Strange behaviour with QPainter

[franco.amato]

Hi to all, I can not update the position of a moving triangle in my paintEvent. It stay at the initial position.
This little triangle is used to simulate a time position so it must move indicating time that move when a sound is playing.

Some code:

// here I determine the current time position ( where I would place the triangle
void WaveWidget::setCurrentTimePosition()
{
	uint newTimeValue = getCurrentPosInSoundCoord();
 
	/* trasform the sound position in widget coordinate */
	qreal samplesPerPixel = m_wave->getSamples() / width();
	qreal x_pos = newTimeValue / samplesPerPixel;
 
	if( x_pos == m_CurrentTimePosition )
		return; //no change
 
       // I inform that time-position is changed
	emit soundPosChanged( m_CurrentTimePosition, x_pos );
 
	/* update current position */
	m_CurrentTimePosition = x_pos;
}

// here I determine the current time position ( where I would place the triangle
void WaveWidget::setCurrentTimePosition()
{
	uint newTimeValue = getCurrentPosInSoundCoord();

	/* trasform the sound position in widget coordinate */
	qreal samplesPerPixel = m_wave->getSamples() / width();
	qreal x_pos = newTimeValue / samplesPerPixel;

	if( x_pos == m_CurrentTimePosition )
		return; //no change

       // I inform that time-position is changed
	emit soundPosChanged( m_CurrentTimePosition, x_pos );

	/* update current position */
	m_CurrentTimePosition = x_pos;
}

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The code of the slot that's informed of the time changes is:

void TimeWidget::soundPosChanged( qreal currentTimePosition, qreal newTimePosition )
{
	int w = width();
	int h = height();
 
	m_CurrentTimePosition = currentTimePosition;
 
	unsigned int rectWidth = ( newTimePosition - m_CurrentTimePosition ) + 4;
	//qDebug() << rectWidth;
 
	// determine the rect to update
	//m_CurrentTimePosition is the triangle central point
	QRect r = QRect( m_CurrentTimePosition - 3, h/2 + 1,  rectWidth, h/2 );
 
	// update it
	update(r);
	// update current position
	m_CurrentTimePosition = newTimePosition;
}

void TimeWidget::soundPosChanged( qreal currentTimePosition, qreal newTimePosition )
{
	int w = width();
	int h = height();

	m_CurrentTimePosition = currentTimePosition;

	unsigned int rectWidth = ( newTimePosition - m_CurrentTimePosition ) + 4;
	//qDebug() << rectWidth;

	// determine the rect to update
	//m_CurrentTimePosition is the triangle central point
	QRect r = QRect( m_CurrentTimePosition - 3, h/2 + 1,  rectWidth, h/2 );

	// update it
	update(r);
	// update current position
	m_CurrentTimePosition = newTimePosition;
}

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Now the code of the paintEvent

void TimeWidget::paintEvent( QPaintEvent* e)
{
	int h = height();
	int w = width();
 
	QPainter p( this );
	p.setRenderHint( QPainter::Antialiasing, true );
	p.fillRect( 0, 0, w, h, Qt::lightGray );
	p.setBrush(Qt::blue);
 
	static const QPointF points[3] = 
	{
		QPointF( m_CurrentTimePosition -2, h/2),
		QPointF( m_CurrentTimePosition + 2, h/2),
		QPointF( m_CurrentTimePosition, h)
	};
 
	qDebug() << "TimeWidget::paintEvent - centralPoint:" << m_CurrentTimePosition;
 
	p.drawPolygon( points, 3 );
}

void TimeWidget::paintEvent( QPaintEvent* e)
{
	int h = height();
	int w = width();

	QPainter p( this );
	p.setRenderHint( QPainter::Antialiasing, true );
	p.fillRect( 0, 0, w, h, Qt::lightGray );
	p.setBrush(Qt::blue);

	static const QPointF points[3] = 
	{
		QPointF( m_CurrentTimePosition -2, h/2),
		QPointF( m_CurrentTimePosition + 2, h/2),
		QPointF( m_CurrentTimePosition, h)
	};

	qDebug() << "TimeWidget::paintEvent - centralPoint:" << m_CurrentTimePosition;

	p.drawPolygon( points, 3 );
}

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I don't know why the triangle doesn't move.
The centralPoint m_CurrentTimePosition is correctly updated as I can see it at shell.

I really don't know where my code is wrong.

I tried to add the following line:

QRect region = e->rect();

QRect region = e->rect();

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as first line in the paintEvent routine and I got the following compiiler error:

error C2027: use of undefined type 'QPaintEvent'

I'm really confused.

Best Regards

[wysota]

I would get rid of the slot and call update() from within the method where you change the value of your property. Calling update() with a parameter doesn't make much sense as you don't use the region in your paintEvent() anyway. As for the error - #include <QPaintEvent>

[franco.amato]

I would get rid of the slot and call update() from within the method where you change the value of your property. Calling update() with a parameter doesn't make much sense as you don't use the region in your paintEvent() anyway. As for the error - #include <QPaintEvent>

OK error solved,
but I changed update( r ) with update and nothing change.
I can not draw the triangle. It stay at position 0.

Maybe a bug of drawPolygon?

[franco.amato]

I think is a drawPolygon problem.

I tried to leave update (r) in the slot and changed

p.drawPolygon( points, 3 );

with

p.drawRect(e->rect()); only for testing and now I can see something moving in the widget so I don't know how to solve it.
And other I think is better call update (r) instead of only update() so only a little portion of widget is updated.

This sould increase speed or am I wrong?

Best

[faldzip]

Just guessing but what if you get rid of "static" before const QPointF points[3]?

[wysota]

It will magically start working


@franco.amato: Please look for errors in your own code instead of blaming others.

[franco.amato]

Just guessing but what if you get rid of "static" before const QPointF points[3]?

Yesssssssssssssssssssssssss it solved my problem.


Best

[faldzip]

So my advice is to read this carefully

[franco.amato]

So my advice is to read this carefully

I still didn't understand why the static gave that behaviour if I update at every call the member variablke

[franco.amato]

It will magically start working


@franco.amato: Please look for errors in your own code instead of blaming others.

Sorry what are you speaking about?

[faldzip]

I still didn't understand why the static gave that behaviour if I update at every call the member variablke

Here is a small example:

void foo() {
  static int counter = 0;
  cout << "foo has been called " << ++counter << " times\n";
}
 
int main() {
  for( int i = 0; i < 10; ++i ) foo();
}

void foo() {
  static int counter = 0;
  cout << "foo has been called " << ++counter << " times\n";
}
 
int main() {
  for( int i = 0; i < 10; ++i ) foo();
}

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it will display:
foo has been called 1 times
foo has been called 2 times
foo has been called 3 times
...
foo has been called 10 times

So, as it can be easily noticed, this line:

static int counter = 0;

static int counter = 0;

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is called ONCE, when this line is rached for the first time. So counter variable is created once and 0 is assigned to it once. Then it remains in memory for every foo() call until the end of app execution.
So in your code point[3] was created and initialized once on the first call of paintEvent() and remained there forever (until program ends).

[franco.amato]

Here is a small example:

void foo() {
  static int counter = 0;
  cout << "foo has been called " << ++counter << " times\n";
}
 
int main() {
  for( int i = 0; i < 10; ++i ) foo();
}

void foo() {
  static int counter = 0;
  cout << "foo has been called " << ++counter << " times\n";
}
 
int main() {
  for( int i = 0; i < 10; ++i ) foo();
}

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it will display:
foo has been called 1 times
foo has been called 2 times
foo has been called 3 times
...
foo has been called 10 times

So, as it can be easily noticed, this line:

static int counter = 0;

static int counter = 0;

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is called ONCE, when this line is rached for the first time. So counter variable is created once and 0 is assigned to it once. Then it remains in memory for every foo() call until the end of app execution.
So in your code point[3] was created and initialized once on the first call of paintEvent() and remained there forever (until program ends).

OK It's all clear.
Thank you again

[wysota]

Sorry what are you speaking about?

Don't you think that if drawPolygon() wasn't working someone would notice it like... a few years ago?