Motion that is too short is not perceived as a transition

What it is

Apparent motion needs enough time samples to encode two positions as one path. Squeeze the duration into a few tens of milliseconds and people do not see travel from here to there — they see that the state has already changed. That is not a slightly weak transition; the path was never built. Subthreshold motion is a hard cut in perception, while still possibly spending a frame or two of compositing.

Why it happens

Discrete frames are seen as continuous travel only when the interval sits inside the apparent-motion window. If the interval is too short, the two positions collapse in time and the system treats them as a new object replacing an old one, not as one object moving. Visual persistence does not rescue this: afterimages can glue a brief flash into stable brightness, they cannot invent a spatial trajectory.

So a 40 ms displacement often has neither the orienting benefit of a transient (the whole event ends inside one fixation, too fast to steer attention) nor path information about belonging. Engineering can report that an animation ran; perception cannot tell it from a display-property swap. If that clip also triggered layout, the only residue is jank risk.

Studying it

Classic apparent-motion work varies the temporal gap and spatial separation of two dots; report of continuous motion drops when the gap is too short or too long. On interfaces, a staircase on duration with a three-way judgment — cut / saw travel / saw something but cannot describe the path — is enough.

Independent variables: duration, travel distance, whether opacity rides along. Dependent variables: proportion judged as “movement,” accuracy of reported path direction. Do not only ask “was there an animation”: compositor crumbs are not a built path.

Where it stops holding

• A very short opacity change can still work as a marker, because it does not need a spatial path, only a luminance transient. Displacement is the failure point here.
• If the user is already looking at the object, a short displacement may still be seen as a jump rather than travel, but the jump itself is visible.
• A higher refresh rate does not turn 40 ms into a trackable path; it only slices the same interval into more frames.

Applying it

• For travel transitions, ask whether the path can be reported, not whether the timeline has keyframes.
• Displacements shorter than about 100 ms are better as a cut or a marker than as an invisible animation.
• Check: being able to see the path at 0.25× speed does not mean it is visible at normal speed. Run a direction-report task at full speed.

Related

• Same group: F7.02.2 Motion that runs too long blocks the next action · F7.02.3 Larger travel needs a longer duration
• Nearby: A1.14.2 Apparent motion makes discrete position changes look continuous · A5.04 Change blindness · F7.12 Performance cost of motion
• Search terms: apparent motion · subthreshold duration · motion transient