title: Length measurement along a B-spline
body: <p>I'm currently working with paths defined on base of a unique B-spline. The B-spline is given by a set of control points and knot values. The number of control points and knots may vary (and remain coherent).</p>
  
  <p>Using the De Boor's algorithm, I am able to evaluate positions on the curve and to get the tangent value for those positions.
  So far so good.</p>
  
  <p>Now how to measure (/approximate) arc length along the B-spline path ?</p>