An ease-in-out curve that has customizable steepness.

sigmoidfactory.js

/* sigmoidFactory
 *
 * Generate an ease-in-out function with desired steepness.
 * Accompanying article: https://medium.com/analytic-animations/ease-in-out-the-sigmoid-factory-c5116d8abce9
 *
 * Required:
 *   k: (float != 0), sharpness of ease
 *
 * Return: f(t), t in 0..1
 */
function sigmoidFactory (k) {
	function base (t) {
		return (1 / (1 + Math.exp(-k * t))) - 0.5;
	}

	var correction = 0.5 / base(1);

	return function (t) {
		t = clamp(t, 0, 1);
		return correction * base(2 * t - 1) + 0.5;
	};
}

function clamp (val, lower, upper) {
	return Math.max(Math.min(val, upper), lower);
}