public function transformVertex(Vertex $v)
{
$x = $this->_matrix['x']['x'] * $v->get_x() + $this->_matrix['x']['y'] * $v->get_y();
$x += $this->_matrix['x']['z'] * $v->get_z() + $this->_matrix['x']['w'] * $v->get_w();
$y = $this->_matrix['y']['x'] * $v->get_x() + $this->_matrix['y']['y'] * $v->get_y();
$y += $this->_matrix['y']['z'] * $v->get_z() + $this->_matrix['y']['w'] * $v->get_w();
$z = $this->_matrix['z']['x'] * $v->get_x() + $this->_matrix['z']['y'] * $v->get_y();
$z += $this->_matrix['z']['z'] * $v->get_z() + $this->_matrix['z']['w'] * $v->get_w();
$w = $this->_matrix['w']['x'] * $v->get_x() + $this->_matrix['w']['y'] * $v->get_y();
$w += $this->_matrix['w']['z'] * $v->get_z() + $this->_matrix['w']['w'] * $v->get_w();
$res = new Vertex(array('x' => $x, 'y' => $y, 'z' => $z, 'w' => $w));
return $res;
}