ccDrawPoly的相对坐标问题

Cocos2d-x
#1 
SYShpLayer* ShpLayer = new SYShpLayer;                //SYShpLayer是继承CCLayer
cocos2d::CCRenderTexture* m_pRenderTex = CCRenderTexture::create( 256, 256, kCCTexture2DPixelFormat_RGBA8888 );
m_pRenderTex->setPosition(50,50);
m_pRenderTex->beginWithClear(100,100,100,100);

ccDrawColor4B(0, 0, 255, 255);
glLineWidth(10);
CCPoint vertices] = { ccp(0,0), ccp(50,50), ccp(100,50), ccp(100,100), ccp(50,100) };
ccDrawPoly( vertices, 5, false);

m_pRenderTex->end();
m_pRenderTex->saveToFile("test12345.png");


```


无论m_pRenderTex的位置设为多少（m_pRenderTex->setPosition(50,50)），画在图片中的多边形都是一样的，这是为什么呢？