click the middle of a straight side to add a point, then drag it outward to make a gable.
- Turn ‘Add points on click’ off and tap along an outline: nothing is added, but a drag still bends it.
Going further
See the tools themselves: Points, Curves and Pen. For how a freehand line gets its points in the first place, see Drawing.