Perceptual duration

A tween has a number written on it. A spring does not: it is described by mass, stiffness and damping, it approaches its target asymptotically, and formally it never arrives. That is fine as physics and useless as a specification, because the moment anyone asks how the spring compares with the 400 millisecond tween beside it, a number is needed. The perceptual duration is that number: the time from the start to the last movement a person could actually see, with everything after it counted as arrived.

The measurement needs a threshold, and the threshold is a choice rather than a constant. Common ones are the point where the remaining distance falls under one percent of the travel, or under half a pixel on the screen it will run on, or where velocity drops below some small figure and stays there. Different tools answer differently, which is why two libraries can report different durations for the same three parameters and both be right. State the threshold when the number matters, treat it as approximate when it does not, and remember that a springier setting can look slower than a stiffer one even when its measured time is shorter, because the wobble at the end keeps drawing the eye.

The word perceptual is doing a second job too, and it is worth keeping the two apart. Felt time is not clock time: a well eased move reads as quicker than a linear one of identical length, an interruptible move reads as quicker than one that has to finish, and motion that starts the instant a control is pressed reads as quicker than motion of the same length that begins after a delay. That is craft lore with attention research behind it rather than a law, and the honest anchors are the response-time thresholds: under roughly 100 milliseconds a result feels instantaneous, and past about a second a person’s attention has started to wander off the task.

Use the figure the way any other duration is used. Quote it in a review, compare it against the timed transitions it will sit beside, and once a family of springs has been chosen, write the settled numbers down where the rest of the system can spend them, which is what a motion token is for. A spring that measures 500 milliseconds and a tween of 400 belong to the same conversation. A spring nobody has measured belongs to no conversation at all.