cocos2d-x CCSprite自动拉伸全屏

CCSize size = CCDirector::sharedDirector()->getWinSize();
 CCSprite* pSprite = CCSprite::create("bg.jpg");
 pSprite->setPosition(ccp(size.width/2, size.height/2));
 float winw = size.width;
 float winh = size.height;

 float spx = pSprite->getTextureRect().getMaxX();
 float spy = pSprite->getTextureRect().getMaxY();

 pSprite->setScaleX(winw/spx);
 pSprite->setScaleY(winh/spy);