Easing

Easing functions control how values interpolate between the start and end of a tween. All built-in easings are pure functions mapping [0, 1] → [0, 1]. They are stored in an indexed table (EasingTable) for fast lookup during batch iteration — the Tween component stores the easing as an Int32 index, not a string.

Built-in easings

31 easing functions are included, matching anime.js naming conventions:

Name	Description
linear	Constant speed
inQuad	Accelerating from zero velocity
outQuad	Decelerating to zero velocity
inOutQuad	Acceleration until halfway, then deceleration
inCubic	Accelerating from zero velocity (cubic)
outCubic	Decelerating to zero velocity (cubic)
inOutCubic	Acceleration then deceleration (cubic)
inQuart	Accelerating from zero velocity (quartic)
outQuart	Decelerating to zero velocity (quartic)
inOutQuart	Acceleration then deceleration (quartic)
inQuint	Accelerating from zero velocity (quintic)
outQuint	Decelerating to zero velocity (quintic)
inOutQuint	Acceleration then deceleration (quintic)
inSine	Accelerating from zero velocity (sine)
outSine	Decelerating to zero velocity (sine)
inOutSine	Acceleration then deceleration (sine)
inExpo	Exponential acceleration
outExpo	Exponential deceleration
inOutExpo	Exponential acceleration then deceleration
inCirc	Circular acceleration
outCirc	Circular deceleration
inOutCirc	Circular acceleration then deceleration
inBack	Overshoots start, then accelerates
outBack	Overshoots end, then decelerates
inOutBack	Overshoots both ends
inElastic	Elastic wobble at start
outElastic	Elastic wobble at end
inOutElastic	Elastic wobble at both ends
inBounce	Bouncing at start
outBounce	Bouncing at end
inOutBounce	Bouncing at both ends

Usage

Pass the easing name as a string in TweenOptions:

animate.tween(entity, Transform2D, {
  x: 100,
  duration: 500,
  ease: 'outElastic',
});

Custom easing

Register custom easing functions at runtime:

const index = animate.registerEasing('myEase', (t) => t * t);

animate.tween(entity, Transform2D, {
  x: 100,
  ease: 'myEase',
});

Custom easing functions must:

• Accept a single number parameter t in the range [0, 1]
• Return a number in the range [0, 1]
• Be pure (no side effects)

Type definitions

export type EasingFn = (t: number) => number;
export type EasingName = keyof typeof EasingIndex;

export const EasingTable: EasingFn[];
export const EasingIndex: Record<string, number>;

EasingIndex maps easing names to their numeric indices in EasingTable. Runtime-registered easings are appended to EasingTable and tracked internally by the Animate resource.