I'm replying to my own post.
The problem is solved with the following code:
//Scaled polygon
//Current polygon
//One polygon in the scene
tnkPolygonSymbol *item;
//Gets one polygon from the scene
item = qgraphicsitem_cast<tnkPolygonSymbol *>(scenes[currentPage]->items()[0]);
//Gets the current polygon from the item
cpoly = item->polygon();
//Transformation matrix
QTranform trans;
//Transform the matrix to an scale of double it size;
trans = trans.scale(2.0,2.0);
//Scale the current polygon to the scaled polygon using the tranformation matrix
spoly = trans.map(cpoly)
//Replaces the current polygon with the scaled polygon
item->setPolygon(spoly);
//Scaled polygon
QPolygonF spoly;
//Current polygon
QPolygonF cpoly;
//One polygon in the scene
tnkPolygonSymbol *item;
//Gets one polygon from the scene
item = qgraphicsitem_cast<tnkPolygonSymbol *>(scenes[currentPage]->items()[0]);
//Gets the current polygon from the item
cpoly = item->polygon();
//Transformation matrix
QTranform trans;
//Transform the matrix to an scale of double it size;
trans = trans.scale(2.0,2.0);
//Scale the current polygon to the scaled polygon using the tranformation matrix
spoly = trans.map(cpoly)
//Replaces the current polygon with the scaled polygon
item->setPolygon(spoly);
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With this the item will have the current scaled polygon and the bounding rectangle will adapt to it.
Thanks for your replies.
Carlos.
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