Easing

{ value: 1, duration: 1000, easing: "easeInOut" }

Each keyframe can specify an easing curve that controls how values interpolate over the transition duration. The easing function maps normalized time (0-1) to an output value that blends between the previous keyframe's value and the current target.

Curves

linear -- Constant rate of change. The default when easing is omitted.

easeIn -- Cubic acceleration (t^3). Starts slow, finishes fast. Used by the ramp_up preset for gradual intensity builds.

easeOut -- Cubic deceleration (1 - (1-t)^3). Starts fast, finishes slow. Used by ramp_down for natural wind-downs.

easeInOut -- Smooth cubic acceleration followed by deceleration. Produces the most natural-feeling transitions. Both the wave and stroke presets use this curve.

step -- Discrete jump at the end of the duration. Holds the previous value until the transition completes, then snaps to the target (t < 1 ? 0 : 1). Useful for timed delays before an instant change.

Usage in Keyframes

Easing is optional on every Keyframe. When omitted, linear is used. Zero-duration keyframes ignore easing entirely since there is no transition to interpolate.

const keyframes = [
  { value: 0, duration: 0 },                       // instant start
  { value: 1, duration: 1000, easing: "easeIn" },  // accelerate up
  { value: 1, duration: 500 },                      // hold (linear)
  { value: 0, duration: 1000, easing: "easeOut" },  // decelerate down
];

Mixing easing curves within a single track creates complex feel profiles. Combine easeIn on the attack with easeOut on the release for a surge-like shape, or use step to hold a value for a precise delay before transitioning.

Runtime Access

The EASING_FUNCTIONS export provides direct access to each curve as a (t: number) => number function, keyed by name.

import { EASING_FUNCTIONS } from "@zendrex/buttplug.js";

const eased = EASING_FUNCTIONS.easeInOut(0.5); // 0.5
const stepped = EASING_FUNCTIONS.step(0.99);   // 0

All functions accept values in the 0-1 range and clamp out-of-bounds inputs. See the PatternEngine reference for the full Easing type definition.