QGLWidget doesn't update

[qtbnl]

Hi,

I modified the example project 2dpainting coming with QT. If I send the timeOut signal directly to the slot animate() in GLWidget class, everything works fine. However, if I send the timeOut signal to another slot calc() in main window, do something in calc(), then emit a new signal newData() to animate(), the program got crashed.

Could you please take a look at the code? I attached it to this post.

Thanks in advance for the help.

QtBNL

[franz]

Your GLWidget::animate() function assumes that sender() is a QTimer. You might consider changing that.

On a side note, this was fairly easy to figure out using a debugger. QtCreator can use one as well.

[qtbnl]

Franz,

Thanks for your reply. I am kind of newBie to Qt and appreciate your help if you explain more for me.

Basically, there are three "connect" statements in the constructor of Window class. If I use the first one only (that's the case of emitting timeOut signal directly to animate), everything works.

If I comment out the first statements and use the next two statements, I am emitting timeOut to calc, and from there emitting newData to animate. In this case, the program crashes.

Could you please tell me what's wrong?

Thanks,
QtBNL

[qtbnl]

Dear All,
I am still waiting for somebody to help me with this issue. As a newBie to Qt, I and don't know too much about the internal of Qt and completely lost my way at this point.

I appreciate your help in advance.

QtBNL

[wysota]

What's wrong with the reply you were given?

[qtbnl]

wysota,

Franz's first reply didn't solve the problem I have. Franz said "Your GLWidget::animate() function assumes that sender() is a QTimer". Actually that's what I need if I want to send timeOut signal directly to animate. For the other case that I want timeOut signal to trigger a data processing function, e.g. calc() and then emit newData signal to animate after the data processing is done, I have to comment out that statement and enable the other two as shown below. In this case, my program crashes.

// connect(timer, SIGNAL(timeout()), openGL, SLOT(animate()));
connect(timer, SIGNAL(timeout()), this, SLOT(calc()));
connect(this, SIGNAL(newData()), openGL, SLOT(animate()));

For some reasons, my attachment is corrupted. I am attaching it again.

Thanks,
QtBNL

[wysota]

I don't see your point. The answer you were given is 100% precise. If you reemit the signal from a different class than QTimer then qobject_cast will return 0 and your modified application will crash.

[qtbnl]

wysota,

Let's see the following code

case 1.

connect(timer, SIGNAL(timeout()), openGL, SLOT(animate()));
//connect(timer, SIGNAL(timeout()), this, SLOT(calc()));
//connect(this, SIGNAL(newData()), openGL, SLOT(animate()));

case 2.

// connect(timer, SIGNAL(timeout()), openGL, SLOT(animate()));
connect(timer, SIGNAL(timeout()), this, SLOT(calc()));
connect(this, SIGNAL(newData()), openGL, SLOT(animate()));

with calc() as

void Window::calc()
{
{
do somthing here for data processing;
}

emit newData();//data);
}


What's the difference between 1 and 2? Why does 2 cause crash?

Thanks,
QtBNL

[stampede]

I guess more people will be able to help if you post your code for 'animate' and 'calc' methods.
Previous answer from wysota explains everything. Look, if you do this:

void Window::calc()
{
//...
emit newData();
}
then "sender()" in 'animate()' is not a QTimer anymore, its now an instance of Window, so qobject_cast<QTimer*>(sender()) will return 0.

[qtbnl]

I am attached my code.

window.cpp

Window::Window()
: QWidget()
{
GLWidget *openGL = new GLWidget(&helper, this);

QTimer *timer = new QTimer(this);
// connect(timer, SIGNAL(timeout()), openGL, SLOT(animate()));
connect(timer, SIGNAL(timeout()), this, SLOT(calc()));
connect(this, SIGNAL(newData()), openGL, SLOT(animate()));
timer->start(250);

ellipse = 0.0;

setWindowTitle(tr("2D Painting on Native and OpenGL Widgets"));
}
//! [0]

void Window::calc()
{
// ...
emit newData();
}



GLWidget::GLWidget(Helper *helper, QWidget *parent)
: QGLWidget(QGLFormat(QGL::SampleBuffers), parent)
{
elapsed = 0;
setFixedSize(600, 600);
setAutoFillBackground(false);
}

void GLWidget::animate()
{
elapsed = (elapsed + qobject_cast<QTimer*>(sender())->interval()) % 1000;
repaint();
}

void GLWidget::paintEvent(QPaintEvent *event)
{
QImage image(100, 100, QImage::Format_RGB16);
for (int i = 0; i < 100; i++) {
for (int j = 0; j < 100; j++) {
image.setPixel(i,j,rand());//data[i][j]);
// qDebug() << image.pixel(i,j);
}
}
QPainter painter;
painter.begin(this);
painter.setRenderHint(QPainter::Antialiasing);
painter.setViewport(0,0,600,600);
painter.scale(6,6);
painter.fillRect(event->rect(), QBrush(QColor(64, 32, 64)));
painter.drawImage(0,0,image);
painter.end();

}

Thanks in advance for the help.
QtBNL

[totem]

Has said at least twice, in void GLWidget::animate() qobject_cast<QTimer*>(sender()) will probably return NULL pointer

[wysota]

I'm in a good mood, let's analyze the code together.

I'm skipping irrelevant parts of the code.


Window::Window()
: QWidget()
{
GLWidget *openGL = new GLWidget(&helper, this);

QTimer *timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(calc()));
connect(this, SIGNAL(newData()), openGL, SLOT(animate()));
timer->start(250);
}

The above code makes two connections which are quite obvious so let's not explain them.


void Window::calc()
{
// ...
emit newData();//data);
}

The calc() routine emits "newData()" signal on behalf of an instance of a Window class.


void GLWidget::animate()
{
elapsed = (elapsed + qobject_cast<QTimer*>(sender())->interval()) % 1000;
repaint();
}

Now the hard part, let's go step by step. The animate() routine says


qobject_cast<QTimer*>(sender())
"if sender() is a QTimer() then return a pointer to it cast to QTimer, otherwise return 0".


elapsed = (elapsed + qobject_cast<QTimer*>(sender())->interval()) % 1000;
"Do some math asking for QTimer::interval() as part of the calculation".

Since sender() is Window and not QTimer, qobject_cast returns 0 which effectively transforms the last expression to:

elapsed = (elapsed + null->interval()) % 1000;
Since null is not QTimer, then asking for its interval() member fails and causes a crash.

Do you understand what the problem is?

[qtbnl]

Wysota,

I appreciate your help with such detailed explanation. You're a nice person.

QtBNL