Question

I am currently playing with Qt trying to set up a small particle system. Therein I’ve subclassed the GLWidget and hacked away at it. Everything was going well until I made some unknown change and now the widget only repaints when I move the mouse (it should be doing it all the time due to the QTimer I have firing). Relevant code:

OpenGLWidget.h

class OpenGLWidget :  public QGLWidget  {

   Q_OBJECT

public:
   OpenGLWidget(QWidget * parent = 0);
   ~OpenGLWidget();

public slots:
   void toggleEmitter();

protected:
   void initializeGL();
   void paintGL();
   void resizeGL(int width, int height);
   QSize minimumSizeHint() const;
   QSize sizeHint() const;
   void mousePressEvent(QMouseEvent *event);
   void mouseMoveEvent(QMouseEvent *event);

protected slots:
   void timeOut();

private:

   void testFunc(Particle & p, unsigned int t);

   QTime time;
   QTimer timer;

   Emitter emitter;

Relevant code from the .cpp

// in the constructor
time.start();
connect(&timer, SIGNAL(timeout()), this, SLOT(timeOut()));
timer.start(0);

void OpenGLWidget::initializeGL()  {
   GLuint tex = 0;

   qglClearColor(bgColor);
   glEnable(GL_DEPTH_TEST);   
   glClearDepth(1.0f);
    glDepthFunc(GL_LEQUAL);
   glShadeModel(GL_SMOOTH); 

   glDisable(GL_CULL_FACE);
   glEnable(GL_TEXTURE_2D); 
   glEnable( GL_BLEND );
   glBlendFunc( GL_SRC_ALPHA, GL_ONE); // _MINUS_SRC_ALPHA );

   glPointSize(3.0f);

   tex = bindTexture(QPixmap(QString("testparticle2.png")), GL_TEXTURE_2D);
   emitter = Emitter(Vector(0, 0, 0.0f), tex, 50, fastdelegate::FastDelegate2<Particle &, unsigned int>(this, &OpenGLWidget::testFunc));
}

void OpenGLWidget::paintGL()  {
   makeCurrent();
   glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

   mainCam.SetView();

   glRotatef(xRot, 1, 0, 0);
   glRotatef(yRot, 0, 1, 0);

   emitter.Process(time.elapsed());

   totalTime += time.elapsed();
   time.restart();

}

void OpenGLWidget::resizeGL(int width, int height)  {
   contextWidth = width;
   contextHeight = height;

   glViewport(0, 0, width, height);

   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   gluPerspective(55.0f, width / (float) height, 0.01f, 100.0f);

   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();

}

void OpenGLWidget::timeOut()  {
   updateGL();
}

void OpenGLWidget::mousePressEvent(QMouseEvent *event)  {
    lastX = event->pos().x();
    lastY = event->pos().y();
}

void OpenGLWidget::mouseMoveEvent(QMouseEvent *event)  {
    int dx = event->x() - lastX;
    int dy = event->y() - lastY;

    if (event->buttons() & Qt::LeftButton) {
        xRot += 3 * dy;
        yRot += 3 * dx;
    } else if (event->buttons() & Qt::RightButton) {
        xRot += 3 * dy;
        yRot += 3 * dx;
    }

    lastX = event->pos().x();
    lastY = event->pos().y();

}

Answer 1

A QTimer with 0 second time out only fires when the event loop gets the control. I suspect this is not the case. However, when you move the mouse, the event loop processes some events (again) and then delivers the pending timer event(s). Maybe you need to invoke processEvents() in your update loop to make sure that the pending timer event from QTimer gets processed.