Scale is set continuously by change in the two-finger span

What it is

As two fingers close (a pinch), the system writes current span over starting span continuously as a scale factor. The span shrinks, the picture shrinks, more context enters the viewport—that is pinch as a zoom-out primitive. Scale is not a detent switch; every frame’s span edits it. Turning a pinch into “pinch once, jump to 50%” throws the continuous channel away.

Why it happens

At two-finger down the system records span0 and scale0, then scale = scale0 × (span / span0) (or a mild exponential to match felt magnification). Controlling span is grabbing the picture’s width. Continuous mapping makes scale an aimable analog: stop when it looks right, without counting pinches. Filtering must kill span flicker from two independent jitters, but heavy filtering makes zoom-out tracking sticky. One-finger motion is pan; span change is this channel. Integrating midpoint translation with span change is what lets a zoom-out also move the viewport.

Studying it

Have people pinch a large image until “the whole building just fits,” and record final scale, overshoot-then-recover counts, and the time-series correlation of span with scale. Independent variables include linear versus exponential ratio, filter window, and whether pan is allowed during the pinch. Button step-zoom is the control. Low correlation means filtering or gesture contention broke the continuous channel.

Where it stops holding

When two fingers translate almost in parallel, span change is near zero and should be a two-finger pan, not scale jitter. Mixed stylus-plus-finger contacts make span semantically unstable. Binding pinch to a non-scalable control (button, switch) leaves the continuous factor nowhere to write. System-wide magnification uses another scale range; do not share one scale with in-app pinch.

Applying it

• Use starting span as the denominator of a continuous factor; closing fingers zooms out. Do not quantize one pinch into a single jump.
• Smooth span over a short window, but keep tracking delay below a visible chase.
• Have people pinch until a named object just fills the width. Scale should shrink monotonically with span. If fingers have already closed and the picture still jumps in steps, or span is still and the picture still shrinks, the mapping is broken.

Related

• Same group: C3.08.2 The zoom anchor is the two-finger midpoint so content under the pinch does not drift · C3.08.3 Min and max scale should rubber-band at the limit · C3.08.4 Lift keeps the current scale; it does not reset · C3.08.5 Pinch is unavailable in a one-handed grip and needs an alternative path
• Adjacent: C3.09 Spread-to-zoom in · C3.07 Pan
• Search: pinch-to-zoom · scale factor · inter-finger distance