Interpolated

Interpolated stores the previous and current fixed-step samples. Call push() after each fixed step and at(schedule.alpha) when rendering.

import { Interpolated, lerpNumber } from "@jolly-pixel/loop";

const height = new Interpolated(0, lerpNumber);

loop.start({
  fixedUpdate: (fixedDeltaMs) => {
    simulate(fixedDeltaMs / 1000);
    height.push(player.y);
  },
  update: (_frameDeltaMs, alpha) => {
    sprite.y = height.at(alpha);
  }
});

The lerp is supplied by the caller, so this class carries no knowledge of what it interpolates:

export type Lerp<T> = (previous: T, current: T, alpha: number) => T;

export function lerpNumber(
  previous: number,
  current: number,
  alpha: number
): number;

const position = new Interpolated({ x: 0, y: 0 }, (previous, current, alpha) => ({
  x: lerpNumber(previous.x, current.x, alpha),
  y: lerpNumber(previous.y, current.y, alpha)
}));

The caller supplies interpolation for other value types. The package has no three dependency.

Constructor

new Interpolated(initial, lerp)

new Interpolated<T>(initial: T, lerp: Lerp<T>);

Both samples start at initial.

API

Member	Description
previous	Read-only sample before current.
current	Read-only latest sample.
push(value: T): this	Shifts current to previous and stores value.
reset(value: T): this	Replaces both samples.
at(alpha: number): T	Blends with alpha clamped to [0, 1].

at() returns an endpoint without calling the lerp when alpha <= 0 or alpha >= 1. Interpolated rendering trails simulation by at most one fixed step.

NOTE

Engine-wide transform interpolation requires fixed steps to be the only transform writers. Existing components also write transforms in update(), so automatic interpolation remains out of scope for this package.