QtQuick中集成OSG渲染

從目前網上介紹OSG與Qml集成有兩種方式：

在[QtQuick2OSGItem](https://bitbucket.org/leon_manukyan/qtquick2osgitem/src/master/sampleapp/)通過繼承抽象類__QQuickFramebufferObject::Renderer__來重寫渲染接口，在__QQuickFramebufferObject__渲染成__Frame Buffer Object(FBO)__在目標Item顯示
繼承QQuickItem，Osg先初始化，渲染成FBO，通過接口__updatePaintNode()__來更新FBO數據.

第一種方法實現起來比較優雅方便，但是不支持多pass渲染，本文主要介紹第二種。在本文用EventAdapter繼承QQuickItem作爲連接QtQuick與Osg的一個基類，後面只要繼承這個類並註冊到qml就可以了。

__QQuickItem__繼承自QObject和QQmlParserStatus，前者是Qt的基類，可以說在Qt中玩物起源於QObject，但這個類和Osg沒有直接的關係，而QQmlParserStatus描述的是在Qml中的狀態關係，所以從QQuickItem需要一個實體來描述這個窗體，從QQuickItem的接口看來，這個窗體的創建就是QQuickWindow，所以我們可以通過QQuickWindow將Osg創建出來的FrameBuffer綁定到QQuickWindow上，在QQuickItem的信號中，我們可以通過windowChanged()來判斷窗體的產生，從而初始化OpenGL的上下文（也即初始化osg）
要向Osg傳遞鼠標和鍵盤之間，需要重寫__QQuickItem__的鼠標接口和按鍵接口，這個我們在處理特定的鼠標或者按鍵事件的原理是一樣的
在本文中將fbo渲染到textureNode中，所以在重載接口updatePaintNode()需要判斷old是否更新，如果更新，將器刪除。
如果窗口大小發生了改變，要在geometryChanged()去更新fbo
利用QQuickWindow在每一幀渲染前(beforeRendering())，渲染每一幀的fbo

最後看代碼

#pragma once

#include <QtQuick/QQuickItem>
#include <QtQuick/QQuickWindow>
#include <QtGui/QOpenGLFramebufferObject>
#include <QtQuick/QSGSimpleTextureNode>
#include <QtCore/QDebug>
#include <QtCore/QObject>
#include <QtGui/QOpenGLFramebufferObject>
#include <QtQuick/QSGSimpleTextureNode>
#include <QtQuick/QQuickWindow>
#include <QtQuick/QQuickItem>

#include <QtGui/QOpenGLContext>
#include <QtGui/QOpenGLFunctions>

#include <osgViewer/CompositeViewer>
#include <osgViewer/ViewerEventHandlers>
#include <osgGA/TrackballManipulator>
#include <osgDB/ReadFile>

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<osgGA/StateSetManipulator>

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：

由於Qml默認QSG_RENDER_LOOP是threaded，所以需要在環境變量設置QSG_RENDER_LOOP爲basic或者window，要不然會有崩潰現象，關於QtQuick的渲染機制，具體可以參考QtQuick基礎教程（四）—場景渲染(Scene Graph)
需要將EventAdapter或者其子類註冊到Qml中

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

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