RailwayRouteOptimization/scripts/Draw/Spline/SplineControlPoints/doc/SplineControlPoints_proc_ru.html

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<ol>
  <li>Choose the degree of the spline in the options tool bar. Supported degrees
  are 2 (quadratic b-spline) and 3 (cubic b-spline). The higher the degree, the
  'smoother' the curve becomes. 
  <br />Note that a quadratic b-spline requires by definition at least 3
  control points while a cubic b-spline has at least 4 control points.</li>
  <li>Check the 'Closed' check box in the options tool bar if you want to create
  a closed spline. Closed splines are continuous closed loops.</li>
  <li>Specify the control points. After defining the third (for quadratic
  b-splines) or fourth (for cubic b-splines) control point, a preview will be
  shown. You can remove the last control point again by clicking the 'Undo' 
  button in the options tool bar.</li>
</ol>

<p>
Example for two open splines (quadratic and cubic):
<br/>
<img src="SplineControlPoints01.png" class="figure"/>
<br/>
Example for two closed, periodic splines (quadratic and cubic):
<br/>
<img src="SplineControlPoints02.png" class="figure"/>
</p>
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