code: function(startPoint, endPoint) {
                  startPoint = vector.create(startPoint).to3D();
                  endPoint = vector.create(endPoint).to3D();
                  if (startPoint === null || endPoint === null) { return null; }
                  this.line = line.create(startPoint, endPoint.subtract(startPoint));
                  this.start = startPoint;
                  this.end = endPoint;
                  return this;
                }
docs: Set the start and end-points of the segment