请问怎样才能选定区域截图？

Cocos2d-x
#1

我用CCRenderTexture, 但总是从0,0位置开始截图， 下面是代码
CCRenderTexture *screenshot = CCRenderTexture::create(100, 100);
screenshot->setPosition(CCPointMake(300, 100));
screenshot->begin();
CCDirector::sharedDirector()->getRunningScene()->visit();
screenshot->end();
不知道哪里错了，哪位能帮忙看下， 谢谢~~