关于相机有几点不明白，望前辈们帮忙指点迷津

liyang2014

在编程指南第九章中有如下一段代码：Main函数1
int main()
{
        osg::ref_ptr<osgViewer::Viewer> viewer = new osgViewer::Viewer();
        //创建各种文字效果
        osg::ref_ptr<osg::Camera> camera = createAllKindText();
        //添加到根节点
        osg::ref_ptr<osg::Group> root = new osg::Group() ;
        root->addChild(createAllKindText());
        //优化场景数据
        osgUtil::Optimizer optimizer ;
        optimizer.optimize(root.get()) ;
        viewer->setSceneData(root.get());
        viewer->realize();
        viewer->run();
        return 0 ;
}
这个camera应该是新建的从相机？其中creatAllKindText返回是一个从相机
但是viewer->getCamera()这个主相机应该默认就存在吧？
osg::ref_ptr<osg::Camera> createAllKindText()
{
        osg::ref_ptr<osg::Camera> camera = new osg::Camera() ;

        //设置投影矩阵
        camera->setProjectionMatrix(osg::Matrix:rtho2D(0,1280,0,800));

        //设置视图矩阵  
        camera->setReferenceFrame(osg::Transform::ABSOLUTE_RF);
        camera->setViewMatrix(osg::Matrix::identity());

        //清除深度缓存
        camera->setClearMask(GL_DEPTH_BUFFER_BIT);

        //设置渲染顺寻，在主摄像机之前渲染
        camera->setRenderOrder(osg::Camera:OST_RENDER);

        //设置为不接收事件，始终不得到焦点
        camera->setAllowEventFocus(false);

        osg::ref_ptr<osgText::Font> font = osgText::readFontFile("fonts/arial.ttf");

        osg::ref_ptr<osg::Geode> geode  = new osg::Geode;

        osg::ref_ptr<osg::StateSet> stateset = geode->getOrCreateStateSet();
        stateset->setMode(GL_LIGHTING,osg::StateAttribute::OFF);

        //-----------------------------------------------//-----------------------------------------------
        //设置文字输出的格式

        //设置格式为LEFT_TO_RIGHT，从左到右
        {
                osg::ref_ptr<osgText::Text> text = new osgText::Text;
                text->setFont(font.get());
                text->setColor(osg::Vec4(1.0f,1.0f,0.0f,1.0f));
                text->setCharacterSize(20.0f);
                text->setPosition(osg::Vec3(50.0f,750.0f,0.0f));

                text->setLayout(osgText::Text:EFT_TO_RIGHT);

                text->setText("text->setLayout(osgText::Text::LEFT_TO_RIGHT);");
                geode->addDrawable(text.get());
        }

        //设置格式为RIGHT_TO_LEFT，从右到左
        {
                osg::ref_ptr<osgText::Text> text = new osgText::Text;
                text->setFont(font.get());
                text->setColor(osg::Vec4(1.0f,1.0f,0.0f,1.0f));
                text->setCharacterSize(30.0f);
                text->setPosition(osg::Vec3(1200.0f,750.0f,0.0f));

                text->setLayout(osgText::Text::RIGHT_TO_LEFT);
                text->setAlignment(osgText::Text::RIGHT_BASE_LINE);

                text->setText("text->setLayout(osgText::Text::RIGHT_TO_LEFT);");

                geode->addDrawable(text.get());
        }

        //设置格式为VERTICAL，垂直
        {
                osg::ref_ptr<osgText::Text> text = new osgText::Text;
                text->setFont(font.get());
                text->setColor(osg::Vec4(1.0f,1.0f,0.0f,1.0f));
                text->setCharacterSize(20.0f);
                text->setPosition(osg::Vec3(50.0f,750.0f,0.0f));

                text->setLayout(osgText::Text::VERTICAL);

                text->setText("text->setLayout(osgText::Text::VERTICAL);");

                geode->addDrawable(text.get());
        }

        //-----------------------------------------------//-----------------------------------------------
        //设置阴影
        {
                osg::ref_ptr<osgText::Text> text = new osgText::Text;
                text->setFont(font.get());
                text->setColor(osg::Vec4(1.0f,1.0f,0.0f,1.0f));
                text->setCharacterSize(40.0f);
                text->setPosition(osg::Vec3(100.0f,650.0f,0.0f));

                //设置阴影为DROP_SHADOW_BOTTOM_RIGHT
                text->setBackdropType(osgText::Text:ROP_SHADOW_BOTTOM_RIGHT);
                text->setBackdropColor(osg::Vec4(0.0f,1.0f,0.0f,1.0f));
                text->setBackdropOffset(0.1f,-0.1f);

                text->setText("text->setBackdropType(osgText::Text::DROP_SHADOW_BOTTOM_RIGHT);");

                geode->addDrawable(text.get());
        }

        //-----------------------------------------------//-----------------------------------------------
        //设置边框
        {
                osg::ref_ptr<osgText::Text> text = new osgText::Text;
                text->setFont(font.get());
                text->setColor(osg::Vec4(1.0f,1.0f,0.0f,1.0f));
                text->setCharacterSize(30.0f);
                text->setPosition(osg::Vec3(100.0f,450.0f,0.0f));

                //设置边框对齐绘制
                text->setDrawMode(osgText::Text::TEXT | osgText::Text::BOUNDINGBOX | osgText::Text::ALIGNMENT);

                text->setText("text->setDrawMode(osgText::Text::TEXT | osgText::Text::BOUNDINGBOX | osgText::Text::ALIGNMENT);");

                geode->addDrawable(text.get());
        }

        //-----------------------------------------------//-----------------------------------------------
        //设置颜色倾斜模式
        {
                osg::ref_ptr<osgText::Text> text = new osgText::Text;
                text->setFont(font.get());
                text->setColor(osg::Vec4(1.0f,1.0f,0.0f,1.0f));
                text->setCharacterSize(40.0f);
                text->setPosition(osg::Vec3(100.0f,250.0f,0.0f));
                //设置颜色倾斜模式为PER_CHARACTER
                text->setColorGradientMode(osgText::Text::PER_CHARACTER);
                //设置倾斜四个角落的颜色
                text->setColorGradientCorners(osg::Vec4(1.0f,0.0f,0.0f,1.0f),osg::Vec4(0.0f,1.0f,0.0f,1.0f),
                        osg::Vec4(0.0f,0.0f,1.0f,1.0f),osg::Vec4(1.0f,1.0f,1.0f,1.0f));
                text->setText("text->setColorGradientMode(osgText::Text::SOLID);");
                geode->addDrawable(text.get());
        }

        camera->addChild(geode.get());

        return camera.get() ;
}
为什么说是在主摄相机之前渲染？ 从摄像机和主摄像机之间关系怎样理解？
我现在只明白应该是在camera相机内部最后一个渲染，但是我不理解FlySky为什么说是在 主摄像机之前渲染。我想这里有一个我不明白的知识点吧，望高人指点迷津。
Main函数2
int main()
{
//根节点
    osg::Group* root = new osg::Group();
root = createTrans();
    osgViewer::Viewer viewer;
    //创建节点到场景中
    viewer.setSceneData(root);
    viewer.realize();
    return viewer.run();
}

另外像这样相机节点是加在哪里，这个流程原理不明白，上面的Main函数中camera节点下挂实体节点，这个main函数的相机节点跟root节点是怎么关联的？