function soften(factor) {
  return (onRefs, onOffsets, offOffsets) => {
    const scaledOffOffsets = offOffsets.map(pt => 
      [ pt[0] * factor, pt[1] * factor ]);
    return [ onRefs, onOffsets, scaledOffOffsets ];
  }
}
Softness control
@param { number } factor Softness control factor
@returns { Filter }