Rotation angle is set by change in the two-finger connecting line
Aliases: two-finger rotate · twist gesture · rotate with pinch
What it is
Two-finger rotation does not read one finger’s arc. It reads how far the line between two fingers has turned from its starting line. The line’s turn is the object’s turn. Drawing a rotation with one finger circling fights pens and lassos for the path; two fingers lift rotation out of path-following into a pose quantity.
Why it happens
Each finger supplies a point; the line has a direction. At down the system records θ0, then each frame Δθ = θnow − θ0 is added to the object’s heading. The midpoint is also the pivot, so a twist turns around the center between the fingers, not around the canvas origin. Angle must be unwrapped through −π to π, or crossing 180° jumps half a turn. One finger almost still and the other circling still turns the line; that fits hands better than demanding a symmetric crank. Sampling noise blows up when the span is short—direction of two close points is unstable—so too-near fingers should refuse rotation and keep only pan or scale.
Studying it
Have people turn a photo to a named heading (horizon level) and record final error, 180° jumps, and how angle jitter relates to span. Independent variables include minimum span, whether the midpoint is the pivot, and angle filtering. Control: a one-finger handle dragged around a center. Variance at short span is the hardest evidence for this mechanism.
Where it stops holding
A map locked to north either disables rotation or previews it briefly and springs back to north. Two fingers on a 3D object rotate the projection, not an arbitrary axis; writing Δθ straight into yaw looks right in plan view and wrong in elevation. A one-finger drawing tool has no second finger; rotation should be a dedicated handle. Gloves or moisture that merge contacts collapse the line to a point.
Applying it
- Drive heading from the unwrapped angle of the two-finger line, pivot at the current midpoint, and emit no angle below a span threshold.
- Unwrap across ±180°; never allow a half-turn jump in one frame.
- Have people twist a horizon to level. The photo should turn between the fingers. If it slings around a screen corner or flips at some heading, inspect unwrapping and the pivot.