OSG嵌入QT的简明总结

1.解决方案

不得不说关于OSG的资料实在太零散了，搜索了很多关于OSG在QT下的解决方案，都是各有各的说法，有的说的不是很清楚，有的已经过时了。这里提供一下自己的解决方案吧。
在远古的OSG里面，应该是提供对QT的支持的；不过应该是由于QT版本变动比较大，现在的OSG版本应该已经没有了。但是在GitHub上又有了新的osgQt项目（地址：https://github.com/openscenegraph/osgQt） 用来解决这个问题。
osgQt是个简单的小项目，其实没有必要额外的编译，最核心的是个名为GraphicsWindowQt的类，只需要复制GraphicsWindowQt.h和GraphicsWindowQt.cpp到QT工程里面就可以使用了。同时osgQt给出了一个名为osgviewerQt的样例，也仅仅只是个cpp文件。结合两者，一个简单的示例就出来了。
QT工程如下：

编译运行后如下：

2.存在问题

1) 警告提示

上述项目直接运行，会出现诸如“QOpenGLContext::swapBuffers() called with non-exposed window, behavior is undefined”的警告。查阅网上的英文资料，大意说是因为OpenGL环境未初始化产生的。在osgviewerQt中绑定了一个定时器，每隔10ms就调用frame()来绘制一帧，而这个定时器是在构造函数的时候就开始调用了，没有等待QT中OpenGL环境的生成。在这里我把定时器的部分给改进了一下，等待OSG的环境初始化完成在启动定时器，这个警告就没有了。osgviewerQt改进后的代码如下：

#include 
#include 
#include 

#include 
#include 

#include 

#include 

#include "GraphicsWindowQt"

#include 

class ViewerWidget : public QWidget, public osgViewer::CompositeViewer
{
public:
    ViewerWidget(QWidget* parent = 0, Qt::WindowFlags f = 0, osgViewer::ViewerBase::ThreadingModel threadingModel=osgViewer::CompositeViewer::SingleThreaded) : QWidget(parent, f)
    {
        setThreadingModel(threadingModel);

        // disable the default setting of viewer.done() by pressing Escape.
        setKeyEventSetsDone(0);

        QWidget* widget1 = addViewWidget( createGraphicsWindow(0,0,100,100), osgDB::readRefNodeFile("D:/Work/OSGBuild/OpenSceneGraph-Data/cow.osgt") );
        QWidget* widget2 = addViewWidget( createGraphicsWindow(0,0,100,100), osgDB::readRefNodeFile("D:/Work/OSGBuild/OpenSceneGraph-Data/glider.osgt") );
        QWidget* widget3 = addViewWidget( createGraphicsWindow(0,0,100,100), osgDB::readRefNodeFile("D:/Work/OSGBuild/OpenSceneGraph-Data/axes.osgt") );
        QWidget* widget4 = addViewWidget( createGraphicsWindow(0,0,100,100), osgDB::readRefNodeFile("D:/Work/OSGBuild/OpenSceneGraph-Data/fountain.osgt") );
//        QWidget* popupWidget = addViewWidget( createGraphicsWindow(900,100,320,240,"Popup window",true), osgDB::readRefNodeFile("D:/Work/OSGBuild/OpenSceneGraph-Data/dumptruck.osgt") );
//        popupWidget->show();

        QGridLayout* grid = new QGridLayout;
        grid->addWidget( widget1, 0, 0 );
        grid->addWidget( widget2, 0, 1 );
        grid->addWidget( widget3, 1, 0 );
        grid->addWidget( widget4, 1, 1 );
        setLayout( grid );

        //connect( &_timer, SIGNAL(timeout()), this, SLOT(update()) );
        //_timer.start( 10 );
    }

    QWidget* addViewWidget( osgQt::GraphicsWindowQt* gw, osg::ref_ptr scene )
    {
        osgViewer::View* view = new osgViewer::View;
        addView( view );

        osg::Camera* camera = view->getCamera();
        camera->setGraphicsContext( gw );

        const osg::GraphicsContext::Traits* traits = gw->getTraits();

        camera->setClearColor( osg::Vec4(0.2, 0.2, 0.6, 1.0) );
        camera->setViewport( new osg::Viewport(0, 0, traits->width, traits->height) );

        // set the draw and read buffers up for a double buffered window with rendering going to back buffer
        camera->setDrawBuffer(GL_BACK);
        camera->setReadBuffer(GL_BACK);

        camera->setProjectionMatrixAsPerspective(30.0f, static_cast(traits->width)/static_cast(traits->height), 1.0f, 10000.0f );

        view->setSceneData( scene );
        view->addEventHandler( new osgViewer::StatsHandler );
        view->setCameraManipulator( new osgGA::MultiTouchTrackballManipulator );
        gw->setTouchEventsEnabled( true );
        return gw->getGLWidget();
    }

    osgQt::GraphicsWindowQt* createGraphicsWindow( int x, int y, int w, int h, const std::string& name="", bool windowDecoration=false )
    {
        osg::DisplaySettings* ds = osg::DisplaySettings::instance().get();
        osg::ref_ptr traits = new osg::GraphicsContext::Traits;
        traits->windowName = name;
        traits->windowDecoration = windowDecoration;
        traits->x = x;
        traits->y = y;
        traits->width = w;
        traits->height = h;
        traits->doubleBuffer = true;
        traits->alpha = ds->getMinimumNumAlphaBits();
        traits->stencil = ds->getMinimumNumStencilBits();
        traits->sampleBuffers = ds->getMultiSamples();
        traits->samples = ds->getNumMultiSamples();

        return new osgQt::GraphicsWindowQt(traits.get());
    }

//    virtual void paintEvent( QPaintEvent* /*event*/ )
//    { frame(); }

    //定时器事件
    void timerEvent(QTimerEvent* )
    {
        frame();
    }

    //启动定时器绘制
    void show()
    {
        QWidget::show();
        _timerID = startTimer(10);
    }


protected:

    //QTimer _timer;
    int _timerID;               //定时器ID
};

int main( int argc, char** argv )
{
    osg::ArgumentParser arguments(&argc, argv);

#if QT_VERSION >= 0x050000
    // Qt5 is currently crashing and reporting "Cannot make QOpenGLContext current in a different thread" when the viewer is run multi-threaded, this is regression from Qt4
    osgViewer::ViewerBase::ThreadingModel threadingModel = osgViewer::ViewerBase::SingleThreaded;
#else
    osgViewer::ViewerBase::ThreadingModel threadingModel = osgViewer::ViewerBase::CullDrawThreadPerContext;
#endif

    while (arguments.read("--SingleThreaded")) threadingModel = osgViewer::ViewerBase::SingleThreaded;
    while (arguments.read("--CullDrawThreadPerContext")) threadingModel = osgViewer::ViewerBase::CullDrawThreadPerContext;
    while (arguments.read("--DrawThreadPerContext")) threadingModel = osgViewer::ViewerBase::DrawThreadPerContext;
    while (arguments.read("--CullThreadPerCameraDrawThreadPerContext")) threadingModel = osgViewer::ViewerBase::CullThreadPerCameraDrawThreadPerContext;

#if QT_VERSION >= 0x040800
    // Required for multithreaded QGLWidget on Linux/X11, see http://blog.qt.io/blog/2011/06/03/threaded-opengl-in-4-8/
    if (threadingModel != osgViewer::ViewerBase::SingleThreaded)
        QApplication::setAttribute(Qt::AA_X11InitThreads);
#endif

    QApplication app(argc, argv);
    ViewerWidget* viewWidget = new ViewerWidget(0, Qt::Widget, threadingModel);
    viewWidget->setGeometry( 100, 100, 800, 600 );
    viewWidget->show();
    return app.exec();
}

2) 多线程问题

在OSG中提供了诸如CullDrawThreadPerContext等多线程模式，但是在这里是没办法支持这些多线程模式的，只能支持单线程。在网上查阅了一些解决方案，但是最后都不是很完美，有空再把其解决方案写出来。

3) 其他

GraphicsWindowQt最终继承的还是QT中的QGLWidget类，提供OpenGL功能。而在新版本的QT中，这个类已经被废弃了，取而代之的是一个叫做QOpenGLWidget的类。所以这里面问题还是不少的，好在内容相对较少，以后有空可以自己改进。