void HelloWorld::setBackGround(const char *filename){

    if(backImg == 0){

        backImg=Sprite::create(filename);

        backImg->setAnchorPoint(Point::ZERO);

        backImg->setPosition(Point::ZERO);

        Size mywinsize=Director::getInstance()->getWinSize();

        float winw = mywinsize.width;

        float winh = mywinsize.height;

        float spx = backImg->getTextureRect().getMaxX();

        float spy = backImg->getTextureRect().getMaxY();

        backImg->setScaleX(winw/spx);

        backImg->setScaleY(winh/spy);

        this->addChild(backImg);

        return;

    }

    backImg->setTexture(filename);

}