The eye tends to continue along a smooth path
Aliases: Gestalt continuity · continuity principle
What it is
Two lines cross, or a line is briefly blocked by something in front of it, and people don't read the scene as "meeting at that point and each turning off in a new direction" — each line is read as continuing smoothly through in its original direction, and the hidden portion is filled in as continuing that same way. This cue is continuity, also called good continuation.
It's easy to confuse with closure, but the criterion differs: continuity only requires a smooth change in direction; it doesn't require the path to eventually form a closed shape. A curve that keeps extending past an occluder without ever meeting itself still falls under continuity — closure isn't needed.
Why it happens
At a crossing point or a break, the same visual input can be stitched together in several equally "legal" ways, but the visual system picks the reading that keeps the direction change smallest, rather than choosing randomly or picking the geometrically simplest segmentation. For two crossing curves, reading each one as continuing in its original direction keeps each curve's curvature changing continuously through the crossing; reading it instead as each curve turning off to follow the other makes direction snap abruptly at that point. The system defaults to the first reading, because an abrupt direction change in natural scenes usually signals that two separate object boundaries happen to meet there, not that a single line is genuinely bending.
Whether an occluded segment gets filled in as a continuation also depends on whether the endpoint angles and positions on either side of the occluder line up with each other — only when the two visible segments are close enough in direction and position to "match" does the system judge them as one line continuing behind the occluder, rather than two unrelated segments that happen to sit near each other.
Studying it
This cue was first established through perceptual judgment experiments with crossing lines and occluded figures: observers reported whether a crossing read as "two continuous lines" or "four segments each turning," or judged whether the two visible portions of an occluded object's contour belonged to the same object. Later work quantified this judgment as a function of the angle difference and positional offset between the two contour endpoints — smaller angle and position mismatch predicts a higher rate of the two segments being judged as one continuing contour, and larger mismatch predicts they're seen as unrelated pieces.
This judgment paradigm doesn't come up much in interface research, since static-contour continuity is more a subject of basic vision science; interface design tends to borrow the conclusion directly for layout decisions rather than re-testing it.
Where it stops holding
Continuity only operates on smooth direction changes; if a path has a sharp direction change beyond the normal range of curvature at some point, the system won't read it as part of the same continuing path — it will treat that point as the boundary between two different objects instead.
Once the angle and position mismatch across an occluder exceeds the range that supports stitching, the continuity judgment fails and the two segments are seen as unrelated fragments rather than one line; whether closure or some other cue can take over at that point depends on the rest of the figure's specific conditions, and shouldn't be assumed to always happen.