Min and max scale should rubber-band at the limit

What it is

Scale has a minimum and a maximum. When a pinch drives the picture toward the minimum and scale simply clamps, span is still shrinking and people think the gesture died. Limit rubber-banding lets scale overshoot a little past the legal range and spring back on lift, telling “that’s the end” with space rather than a deadlock. Pinch-to-zoom-out hits the floor first: the whole image already fits, and more pinch does not buy more context.

Why it happens

Inside the legal range, scale follows span. Past it, extra span is crushed into elastic remainder; the picture is slightly smaller (or larger) than the limit, but gain drops fast. Lift springs the remainder away. A hard clamp decouples span from the picture—fingers still move, pixels do not—which is hard to tell from touch latency. The band must match the crossed limit: pinching too far shows a slightly small image with a halo of empty space; spreading too far shows pixels stretched and then recovering. Empty space and stretch are different faces of the two limits; do not draw both as the same translucent veil.

Studying it

Set the floor at “the whole image just fills the width” and have people pinch harder, comparing hard clamp, rubber-band, and no limit. Dependent measures include the judgment “can it get smaller,” extra useless pinches past the bound, and whether lift returns to the floor. Log gain of displayed scale versus span during overshoot. The unlimited condition shrinks the image to an un-aimable dot, which is why floors and ceilings are not arbitrary.

Where it stops holding

A tiled map has data levels at low zoom; besides banding, avoid requesting a level that does not exist. A reader’s “page width” is sometimes a comfort floor rather than a physical one; the spring should return to the comfort detent, not to unreadably small type. With system-wide magnification on, in-app floor and system zoom multiply, and the limit cue points at the wrong object.

Applying it

• Past min or max scale, switch to elastic remainder and spring back on lift. Do not let span keep walking while the picture is frozen.
• Give the two limits different looks: a halo of empty space at the floor, a brief pixel stretch or a light jitter at the ceiling.
• Have people pinch until it can get no smaller, then pinch harder. They should see empty-space banding, not a freeze. If they yank repeatedly and ask whether it is broken, the clamp is too hard.

Related

• Same group: C3.08.1 Scale is set continuously by change in the two-finger span · C3.08.2 The zoom anchor is the two-finger midpoint so content under the pinch does not drift · 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: min scale · max scale · pinch rubber-band