add preset interpolators

Author: timhaywood

src/index.ts

@@ -1,4 +1,5 @@
 import { Layer, Vector } from 'expression-globals-typescript';
+import { Interpolator, interpolators } from './interpolators';
 
 const thisLayer = new Layer();
 
@@ -22,11 +23,7 @@ interface EKey extends InputKey {
 
 interface AnimateOptions {
   inputTime: number;
-  interpolator?: (
-    progress: number,
-    easeOut?: number,
-    easeIn?: number
-  ) => number;
+  interpolator?: Interpolator;
 }
 
 // The function that's called from After Effects
@@ -449,4 +446,4 @@ function animate(
 
 const version: string = '_npmVersion';
 
-export { animate, version };
+export { animate, version, interpolators };

src/interpolators.ts

@@ -0,0 +1,42 @@
+export type Interpolator = (
+  progress: number,
+  easeOut?: number,
+  easeIn?: number
+) => number;
+
+export const interpolators: Record<string, Interpolator> = {
+  // no easing, no acceleration
+  linear: t => t,
+  // accelerating from zero velocity
+  easeInQuad: t => t * t,
+  // decelerating to zero velocity
+  easeOutQuad: t => t * (2 - t),
+  // acceleration until halfway, then deceleration
+  easeInOutQuad: t => (t < 0.5 ? 2 * t * t : -1 + (4 - 2 * t) * t),
+  // accelerating from zero velocity
+  easeInCubic: t => t * t * t,
+  // decelerating to zero velocity
+  easeOutCubic: t => --t * t * t + 1,
+  // acceleration until halfway, then deceleration
+  easeInOutCubic: t =>
+    t < 0.5 ? 4 * t * t * t : (t - 1) * (2 * t - 2) * (2 * t - 2) + 1,
+  // accelerating from zero velocity
+  easeInQuart: t => t * t * t * t,
+  // decelerating to zero velocity
+  easeOutQuart: t => 1 - --t * t * t * t,
+  // acceleration until halfway, then deceleration
+  easeInOutQuart: t => (t < 0.5 ? 8 * t * t * t * t : 1 - 8 * --t * t * t * t),
+  // accelerating from zero velocity
+  easeInQuint: t => t * t * t * t * t,
+  // decelerating to zero velocity
+  easeOutQuint: t => 1 + --t * t * t * t * t,
+  // acceleration until halfway, then deceleration
+  easeInOutQuint: t =>
+    t < 0.5 ? 16 * t * t * t * t * t : 1 + 16 * --t * t * t * t * t,
+  easeInElastic: t => (0.04 - 0.04 / t) * Math.sin(25 * t) + 1,
+  easeOutElastic: t => ((0.04 * t) / --t) * Math.sin(25 * t),
+  easeInOutElastic: t =>
+    (t -= 0.5) < 0
+      ? (0.02 + 0.01 / t) * Math.sin(50 * t)
+      : (0.02 - 0.01 / t) * Math.sin(50 * t) + 1,
+};