经历几次修改，总算实现了感觉还行的效果。下面是我的实现代码，高手勿喷！！！！

1. Erasure.hpp

//
// Tailor.hpp
// Hello-mobile

#ifndef Erasure_hpp
#define Erasure_hpp

#include <stdio.h>
#include <cocos2d.h>
USING_NS_CC;
class Erasure : public Layer
{
public:
    Erasure();
    ~Erasure();
    bool init();
    static Scene *scene();

    virtual bool onTouchBegan(Touch *touch,Event *unused_event);
    virtual void onTouchMoved(Touch *touch,Event *unused_event);
    virtual void onTouchEnded(Touch *touch,Event *unused_event);
    virtual void onTouchCancelled(Touch *touch,Event *unused_event);
    CREATE_FUNC(Erasure);
public:
    void initializeElement();
    void initializeDrawNode();

private:
    Sprite *m_pBottom;
    Sprite *m_pImage;

   // DrawNode *m_pDotNode;
    RenderTexture *m_pRenderTexture;
};
#endif /* Tailor_hpp */

3.Erasure.cpp

//
// Tailor.cpp
// Hello-mobile

#include "Erasure.hpp"
Erasure::Erasure()
{
    m_pImage = NULL;
    m_pBottom = NULL;
}
Erasure::~Erasure()
{
    m_pImage = NULL;
    m_pBottom = NULL;
}
Scene *Erasure::scene()
{
    Erasure *pLayer = Erasure::create();
    Scene *scene = Scene::create();
    scene->addChild(pLayer);
    return scene;
}
bool Erasure::init()
{
    if (!Layer::init())
    {
        return false;
    }

    initializeElement();
    initializeDrawNode();

    EventListenerTouchOneByOne *listener = EventListenerTouchOneByOne::create();
    listener->onTouchBegan = CC_CALLBACK_2(Erasure::onTouchBegan,this);
    listener->onTouchMoved = CC_CALLBACK_2(Erasure::onTouchMoved,this);
    listener->onTouchEnded = CC_CALLBACK_2(Erasure::onTouchEnded,this);
    listener->onTouchCancelled = CC_CALLBACK_2(Erasure::onTouchCancelled,this);
    _eventDispatcher->addEventListenerWithSceneGraPHPriority(listener,this);



    return true;
}
void Erasure::initializeElement()
{
    Size winSize = Director::getInstance()->getWinSize();
    m_pBottom = Sprite::create("white.jpeg");
    m_pBottom->setPosition(Vec2(winSize.width/2,winSize.height/2));
    m_pBottom->setScale(5.0f);
    addChild(m_pBottom,0);

    m_pImage = Sprite::create("image.jpg");
    m_pImage->setScale(2.0f);
    m_pImage->setAnchorPoint(Vec2(0.5f,0.5f));
    m_pImage->setPosition(Vec2(winSize.width/2,winSize.height/2));
// addChild(m_pImage); //注意这里不要把遮挡精灵加入父节点，当时在这个地方卡了很久，
//实现不了效果
}
void Erasure::initializeDrawNode()
{
    Size size = Director::getInstance()->getWinSize();

    //m_pDotNode = DrawNode::create();
    //m_pDotNode->retain();
    //m_pDotNode->drawDot(Point(0,0),3.0f,Color4F(255,255));

    m_pRenderTexture = RenderTexture::create(size.width,size.height);
    m_pRenderTexture->setPosition(Vec2(size.width/2,size.height/2));
    this->addChild(m_pRenderTexture);
    //通过RenderTexture将m_pImage渲染到节点上
    m_pRenderTexture->begin();
    m_pImage->visit();
    m_pRenderTexture->end();
}
bool Erasure::onTouchBegan(cocos2d::Touch *touch,cocos2d::Event *unused_event)
{
    return true;
}
void Erasure::onTouchMoved(cocos2d::Touch *touch,cocos2d::Event *unused_event)
{
    Point pt = touch->getLocation();
    float distance = pt.distance(m_ptDragStart);
    if (m_pRenderTexture && m_pRenderTexture->isVisible())
    {
        if (distance > 1)
        {
            m_pRenderTexture->begin();
            int d = (int)distance;
            for (int i = 0; i < d; i++)
            {
                //橡皮擦
                blendFunc.src = GL_ZERO;
                //设置混合模式
                BlendFunc blendFunc;
                blendFunc.src = GL_ZERO;
                blendFunc.dst = GL_ONE_MINUS_DST_ALPHA;
// m_pDotNode->setBlendFunc(blendFunc);
                Sprite *brush = Sprite::create("brush.png");
                brush->setBlendFunc(blendFunc);
                float difx = pt.x - m_ptDragStart.x;
                float dify = pt.y - m_ptDragStart.y;
                float delta = (float)i / distance;
                Point pos = Point(m_ptDragStart.x + (difx * delta),m_ptDragStart.y + (dify * delta));
// m_pDotNode->setPosition(pos);
// m_pDotNode->visit();
                brush->setPosition(pos);
                brush->visit();
            }
            m_pRenderTexture->end();
        }
    }
    m_ptDragStart = touch->getLocation();
}
void Erasure::onTouchEnded(cocos2d::Touch *touch,cocos2d::Event *unused_event)
{

}
void Erasure::onTouchCancelled(cocos2d::Touch *touch,cocos2d::Event *unused_event)
{

}

参考文章：

1.http://www.mamicode.com/info-detail-193798.html