function quartic( u ) {
	if ( isnan( u ) ) {
		return NaN;
	}
	if ( abs( u ) > 1.0 ) {
		return 0.0;
	}
	return ( 15.0 / 16.0 ) * pow( 1.0 - pow( u, 2.0 ), 2.0 );
}
Computes the quartic kernel for a value.

@private
@param {number} u - input value
@returns {number} kernel value at u

@example
var u = 5;
var out = quartic(u);
// returns 0.0