The zoom anchor is the two-finger midpoint so content under the pinch does not drift

What it is

Which point a zoom-out happens around decides whether the picture runs out from between the fingers. Using the two-finger midpoint (or contact centroid) as the scale anchor keeps the content at the pinch between the fingers as scale drops. Anchor at the viewport center or the top-left, and the patch someone is watching slides away during the shrink, as if the map moved on its own.

Why it happens

Scaling is a similarity transform and needs a fixed point. Choose the midpoint and the midpoint of the finger line stays still in content coordinates; shrink looks like surroundings gathering in, matching “what I am holding is the focus.” Choose the viewport center and the building under the fingers flies toward screen center; people have to chase while pinching. Zoom-out is especially sensitive: as scale drops the field grows fast, and anchor error is magnified into a landmark translation that looks like an accidental pan. The midpoint itself moves (pinch and slide at once), so each frame must use the current midpoint, not a midpoint frozen at down—a frozen anchor drags content out from under fingers that have translated.

Studying it

Mark a point on an image, have people put a finger on each side and pinch, and measure that point’s drift relative to the midpoint. Independent variables: anchor at midpoint, at viewport center, at a midpoint frozen at down. Dependent measures include drift pixels, extra pan to catch the point, and reports that “the picture is sliding.” High-speed video shows whether midpoint and landmark move together.

Where it stops holding

When the two fingers are badly asymmetric (one almost still, the other sweeping a large arc), the geometric midpoint and the felt “place I’m holding” part company; the centroid or the steadier finger may be closer to the subjective anchor. If scale is coupled to rotation, the fixed point must also satisfy the angle; the midpoint is still the cheapest choice. Keyboard or button zoom has no two-finger midpoint; the anchor should be viewport center or the current selection, not a pretend pinch midpoint.

Applying it

• Each frame, scale around the current two-finger midpoint, and add the midpoint’s translation to the viewport so a pinch-and-slide does not tear the stick.
• Do not freeze the anchor at layout top-left or screen center unless this is button zoom.
• Park two fingers at the ends of a bridge and pinch. The bridge should stay between them as it shrinks. If it slides to the other side of the screen, the anchor was pinned wrong.

Related

• Same group: C3.08.1 Scale is set continuously by change in the two-finger span · 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 · C3.10 Two-finger rotation
• Search: zoom anchor · pinch centroid · scale origin