 with alternating straight
    line segments coloured black and white; the segments are shorter
    on the sharper bends. 
  </p>

    <p class="img">
      <img src="figure/render-x-spline-1.svg" width="30%">
    </p>
    <p>
    The offset curve is drawn by evaluating the tangent at the end
    of each of these straight line segments, dividing by the length of the
    tangent, rotating 90 degrees to get the unit normal, then 
    multiplying by the appropriate offset.   The diagram below 
    shows the ca