const unsigned int SHOW_ENTITY_ON_MAIN_VIEWPORT_FLAG    = 1;
const unsigned int SHOW_ENTITY_ON_OTHER_VIEWPORT_FLAG    = 1 << 1; //or 1 << n
const unsigned int SHOW_ENTITY_ON_ALL_VIEWPORT_FLAG    = 0xFFFFFFFF;

void createCameraLook(const std::string& CameraName, const std::string& TextureName, 
              unsigned int mask = 1 << 1, 
                      const Ogre::Vector3& CameraPos = Ogre::Vectir3(0,1,3), 
                      const Ogre::Vector3& lookPos = Ogre::Vector3(0,1,0))
{
        //create a camera look this scenenode(attached entitys)
        Ogre::Camera* pCameraLookMe = pOgreSceneMgr->createCamera(CameraName);
    pCameraLookMe->setPosition(CameraPos);
    pCameraLookMe->lookAt(lookPos);
    pCameraLookMe->setAutoTracking(true, m_pSceneNode, lookPos);
    pCameraLookMe->setFarClipDistance(0);
    pCameraLookMe->setNearClipDistance(1.0);
    pCameraLookMe->setAutoAspectRatio(true);
    pSceneNode->attachObject(m_pCameraLookMe);

        //let's create a rttTex and take the camera
    Ogre::TexturePtr pRttCameraLookMe = Ogre::TextureManager::getSingleton().createManual( TextureName, 
        Ogre::ResourceGroupManager::DEFAULT_RESOURCE_GROUP_NAME, Ogre::TEX_TYPE_2D, 
        128, 128, 0, Ogre::PF_DXT5, Ogre::TU_RENDERTARGET );
    Ogre::RenderTarget *rttTex = pRttCameraLookMe->getBuffer()->getRenderTarget();

    Ogre::Viewport* pVp = rttTex->addViewport(pCameraLookMe);
        //alpha background
    pVp->setBackgroundColour(Ogre::ColourValue(0,0,0,0));
        //i just set the mainRenderWnd's viewport's visibilitymask to 1, 
        //so this viewport need to set 1 << 1, if you add more the mask must 1 << n
    pVp->setVisibilityMask(mask);
        //this viewport don't need sky,shadow,overlay
    pVp->setOverlaysEnabled(false);
    pVp->setSkiesEnabled(false);
    pVp->setShadowsEnabled(false);
        //every frame render,or not
    rttTex->setAutoUpdated(true);
}

//after, you can set the entity's visibilityFlag as you need it to show on which viewport.
pEntity->setVisibilityFlags(SHOW_ENTITY_ON_MAIN_VIEWPORT_FLAG | (1 << 1));