function pulse_( x ) {
	let val;
	let start;
	let expx;
	// test
	x = x * options.pulseScale;

	if ( x < 1 ) {
		// acceleration
		val = x - ( 1 - Math.exp( -x ) );
	} else {
		// tail
		// the previous animation ended here:
		start = Math.exp( -1 );
		// simple viscous drag
		x -= 1;
		expx = 1 - Math.exp( -x );
		val = start + ( expx * ( 1 - start ) );
	}

	return val * options.pulseNormalize;
}
Viscous fluid with a pulse for part and decay for the rest.
- Applies a fixed force over an interval (a damped acceleration), and
- Lets the exponential bleed away the velocity over a longer interval
- Michael Herf, http://stereopsis.com/stopping/