Easing

Easing is a technique used to control the rate of change of a parameter, most often time, during an animation or transition. Rather than moving at a constant speed, easing adjusts the motion so that it starts slowly, speeds up, and then slows down again, creating a more natural feel.

Practically, easing uses a function that converts a normalized progress value t in the interval [0, 1] into another value f(t) in the same interval. This function shapes the trajectory of the animated property, such as position, opacity, or rotation. The same concept applies to physics simulations and robotics control where smooth transitions are desirable.

Common easing categories include ease-in, ease-out, and ease-in-out. Ease-in starts the motion slowly and accelerates; ease-out starts fast and decelerates toward the end; ease-in-out does both. Easing functions may be polynomial (quadratic, cubic, quartic, quintic), trigonometric (sine, cosine), exponential, or circular in form, and many systems use cubic-bezier curves to define custom easing.

In software and web development, easing appears in animation libraries and CSS timing functions, where presets such as linear, ease, ease-in, ease-out, and ease-in-out are common, and developers may supply custom cubic-bezier curves.

Applications include user interface motion to communicate state changes, game character movement, and film or game special effects. Proper easing enhances perceived physical realism and responsiveness, but ill-chosen curves can feel jerky or artificial.