public function drawClippingCircle($x, $y, $w)
{
$this->curClippingPath = count($this->clippingPaths);
$defs = $this->resource->addChild('defs');
$clip = $defs->addChild('clipPath');
$clip->addAttribute('id', 'clip' . $this->curClippingPath);
$circle = $clip->addChild('circle');
$circle->addAttribute('cx', $x . $this->units);
$circle->addAttribute('cy', $y . $this->units);
$circle->addAttribute('r', $w . $this->units);
return $this;
}