Issues with QGLWidget as QMainWindow central widget

[wysota]

This code doesn't make sense:

QSizePolicy sizeHint() {
        return QSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
    }

QSizePolicy sizeHint() {
        return QSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
    }

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I don't know what you are trying to obtain with this, but sizeHint() returns QSize and not QSizePolicy and it's a const method too.

See if it helps if you call update() at the end of resizeGL().

[Zyl]

I don't know what you are trying to obtain with this, but sizeHint() returns QSize and not QSizePolicy and it's a const method too.

Oops! Thanks. That explains a few things. I changed the public methods to this:

public:
    explicit ShardIRCClient(QWidget *parent = NULL);
 
    QSize sizeHint() const {
        return QSize(800, 600);
    }
 
    QSize minimumSizeHint() const {
        return QSize(640, 480);
    }

public:
    explicit ShardIRCClient(QWidget *parent = NULL);

    QSize sizeHint() const {
        return QSize(800, 600);
    }

    QSize minimumSizeHint() const {
        return QSize(640, 480);
    }

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The window can now not be made smaller than a size to allow a 640*480 widget to fit, so that works properly. (QSizeHint::Preferred)

See if it helps if you call update() at the end of resizeGL().

Assuming you meant updateGL(): Yes and no. The widget now redraws when resizing it larger. It won't react if resized smaller and its overall performance is still oddly poor.

[wysota]

Assuming you meant updateGL(): Yes and no.

update() calls updateGL() so, no, I meant update()

The widget now redraws when resizing it larger. It won't react if resized smaller and its overall performance is still oddly poor.

Please provide a minimal compilable example reproducing the problem.

[Zyl]

This is the most minimal example I could make: Link
Hope that shows what I mean.

[wysota]

I tested your app on Linux and I can't see any unexpected behaviour. When I resize the window (up or down), paintGL gets called.

As for your note regarding updateGL()... Here is what paintEvent for QGLWidget looks like (which is what gets called upon calling update()):

void QGLWidget::paintEvent(QPaintEvent *)
{
    if (updatesEnabled()) {
        glDraw();
        updateOverlayGL();
    }
}

void QGLWidget::paintEvent(QPaintEvent *)
{
    if (updatesEnabled()) {
        glDraw();
        updateOverlayGL();
    }
}

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and this is updateGL():

void QGLWidget::updateGL()
{ 
    if (updatesEnabled())
        glDraw();
}

void QGLWidget::updateGL()
{ 
    if (updatesEnabled())
        glDraw();
}

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[Zyl]

Strange. Using update() I get not additional paintGL()-calls, while with using updateGL() I do.
Also I already do get automatic calls to paintGL() when resizing the window, without needing to set a call to update() nor updateGL(). However, although paintGL() is definitely called, there is no change visible unless I stop moving the mouse. Only an additional updateGL() call in resizeGL() causes a visible change, BUT ONLY when resizing the window larger. It really feels as if something's running on the wrong thread, and I don't see where I'd ever have to do with that. (Win7 x64 btw)

[wysota]

Unless you are using threads yourself, there is no threading involved. What you observe might be somehow caused by your system settings.

[Zyl]

I know of no system setting regarding the rendering mechanism of OpenGL in a window.

If I remove this line from MainWindow::MainWindow

this->setCentralWidget(widget);

this->setCentralWidget(widget);

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the problem no longer occurs. However, no layouting is performed and the widget keeps its initial size.

Doing layouting results in the whole problem returning however:

#include <QHBoxLayout>
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QWidget* center = new QWidget(this);
    QGLWidget* widget = new MyGLWidget(center);
    QHBoxLayout* layout = new QHBoxLayout(center);
    layout->setMargin(0);
    center->setLayout(layout);
    center->layout()->addWidget(widget);
    this->setCentralWidget(center);
}

#include <QHBoxLayout>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QWidget* center = new QWidget(this);
    QGLWidget* widget = new MyGLWidget(center);
    QHBoxLayout* layout = new QHBoxLayout(center);
    layout->setMargin(0);
    center->setLayout(layout);
    center->layout()->addWidget(widget);
    this->setCentralWidget(center);
}

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[wysota]

I didn't say the system setting has to be related to rendering of OpenGL in a window. You might have some system tweak installed that causes such behaviour. If what you observe was a common problem, it would have been identified and resolved a long time ago since it's quite common to use OpenGL child windows. Does it change anything if you call center->winId() (you can ignore the result) as part of your last code snippet?

[Zyl]

I don't think I have any outrageous system tweaks installed. The hackiest thing on this machine is probably Dropbox with its random folder scans.
I tried adding center->winId() at various lines (and even multiple times), but there was no change.

It might be worth working with the hint that just doing

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QGLWidget* widget = new MyGLWidget(this);
    widget->setGeometry(0, 0, 800, 600);
}

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QGLWidget* widget = new MyGLWidget(this);
    widget->setGeometry(0, 0, 800, 600);
}

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presents me the widget with proper performance. I can scale the window larger and as long as new parts of the 800*600 rectangle area of the widget are revealed paintGL() gets called and I see the redrawn content immediately, at what appears to be 60Hz - my screen's refresh rate; exactly what I want and one should expect. I can't help but think the problem to lie somewhere in the Qt layout classes.

[wysota]

I don't think I have any outrageous system tweaks installed. The hackiest thing on this machine is probably Dropbox with its random folder scans.

You'd be suprised how things tend to influence stuff logically unrelated to what the things are meant to do, especially when it comes to Windows.

I tried adding center->winId() at various lines (and even multiple times), but there was no change.

It was only needed once and in theory it shouldn't be needed at all

I can't help but think the problem to lie somewhere in the Qt layout classes.

Unlikely, especially since I've been using Qt+OpenGL+layouts and didn't experience any slowdowns.

As an experiment, try this:

class Widget : public QWidget {
public:
  Widget(QWidget *parent = 0) : QWidget(parent) {
    w = new MyGLWidget(this);
  }
protected:
  void resizeEvent(QResizeEvent *re) {
    w->setGeometry(0,0, width(), height());
  }
  MyGLWidget *w;
};
 
int main(...) {
  ...
  Widget w;
  w.show();
  ...
}

class Widget : public QWidget {
public:
  Widget(QWidget *parent = 0) : QWidget(parent) {
    w = new MyGLWidget(this);
  }
protected:
  void resizeEvent(QResizeEvent *re) {
    w->setGeometry(0,0, width(), height());
  }
  MyGLWidget *w;
};

int main(...) {
  ...
  Widget w;
  w.show();
  ...
}

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No layouts, same GL widget code. Is there a slowdown? If yes, what if you replace MyGLWidget with QGLWidget?

[Zyl]

Same slow behaviour with MyGLWidget as well as with QGLWidget.
If this goes on I'll try making a QThread spamming sizeInfo signals about the window size and catch them in a MyGLWidget slot... >_>
EDIT: That actually works, but it acts equally crappy. (Yes I did just try it)