+ Blue polygon is in the attached file "Poly_chapeau.txt" + Cross positions are in the attached file "Parcours.txt" + Square polygon (structured element) is in the attached file "Poly_element_carre_d2_2.txt" My source code look like this :     // Erosion     QPolygonF poly_resultat = poly_ensemble;// blue polygon     CPolygone poly_es_translate;// CPolygon is derived from QPolygon     QPolygonF::Iterator itParcours = poly_parcours.begin();     int nCpt = 1;     while ( itParcours < poly_parcours.end() && nCpt < 33 ) {         poly_es_translate = poly_element_structurant;// square strutured element         QPointF tr = *itParcours;         QTransform t;         t.translate( tr.x(), tr.y() );         poly_es_translate = t.map( poly_element_structurant );         //poly_es_translate.translate( tr.x(), tr.y() );         cout << "nCpt = " << nCpt << "\n tx = " << tr.x() << ", ty = " << tr.y() << "\n poly_es_translate = \n" << poly_es_translate << endl;         poly_resultat = poly_resultat.united( poly_es_translate );         // suivant         ++itParcours;         ++nCpt;     }