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