qt quick with opengl underlay

[sajis997]

Hello forum,

I am trying to draw a teapot as opengl underlay rendering and qt quick UI on top of the underlay to interact with the teapot. Since qt quick 2 uses opengl 2 for all of its rendering , i am using the compatibility profile with the modern opengl pipeline .

I am having QQuickView subclass as follows:

class Window : public QQuickView
{
    Q_OBJECT
 
public:
 
    explicit Window(QWindow *parent = 0);
 
public slots:
    void renderOpenGLScene();
 
private:
 
    QScopedPointer<AbstractScene> mScene;
    QTime mTime;
};

class Window : public QQuickView
{
    Q_OBJECT

public:

    explicit Window(QWindow *parent = 0);

public slots:
    void renderOpenGLScene();

private:

    QScopedPointer<AbstractScene> mScene;
    QTime mTime;
};

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All the opengl commands are initiated in the following class :

class TeapotTessellation : public AbstractScene, protected QOpenGLFunctions_4_3_Compatibility
{
    Q_OBJECT
 
public:
 
    explicit TeapotTessellation(QObject *parent = 0);
    ~TeapotTessellation();
 
    //scene related functions
    virtual void initialise();
    virtual void update(float t);
    virtual void render();
    virtual void resize(int w,int h);
 
private:
 
    void loadShaders();
 
    GLSLShader *mTessellationShader;
 
    TeapotVboPatch *mTeapotPatch;
 
    ....................
    ....................
};
 
I can see the teapot, but the QML scene is not behaving properly(spring animation of a rounded rectangle) and i get the following info in the cosole:
 
[code]
QSGContext::initialize: stencil buffer support missing, expect rendering errors

class TeapotTessellation : public AbstractScene, protected QOpenGLFunctions_4_3_Compatibility
{
    Q_OBJECT

public:

    explicit TeapotTessellation(QObject *parent = 0);
    ~TeapotTessellation();

    //scene related functions
    virtual void initialise();
    virtual void update(float t);
    virtual void render();
    virtual void resize(int w,int h);

private:

    void loadShaders();

    GLSLShader *mTessellationShader;

    TeapotVboPatch *mTeapotPatch;

    ....................
    ....................
};

I can see the teapot, but the QML scene is not behaving properly(spring animation of a rounded rectangle) and i get the following info in the cosole:

[code]
QSGContext::initialize: stencil buffer support missing, expect rendering errors

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The qml code is tested separately and i works fine.

Where should i look into the debug this issue ?
[/code]

[wysota]

We cannot see how you get your GL code called. You should connect to the beforeRendering() signal with a slot that performs rendering. Remember to reset the state of the GL engine to what QtQuick expects by calling resetOpenGLState().

[sajis997]

Hi

I am already connecting the signal as follows:

Window::Window(QWindow *parent)
    : QQuickView(parent),
      mScene(new TeapotTessellation(this))
{
    setClearBeforeRendering(false);
 
    QObject::connect(this,SIGNAL(beforeRendering()),SLOT(renderOpenGLScene()),Qt::DirectConnection);
 
    QTimer *timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(update()));
    timer->start();
 
    QSurfaceFormat format;
    format.setMajorVersion(4);
    format.setMinorVersion(3);
    format.setSamples(4);
    format.setProfile(QSurfaceFormat::CompatibilityProfile);
 
    setFormat(format);
}

Window::Window(QWindow *parent)
    : QQuickView(parent),
      mScene(new TeapotTessellation(this))
{
    setClearBeforeRendering(false);

    QObject::connect(this,SIGNAL(beforeRendering()),SLOT(renderOpenGLScene()),Qt::DirectConnection);

    QTimer *timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(update()));
    timer->start();

    QSurfaceFormat format;
    format.setMajorVersion(4);
    format.setMinorVersion(3);
    format.setSamples(4);
    format.setProfile(QSurfaceFormat::CompatibilityProfile);

    setFormat(format);
}

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And after every rendering commands i am unbinding the shaders and vertex array object explicitly and i am not sure where to call this resetOpenGLState(). Check also the following snippet:

void Window::update()
{
    float time = mTime.elapsed() / 1000.0f;
 
    mScene->update(time);
 
    QQuickView::update();
}
 
void Window::renderOpenGLScene()
{
    static bool firstTime = true;
 
    if(firstTime)
    {
        mScene->initialise();
        mScene->resize(width(),height());
        firstTime = false;
    }
 
    mScene->render();
 
    //SHOULD I CALL THE FUNCTION resetOpenGLState() here ?
}

void Window::update()
{
    float time = mTime.elapsed() / 1000.0f;

    mScene->update(time);

    QQuickView::update();
}

void Window::renderOpenGLScene()
{
    static bool firstTime = true;

    if(firstTime)
    {
        mScene->initialise();
        mScene->resize(width(),height());
        firstTime = false;
    }

    mScene->render();

    //SHOULD I CALL THE FUNCTION resetOpenGLState() here ?
}

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[wysota]

Yes, call the function.

And you shouldn't access any of the window's properties from the rendering thread. Better initialize that scene before you start drawing.

[sajis997]

Thanks,

I have managed to render an opengl scene under the qt quick scene. I largely followed the tutorial titled - "Scene Graph - OpenGL Under QML"


Now i want to animate the opengl scene that contains a teapot. I usually rotate the opengl scene using the Qt' s timer event and timer function as follows:

QTimer *timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(updateScene()));
    timer->start();

QTimer *timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()),this,SLOT(updateScene()));
    timer->start();

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And inside the updateScene() slot I have the following :

void GLWindow::updateScene()
{
    float t = mTime.elapsed() * 0.001;
 
    mScene->update(t);
 
    paintGL();
}

void GLWindow::updateScene()
{
    float t = mTime.elapsed() * 0.001;

    mScene->update(t);

    paintGL();
}

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I used the same mechanism while integrating qt quick with opengl underlay, but the teapot only rotates for a while with the mouse click and it stops. I want the teapot to rotate continuously untill the application closes.

Any hint to achieve this ?

Thanks

[wysota]

You have to tell QtQuick scene to update itself. It will call your GL code with the beforeRendering signal.

[sajis997]

Thanks !

Any concrete example to what you have just mentioned ?

[wysota]

QQuickWindow::update().