/**
  * Extend the outline by one pixel.
  *
  * @param  Pixel  $pixel  the next outline pixel
  *
  * @return self
  * 
  */
 public function push(Pixel $pixel)
 {
     $this->boundary->push($pixel);
     if (isset($this->outline[0]) && $this->outline[0]->position() == $pixel->position()) {
         $this->closed = true;
         return $this->clean();
     }
     $this->size++;
     $this->outline[] = $pixel;
     return $this;
 }