public boolean addArc(Point centre, double rayon, double angleOrigine, double ouverture, int skip){
boolean ok = false;
if (elements.size() < positions.size() - 1){
Arc arc = new Arc(positions.get(indexElement), positions.get(indexElement+1+skip), centre, rayon, angleOrigine, ouverture);
arc.calculerLongueur();
this.elements.add(arc);
ok = true;
indexElement += (skip+1);
}