/**
* Split this QuadTree node into four quadrants for NW/NE/SE/SW
*
* @return null
**/
protected function subdivide()
{
$width = $this->boundingBox->getWidth() / 2;
$height = $this->boundingBox->getHeight() / 2;
$centrePointX = $this->boundingBox->getCentrePoint()->getX();
$centrePointY = $this->boundingBox->getCentrePoint()->getY();
$this->northWest = new QuadTree(new QuadTreeBoundingBox(new QuadTreeXYPoint($centrePointX - $width / 2, $centrePointY + $height / 2), $width, $height), $this->maxPoints);
$this->northEast = new QuadTree(new QuadTreeBoundingBox(new QuadTreeXYPoint($centrePointX + $width / 2, $centrePointY + $height / 2), $width, $height), $this->maxPoints);
$this->southWest = new QuadTree(new QuadTreeBoundingBox(new QuadTreeXYPoint($centrePointX - $width / 2, $centrePointY - $height / 2), $width, $height), $this->maxPoints);
$this->southEast = new QuadTree(new QuadTreeBoundingBox(new QuadTreeXYPoint($centrePointX + $width / 2, $centrePointY - $height / 2), $width, $height), $this->maxPoints);
}
