Point2d p2 = model.getMerge().get( atom ).getPoint2d();
// the element size has to be scaled to model space
// so that it can be scaled back to screen space...
PathBuilder pb = new PathBuilder();
pb.color( jcpModel.getHoverOverColor() );
Vector2d vec = new Vector2d();
vec.sub( p2, p1 );
Vector2d per = GeometryTools.calculatePerpendicularUnitVector( p1, p2 );