Pan is continuous manipulation in which content moves with the finger
Aliases: 1:1 pan · canvas drag · tracked panning
What it is
A pan moves a larger surface under the viewport, and while the finger is down the content stays equidistant from the contact: wherever the finger goes, the grabbed point goes. Maps, zoomed photos, and infinite canvases run on it. What moves is the relation between viewport and content, not a droppable object into a slot, and not a page command at lift.
Why it happens
Direct touch makes “grab this point on the paper” real: contact and content coordinates bind at down, then each frame updates with the same offset. At gain 1, that point sticks under the pad and spatial orientation is steadiest. Gain other than 1 (pointer acceleration, or travelling farther with less motion) breaks the stick, and hand–eye ratio has to be rebuilt. Pan needs a visual closed loop for the whole contact. Lift may rest or hand terminal speed to inertia, but the tracking phase itself must stay equidistant, or “holding to look” becomes “pushing a drift.” Drag differs in objecthood: a drag peels a figure off a ground; a pan has no figure, the whole surface moves.
Studying it
On a map or a large image, “keep this landmark under the finger and move it to the crosshair.” Record pixel error between contact and landmark, corrections, and whether the stick broke. Independent variables include gain, latency, and content density. A control task is dropping an icon in a folder—that is drag, with a different error pattern (drop alignment versus grab maintenance). Also test whether one-finger pan and two-finger pan (the other finger often scaling at the same time) steal each other’s bind point.
Where it stops holding
Vertical list scrolling often reuses the same tracker, but the user’s model is “flipping a document,” not “grabbing this row of paper.” On a short list with inertia, the tracking phase is brief and the pan feel is weak. A 3D globe or a tilted camera splits equidistance out of 2D screen coordinates; finger motion on glass no longer matches equidistance on the ground. A mouse grab-tool is indirect pan, with different gain and button arming.
Applying it
- Keep gain 1 on maps, large images, and canvases while one finger is down; the grabbed content point must not slide out from under the pad.
- Do not implement carrying a droppable object as a whole-canvas pan; the object must be grabbable on its own, and only the background pans.
- Have someone hold a landmark to a screen cross. The landmark should stay near the pad the whole way. If it leads, lags, or drifts the other way, check gain, latency, or a scroll parent that stole the contact.