QPainter problem - migration from qt3 [SOLVED]

[tpomorsk]

Hi,
I had the following code in qt3 and it worked perfectly (I removed resize and refresh procedures)
whiteboard.h

class Whiteboard : public QFrame
{
   Q_OBJECT
public:
   QPoint qt_currMosePos;
   QPoint qt_prevMousePos;
   QPen *pen;
   QPainter * paint;
 
   Whiteboard( QWidget *parent=0, const char* name = 0);
   ~Whiteboard()
 
protected:
   void mouseMoveEvent(QMouseEvent *);
   void mousePressEvent(QMouseEvent *);
};

class Whiteboard : public QFrame
{
   Q_OBJECT
public:
   QPoint qt_currMosePos;
   QPoint qt_prevMousePos;
   QPen *pen;
   QPainter * paint;
   
   Whiteboard( QWidget *parent=0, const char* name = 0);
   ~Whiteboard()

protected:
   void mouseMoveEvent(QMouseEvent *);
   void mousePressEvent(QMouseEvent *);
};

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whiteboard.cpp

#include "whiteboard.h"
 
Whiteboard::Whiteboard( QWidget *parent, const char* name) : QFrame(parent,name)
{
   paint = new QPainter(this);
   pen = new QPen(Qt::red,2);
   paint->setPen(*pen);
   this->setBackgroundMode(Qt::NoBackground);
}
Whiteboard::~Whiteboard()
{
   delete paint;
}
 
void Whiteboard::mouseMoveEvent(QMouseEvent *e)
{
   qt_currMousePos = e->pos();
   paint->drawLine(qt_prevMousePos,qt_currMosePos);
   qt_prevMousePos = qt_currMousePos;
}
 
void Whiteboard::mousePressEvent(QMouseEvent *e)
{
   if(e->button() == Qt::LeftButton) {
      qt_prevMousePos = e->pos();
      paint->drawLine(qt_prevMousePos,qt_prevMousePos);
   }
}

#include "whiteboard.h"

Whiteboard::Whiteboard( QWidget *parent, const char* name) : QFrame(parent,name)
{
   paint = new QPainter(this);
   pen = new QPen(Qt::red,2);
   paint->setPen(*pen);
   this->setBackgroundMode(Qt::NoBackground);
}
Whiteboard::~Whiteboard()
{
   delete paint;
}

void Whiteboard::mouseMoveEvent(QMouseEvent *e)
{
   qt_currMousePos = e->pos();
   paint->drawLine(qt_prevMousePos,qt_currMosePos);
   qt_prevMousePos = qt_currMousePos;
}

void Whiteboard::mousePressEvent(QMouseEvent *e)
{
   if(e->button() == Qt::LeftButton) {
      qt_prevMousePos = e->pos();
      paint->drawLine(qt_prevMousePos,qt_prevMousePos);
   }
}

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Basically it allows you to plot on the widget with your mouse.
I compiled it under Solaris and Linux.

Now I migrated to qt4. I ran qt3to4 and recompiled it. No problems.
But when I launch the program I cannot paint and I have the following message in the console:
QPainter::begin: Widget painting can only begin as a result of a paintEvent

What has happend? Are there some new limitations in qt4?
I know that migration from qt3 has been discussed since long time ago but I cannot find any mention of this problem.
Thanks for help

Tomek

[kemp]

Try to use Q3Painter. Maybe this will help.

[tpomorsk]

Thanks for reply.
It didn't help.
Even if I use Q3Frame and Q3Painter I still have the same message in the conosle and still I cannot paint.

Tomek

[high_flyer]

What has happend? Are there some new limitations in qt4?

Yes.
Well, not really limitations, but new ways to do things (and new structure)
And this:

Widget painting can only begin as a result of a paintEvent

being one of them.
read all about it here:
http://doc.trolltech.com/4.0/qt4-arthur.html

[jpn]

Painting using a QPainter can only take place in a paintEvent() or a function called by a paintEvent().

Something like this should be more appropriate for Qt4 (don't expect it to compile nor fully work, it's written on the fly):

Whiteboard::Whiteboard( QWidget *parent) : QFrame(parent)
{
   setBackgroundMode(Qt::NoBackground);
}
 
void Whiteboard::mouseMoveEvent(QMouseEvent *e)
{
   qt_currMousePos = e->pos();
   update(); // schedule a paint event
}
 
void Whiteboard::mousePressEvent(QMouseEvent *e)
{
   if(e->button() == Qt::LeftButton) {
      qt_currMousePos = e->pos();
	  qt_prevMousePos = qt_currMousePos;
	  update(); // schedule a paint event
   }
}
 
void Whiteboard::paintEvent(QPaintEvent *e)
{
	QPainter painter(this);
	painter.setPen(QPen(Qt::red, 2));
	painter.drawLine(qt_prevMousePos, qt_currMosePos);
	qt_prevMousePos = qt_currMousePos;
}

Whiteboard::Whiteboard( QWidget *parent) : QFrame(parent)
{
   setBackgroundMode(Qt::NoBackground);
}
 
void Whiteboard::mouseMoveEvent(QMouseEvent *e)
{
   qt_currMousePos = e->pos();
   update(); // schedule a paint event
}
 
void Whiteboard::mousePressEvent(QMouseEvent *e)
{
   if(e->button() == Qt::LeftButton) {
      qt_currMousePos = e->pos();
	  qt_prevMousePos = qt_currMousePos;
	  update(); // schedule a paint event
   }
}

void Whiteboard::paintEvent(QPaintEvent *e)
{
	QPainter painter(this);
	painter.setPen(QPen(Qt::red, 2));
	painter.drawLine(qt_prevMousePos, qt_currMosePos);
	qt_prevMousePos = qt_currMousePos;
}

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[tpomorsk]

High_flyer,
Thanks for the link. I read it and now I understand the reason for this change.

Jpn,
Thanks for the solution. I tried it but it's too slow. The line which is drawn is not smooth enough. It's much much worse than with qt3 where QPainter could be initialized only once (in a constructor of a widget). I think that the function begin() of QPainter takes too much time.
I tried to initialize QPainter in the constructor and run only begin() in paintEvent but it's still very slow.

Whiteboard::Whiteboard( QWidget *parent) : QFrame(parent)
{
   setBackgroundMode(Qt::NoBackground);
   paint = new QPainter();
}
 
...
 
void Whiteboard::paintEvent(QPaintEvent *e)
{
    paint->begin(this);
    paint->setPen(QPen(Qt::red, 2));
    paint->drawLine(qt_prevMousePos, qt_currMosePos);
    qt_prevMousePos = qt_currMousePos;
    paint->end();
}

Whiteboard::Whiteboard( QWidget *parent) : QFrame(parent)
{
   setBackgroundMode(Qt::NoBackground);
   paint = new QPainter();
}

...

void Whiteboard::paintEvent(QPaintEvent *e)
{
    paint->begin(this);
    paint->setPen(QPen(Qt::red, 2));
    paint->drawLine(qt_prevMousePos, qt_currMosePos);
    qt_prevMousePos = qt_currMousePos;
    paint->end();
}

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Is there another way to optimize it or do I have to play with QPaintEngine?

Tomek

[tpomorsk]

Problem solved.
In fact paintEvent() was slow because of double-buffering, which is turned on by default in Qt4. All I had to do is to turn it off.
I used JPN's code and added the following line in the constructor of my main class (whiteboard)

this->setAttribute(Qt::WA_PaintOnScreen);

this->setAttribute(Qt::WA_PaintOnScreen);

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After that there is no difference in performance between qt4 and qt3.


Thanks for help,
Tomek

[high_flyer]

In fact paintEvent() was slow because of double-buffering, which is turned on by default in Qt4.

Thanks for pointing that out.
Another handy point to check in the future, if painting is slow.