Learning OSG programing---Multi Camera in one window 在单窗口中创建多相机

　　在学习OSG提供的例子osgCamera中，由于例子很长，涉及很多细节，考虑将其分解为几个小例子。本文介绍实现在一个窗口中添加多个相机的功能。

　　此函数接受一个Viewer引用类型参数，设置图形上下文的特征变量traits,并由它建立图形上下文。根据需要差创建的相机数量，循环创建相机变量，并将其添加到观察器变量中，作为从相机。每次创建相机，都需要获取当前图形上下文，并设置相机的起点（左下角）坐标和相机的宽度与高度。相机间的区别仅在于相机起点的不同。完整的程序代码如下。

void singleWindowMultipleCameras(osgViewer::Viewer& viewer)
{
    osg::GraphicsContext::WindowingSystemInterface* wsi = osg::GraphicsContext::getWindowingSystemInterface();
    if (!wsi)
    {   
        osg::notify(osg::NOTICE)<<"Error, no WindowSystemInterface available, cannot create windows."<<std::endl;
        return;
    }

    unsigned int width, height;
    osg::GraphicsContext::ScreenIdentifier main_screen_id;

    main_screen_id.readDISPLAY();
    main_screen_id.setUndefinedScreenDetailsToDefaultScreen();
    wsi->getScreenResolution(main_screen_id, width, height);

    osg::ref_ptr<osg::GraphicsContext::Traits> traits = new osg::GraphicsContext::Traits;
    traits->x = 0;
    traits->y = 0;
    traits->width = width;
    traits->height = height;
    traits->windowDecoration = true;
    traits->doubleBuffer = true;
    traits->sharedContext = 0;
    traits->readDISPLAY();
    traits->setUndefinedScreenDetailsToDefaultScreen();

    osg::ref_ptr<osg::GraphicsContext> gc = osg::GraphicsContext::createGraphicsContext(traits.get());
    if (gc.valid())
    {
        osg::notify(osg::INFO)<<"  GraphicsWindow has been created successfully."<<std::endl;

        // need to ensure that the window is cleared make sure that the complete window is set the correct colour
        // rather than just the parts of the window that are under the camera's viewports
        gc->setClearColor(osg::Vec4f(0.2f,0.2f,0.6f,1.0f));
        gc->setClearMask(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    }
    else
    {
        osg::notify(osg::NOTICE)<<"  GraphicsWindow has not been created successfully."<<std::endl;
    }

    unsigned int numCameras = 3;
    double aspectRatioScale = 1.0;///(double)numCameras;
    for(unsigned int i=0; i<numCameras;++i)
    {
        osg::ref_ptr<osg::Camera> camera = new osg::Camera;
        camera->setGraphicsContext(gc.get());
        camera->setViewport(new osg::Viewport((i*width)/numCameras,(i*height)/numCameras, width/numCameras, height/numCameras));
        GLenum buffer = traits->doubleBuffer ? GL_BACK : GL_FRONT;
        camera->setDrawBuffer(buffer);
        camera->setReadBuffer(buffer);

        viewer.addSlave(camera.get(), osg::Matrixd(), osg::Matrixd::scale(aspectRatioScale,1.0,1.0));
    }
}

　　之后在主函数中调用创建多相机函数，为观察器添加从相机，并向观察器中加入模型，进行显示。主函数代码如下：

int main(){
    osgViewer::Viewer viewer;
    singleWindowMultipleCameras(viewer);
    viewer.setSceneData(osgDB::readRefNodeFile("cow.osg"));
    return viewer.run();
}

　　运行程序，可显示多相机效果，如下所示：

　　可以更改singleWindowMultipleCameras函数中的numCameras变量，创建更多的从相机。

　　创建多相机的关键是相机视点的设置和向观察器中添加从相机。而程序最开始的创建窗口系统接口，图形上下文，图形上下文特征以及相机的缓冲设置，则是一些辅助性工作。