Rotation and scale happen together and need an explicit couple-or-decouple decision
Aliases: rotate-scale lock · decoupled pinch · similarity transform
What it is
Once two fingers are down, span and connecting-line angle almost always both change. The system has to decide: couple them (a similarity transform, turn and scale) or decouple (keep only the larger component, or require a clear rotation threshold before turning). Products that never decide twist a photo when someone only meant to enlarge, or resize it when they only meant to straighten.
Why it happens
Hands rarely produce pure rotation or pure scale: elbow and wrist mix in the other component. Coupling writes one contact as a full 2D similarity, which fits free placement of stickers and photos. Decoupling sets a rotation-threshold angle, drops Δθ inside it and keeps only scale, then writes both once the threshold is crossed—which fits maps (north matters) and documents (a little skew looks wrong). A principal-component lock is a third option: lock to scale when span dominates, to rotate when angle dominates, and ignore the minor component for the rest of the contact. None is universally better. One must be chosen explicitly and shown (a compass on a map, none on a sticker).
Studying it
Run “enlarge only, try not to turn” and “straighten only, try not to resize” under full coupling, an angle threshold, and a principal-component lock. Dependent measures include unwanted heading, unwanted scale change, and time. Decompose traces into span and θ and see whether the lock cuts where people start “seriously turning.” Recruit map users and collage users separately; their tolerances differ completely.
Where it stops holding
A 3D trackball already maps two-finger motion onto two rotation axes; stuffing scale in as well overloads it. System-wide magnification may still rotate the whole screen after the app decoupled, and people cannot tell who turned. Trackpad “rotate” often wants a more theatrical pose; a touchscreen threshold angle does not transfer.
Applying it
- Write the policy per canvas: couple for free stickers; use a rotation threshold on maps or documents, plus a “re-north / straighten” button.
- Once a component is locked, do not hop back and forth inside the same contact, or the picture jitters.
- Ask for “only enlarge this photo.” A few degrees of leftover skew means coupling is too strong or the threshold too low. Then “only level the horizon.” If the picture also grows and shrinks, scale was not dropped.