Rotate gesture

A rotate gesture is arithmetic on a line. Two contacts land, and the surface remembers the angle of the segment between them. As the fingers orbit, that angle changes, and the difference between the current angle and the remembered one is applied to the content. The pivot is the midpoint of the two contacts, not the center of the object, which is why an object turning under two fingers feels pinned to the hand rather than to the canvas. Every part of this is the same recognizer that produces pinch: one pair of contacts yields a distance, an angle, and a midpoint at once, so scale, rotation and pan usually arrive from the same gesture and have to be applied together. See multi-touch for the contact bookkeeping underneath, which is where the pointerId that keeps two fingers apart comes from.

Raw angle is almost never what ships. Fingers wobble, so a rotation applied straight from the sensor drifts a degree or two off true and nothing ever lands square again. The fix is detents: as the value passes within a few degrees of 0, 90, 180 or 270, it sticks there until the fingers pull it decisively away, and good implementations mark the moment with a short haptic tick and a guide line. A second guard is a threshold before rotation begins at all, usually around ten degrees of travel, so a two-finger pan or pinch does not leave the photo half a degree crooked. Both are cheap, and both are the difference between a gesture that feels precise and one people stop using.

Whether rotation should exist at all is a question about the content. A photo, a shape on a design canvas, a 3D model: these have no privileged orientation, so turning them is an edit the reader intends. A map is the harder case. North up is a strong convention and a lot of readers rely on it, so map rotation is normally paired with a compass affordance that both reports the current heading and resets to north on a tap, and plenty of map products keep rotation off until the reader opts in. A document, a list, or a form has no rotate at all, because there is no meaning to give the result.

The trap is accidental rotation. Any two-finger gesture on a surface that recognizes turning will produce a little of it, so a reader who meant to pinch a photo ends up with it three degrees off and no obvious way back. Three habits keep that from being a bug report: give the gesture a threshold, snap to the cardinal angles, and always offer a cheap way home, either a reset control or plain undo. And remember that rotation is a multi-touch gesture with no keyboard or mouse equivalent, so whatever it does has to be reachable another way for anyone who cannot make it.