Spread and pinch are two directions of the same gesture
Aliases: unpinch · spread-to-zoom · reverse pinch
What it is
Spreading two fingers (spread / unpinch) pulls the picture closer; closing them pushes it away. They are not two vocabularies. They are plus and minus on one span channel. Recognizer, anchor, filtering, and scale persist after lift should be symmetric. Making zoom-in a different two-finger routine (must double-tap, then drag) breaks that bidirectional axis.
Why it happens
Span’s rate has a sign: up means scale up, down means scale down. Muscle memory is bidirectional too—open a little more if it is not close enough, close if it is too close—without changing gestures. If zoom-in uses one threshold and zoom-out another, or zoom-in needs extra arming, people stall mid-adjustment. Symmetry includes failure: hitting the ceiling and hitting the floor should both elastic-band, not band on one side and deadlock on the other. Direction may flip during one contact: open then close without lifting should travel back and forth on the same scale curve, not be cut into two independent gestures.
Studying it
Have people enlarge a picture to a detail and shrink it back to the whole, two fingers only, no lifting to change tools. Log mid-contact direction changes, whether scale jumps at the flip, and whether one direction is ignored. Registering “spread” and “pinch” as two recognizers is the control; it usually drops events at the flip. Eye tracking can show whether people hunt for a button before changing direction—evidence they do not believe it is one axis.
Where it stops holding
Some accessibility switches make zoom-in a one-finger double-tap-drag and zoom-out a reverse drag; that is another one-finger axis, and an asymmetric two-finger pinch should not be registered beside it. A game that fires a skill only on spread, with close doing nothing, is ability design, not view scale. If two-finger rotate occupies the contact first, a spread may be eaten as angle; on a rotate-first canvas the bidirectional scale axis must yield or decouple.
Applying it
- Drive scale up and down from the same span. Allow direction to reverse inside one contact; do not restart the gesture at the flip.
- Share limit rubber-banding and lift-persist between zoom-in and zoom-out. Do not make one sticky and the other a jump.
- In one two-finger hold, pull a picture closer then push it away. Scale should pass the peak continuously, not jump to 100% or freeze at the flip. If people lift and switch to a “zoom-in tool,” the bidirectional axis was not perceived.