QtQuick中集成OSG渲染

从目前网上介绍OSG与Qml集成有两种方式：

1. 在[QtQuick2OSGItem](https://bitbucket.org/leon_manukyan/qtquick2osgitem/src/master/sampleapp/)通过继承抽象类__QQuickFramebufferObject::Renderer__来重写渲染接口，在__QQuickFramebufferObject__渲染成__Frame Buffer Object(FBO)__在目标Item显示
2. 继承QQuickItem，Osg先初始化，渲染成FBO，通过接口__updatePaintNode()__来更新FBO数据.

第一种方法实现起来比较优雅方便，但是不支持多pass渲染，本文主要介绍第二种。在本文用EventAdapter继承QQuickItem作为连接QtQuick与Osg的一个基类，后面只要继承这个类并注册到qml就可以了。

• __QQuickItem__继承自QObject和QQmlParserStatus，前者是Qt的基类，可以说在Qt中玩物起源于QObject，但这个类和Osg没有直接的关系，而QQmlParserStatus描述的是在Qml中的状态关系，所以从QQuickItem需要一个实体来描述这个窗体，从QQuickItem的接口看来，这个窗体的创建就是QQuickWindow，所以我们可以通过QQuickWindow将Osg创建出来的FrameBuffer绑定到QQuickWindow上，在QQuickItem的信号中，我们可以通过windowChanged()来判断窗体的产生，从而初始化OpenGL的上下文（也即初始化osg）
• 要向Osg传递鼠标和键盘之间，需要重写__QQuickItem__的鼠标接口和按键接口，这个我们在处理特定的鼠标或者按键事件的原理是一样的
• 在本文中将fbo渲染到textureNode中，所以在重载接口updatePaintNode()需要判断old是否更新，如果更新，将器删除。
• 如果窗口大小发生了改变，要在geometryChanged()去更新fbo
• 利用QQuickWindow在每一帧渲染前(beforeRendering())，渲染每一帧的fbo

最后看代码

#pragma once

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#include 
#include 

#include 
#include 
#include 
#include 

class  EventAdapter : public QQuickItem
{
    Q_OBJECT
public:
    explicit EventAdapter(QQuickItem *parent = nullptr);
    osgViewer::Viewer* getViewer();
public slots:

protected:
    void geometryChanged(const QRectF &newGeometry, const QRectF &oldGeometry);
    void mousePressEvent(QMouseEvent *event);
    void mouseMoveEvent(QMouseEvent *event);
    void mouseReleaseEvent(QMouseEvent *event);
    void wheelEvent(QWheelEvent *event);
    void keyPressEvent(QKeyEvent *event);
    QSGNode* updatePaintNode(QSGNode *oldNode,
                             UpdatePaintNodeData *updatePaintNodeData);

public:
    inline int mouseButton(QMouseEvent *event)
    {
        int button = 0;
        switch (event->button())
        {
            case Qt::LeftButton: button = 1; break;
            case Qt::MidButton: button = 2; break;
            case Qt::RightButton: button = 3; break;
            case Qt::NoButton: button = 0; break;
            default: button = 0; break;
        }
        return button;
    }

public:
    osg::ref_ptr<osgViewer::Viewer> viewer;
    QOpenGLFramebufferObject *fbo;
    QSGTexture *texture;
    QSGSimpleTextureNode *textureNode;
public slots:
    void updateViewport();
private slots:
    void onWindowChanged(QQuickWindow *window);
    void frame();

private:
    void initOSG();
    void initFBO();
    void updateFBO();
};

#include "EventAdapter.h"
#include

EventAdapter::EventAdapter(QQuickItem *parent) :
    QQuickItem(parent),
    fbo(nullptr),
    texture(nullptr),
    textureNode(nullptr)
{
    initOSG();
    connect(this, SIGNAL(windowChanged(QQuickWindow*)),
            this, SLOT(onWindowChanged(QQuickWindow*)));

    setAcceptHoverEvents(true);
    setAcceptedMouseButtons(Qt::AllButtons);
}

void EventAdapter::geometryChanged(const QRectF &newGeometry, const QRectF &oldGeometry)
{
    if (window())
    {
        updateViewport();
    }

    QQuickItem::geometryChanged(newGeometry, oldGeometry);
}

void EventAdapter::mousePressEvent(QMouseEvent *event)
{
    int button = mouseButton(event);
    viewer->getEventQueue()->mouseButtonPress(event->x(), event->y(), button);
}

void EventAdapter::mouseMoveEvent(QMouseEvent *event)
{
    viewer->getEventQueue()->mouseMotion(event->x(), event->y());
}

void EventAdapter::mouseReleaseEvent(QMouseEvent *event)
{
    int button = mouseButton(event);
    viewer->getEventQueue()->mouseButtonRelease(event->x(), event->y(), button);
}

void EventAdapter::keyPressEvent(QKeyEvent *event)
{
    viewer->getEventQueue()->keyPress(static_cast<int>(*(event->text().toLatin1().data())));
}

void EventAdapter::wheelEvent(QWheelEvent *event)
{
    if (event->delta() > 0)
        viewer->getEventQueue()->mouseScroll(osgGA::GUIEventAdapter::SCROLL_UP);
    else
        viewer->getEventQueue()->mouseScroll(osgGA::GUIEventAdapter::SCROLL_DOWN);

}

QSGNode *EventAdapter::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData *updatePaintNodeData)
{
    if (oldNode && oldNode != textureNode) {
        delete oldNode;
    }
    Q_UNUSED(updatePaintNodeData)
    return textureNode;
}

void EventAdapter::updateViewport()
{
    if (!this->window())
        return;

    QSize size(this->boundingRect().size().toSize());
    viewer->getCamera()->getGraphicsContext()->resizedImplementation(0, 0, size.width(), size.height());
    osgGA::GUIEventAdapter *ea = viewer->getEventQueue()->getCurrentEventState();
    ea->setXmin(0);
    ea->setXmax(size.width());
    ea->setYmin(0);
    ea->setYmax(size.height());
    viewer->getCamera()->setViewport(0, 0, size.width(), size.height());
    viewer->getCamera()->setProjectionMatrixAsPerspective(30.0f, static_cast<double>(size.width()) / static_cast<double>(size.height()), 0.1f, 10000.0f);
    // 				view->getCamera()->getGraphicsContext()->getState()->setUseModelViewAndProjectionUniforms(true);
    // 				view->getCamera()->getGraphicsContext()->getState()->setUseVertexAttributeAliasing(true);
    if (texture && texture->textureSize() != size) {
        updateFBO();
    }

}


void EventAdapter::initOSG() {
    viewer = new osgViewer::Viewer();
    viewer->addEventHandler( new osgGA::StateSetManipulator(viewer->getCamera()->getOrCreateStateSet()) );
    viewer->addEventHandler(new osgViewer::StatsHandler);
    //viewer->addEventHandler(new osgViewer::WindowSizeHandler);
    //viewer->getEventQueue()->getCurrentEventState()
}

void EventAdapter::initFBO() {
    QRectF rect = this->boundingRect();
    QOpenGLFramebufferObjectFormat format;
    format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
    QSize size(rect.size().toSize());
    fbo = new QOpenGLFramebufferObject(size, format);
    texture = this->window()->createTextureFromId(fbo->texture(), size);
    textureNode = new QSGSimpleTextureNode();
    textureNode->setRect(0, this->height(), this->width(), -height());
    textureNode->setTexture(texture);
    this->setFlag(QQuickItem::ItemHasContents, true);
    updateViewport();
    this->update();
}

void EventAdapter::updateFBO() {
    if (fbo)
        delete fbo;
    QRectF rect = this->mapRectToItem(nullptr, boundingRect());
    QOpenGLFramebufferObjectFormat format;
    format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
    QSize size(rect.size().toSize());
    fbo = new QOpenGLFramebufferObject(size, format);
    if (texture)
        delete texture;
    texture = window()->createTextureFromId(fbo->texture(), size);
    textureNode = new QSGSimpleTextureNode();
    textureNode->setRect(0, height(), width(), -this->height());
    textureNode->setTexture(texture);
    this->update();
}

void EventAdapter::onWindowChanged(QQuickWindow *window)
{
    if (!window)
    {
        return;
    }
    //std::cout << "onWindowChanged" << std::endl;
    connect(window, SIGNAL(beforeRendering()),
            this, SLOT(frame()), Qt::DirectConnection);
    window->setClearBeforeRendering(false);

    osg::ref_ptr<osgViewer::GraphicsWindowEmbedded> graphicsWindow
        = new osgViewer::GraphicsWindowEmbedded(0, 0, window->width(), window->height());
    graphicsWindow->setClearColor(osg::Vec4(1.0f, 1.0f, 0.0f, 1.0f));
    viewer->getCamera()->setGraphicsContext(graphicsWindow);
    updateViewport();
    connect(window, SIGNAL(widthChanged(int)),
            this, SLOT(updateViewport()));
    connect(window, SIGNAL(heightChanged(int)),
            this, SLOT(updateViewport()));
}

void EventAdapter::frame()
{
    window()->update();
    if(!fbo)
        initFBO();
    if(fbo)
    {
        fbo->bind();
    }
    QOpenGLContext::currentContext()->functions()->glUseProgram(0);
    viewer->frame();
}

osgViewer::Viewer* EventAdapter::getViewer()
{
    return viewer.get();
}

注：

• 由于Qml默认QSG_RENDER_LOOP是threaded，所以需要在环境变量设置QSG_RENDER_LOOP为basic或者window，要不然会有崩溃现象，关于QtQuick的渲染机制，具体可以参考QtQuick基础教程（四）—场景渲染(Scene Graph)
  

• 需要将EventAdapter或者其子类注册到Qml中

qmlRegisterType<SceneItem>("Hohai.Controls", 1, 0, "EventAdapter");