cubic bezier easing

cubic-bezier.js

// simplified version of:
// https://github.com/gre/bezier-easing/blob/master/src/index.js
// https://npmjs.com/package/bezier-easing

function slopeFromT(t, A, B, C) {
	return 1.0 / (3.0 * A * t * t + 2.0 * B * t + C);
}

function xFromT(t, A, B, C, D) {
	return A * (t * t * t) + B * (t * t) + C * t + D;
}

function yFromT(t, E, F, G, H) {
	let tt = t * t;
	return E * (tt * t) + F * tt + G * t + H;
}

function cubicBezier(percent, x0, y0, x1, y1) {
	if (percent <= 0) return 0;
	if (percent >= 1) return 1;
	if (x0 === y0 && x1 === y1) return percent; // linear

	let x0a = 0; // initial x
	let y0a = 0; // initial y
	let x1a = x0; // 1st influence x
	let y1a = y0; // 1st influence y
	let x2a = x1; // 2nd influence x
	let y2a = y1; // 2nd influence y
	let x3a = 1; // final x
	let y3a = 1; // final y

	let A = x3a - 3.0 * x2a + 3.0 * x1a - x0a;
	let B = 3.0 * x2a - 6.0 * x1a + 3.0 * x0a;
	let C = 3.0 * x1a - 3.0 * x0a;
	let D = x0a;

	let E = y3a - 3.0 * y2a + 3.0 * y1a - y0a;
	let F = 3.0 * y2a - 6.0 * y1a + 3.0 * y0a;
	let G = 3.0 * y1a - 3.0 * y0a;
	let H = y0a;

	let current = percent;
	let i, currentx, currentslope;
	for (i = 0; i < 5; i++) {
		currentx = xFromT(current, A, B, C, D);
		currentslope = slopeFromT(current, A, B, C);
		if (currentslope === Infinity) currentslope = percent;
		current -= (currentx - percent) * currentslope;
		current = Math.min(Math.max(current, 0.0), 1.0);
	}

	return yFromT(current, E, F, G, H);
}