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OpenGL gl engine and QT widget

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I use a opengl engine and i would integrate it in a qt widget.
The engine have an infinite loop for drawing items in a scene.
I think to use an opengl qt widget for visualize some 3d data.
My problem is that all the drawing is done in this infinite loop , how i can insert all in the onpaint function of the opengl widget if there are two infinite loop ?The on paint of the qt widget and the ondraw of the engine?

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Do not use a loop for updating the GL widget. Use a timer, and have the timer call updateGL on you widget once every time the timer hits....

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OK, I understand that to elegantly use QT with OpenGL I'd have to connect QTTimer to my draw() function. That's logical.
But what can I do to call lotsOfMath() as often as possible (that is: to call it as soon as previous one has just ended).

The thing is I would like to write a simulation in which some math stuff and collision detection will take place. And it would be nice if I could call sim() "continuously" and call draw() every 50th frame for example.

When using SDL it is quite straightforward, but with QT?...

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I'll try it, but that would require to have something like:

if (!running) {

//my stuff

Hmmm... yes, it certainly is an idea to try. Thanks.

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Original post by Genkaku
I'll try it, but that would require to have something like:

if (!running) {

//my stuff

Hmmm... yes, it certainly is an idea to try. Thanks.

I can't follow you. Why would you need that?

If you want to pause your application, then you can simply stop the timer, until it continues, if you meant that...

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I'm still unsure what you mean. Are you worried about calls from different threads? If so:
QTimer calls all slots on the thread the timer is started. Therefore if you start the QTimer in the main thread, your slots will only be called in the context of the main thread. There will be no concurrent calls.

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for example to integrate Ogre Engine with Qt you have to do someting like this:

switch off qt drawing

QPaintEngine* paintEngine() const { return NULL; }

and overide paintEvent

void GLWidget::paintEvent(QPaintEvent *e)


so that is probably what you want to achieve with your engine ;)
But i don' know how they did this, and I would like to find out as well, someone can help? :)

Best Regards

sorry for my english

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Hi there,

I believe what you are looking for is the class QGLWidget. It is a wrapper for openGL, and the reference documentation is here: [http://doc.trolltech.com/4.3/qglwidget.html#paintGL]

paintGL() is basically where you want to put your drawing code. It is called from drawGL() by the base class behind the scenes. It draws sufficiently fast enough for most applications

Now, if you want to have a separate math update function that updates as fast as possible, then you may look at creating a timer like this:

// Create a derived class of QGLWidget.
class MyWidget : public QGLWidget
public slots:
void updateMath();
// redefine all the virtual base class functions here if you need

QTimer* timer;

timer = new QTimer(parent);
connect(timer, SIGNAL(timeout()), this, SLOT(updateMath()));
timer->start(); // This is intentionally without params to update as fast as possible
{delete timer;}

I hope this helps!

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SiS-Shadowman, yes, those were exactly my fears. Thank you for erradicating them ;)

I have a GLWidget class that inherits from QGLWidget and I implemented
void initializeGL();
void paintGL();
void resizeGL(int width, int height);

Now I can see my 'drawings'.

In the constructor I have
QTimer *timerUpdate = new QTimer(this);
connect(timerUpdate, SIGNAL(timeout()), this, SLOT(updateSim()));
that does the math.

But new problem is upon me... All I see is at most a series of still images, not a motion.
That is I see one frame and the widget updates only when I do something to the main window, like resizing, or using menu bars. That's awkward :> Any ideas?...

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