One more solution, Now this draws so many lines.. I'm trying and let you know if I get it...
Here is the program...

Qt Code: 
  1. void setRasterOpTest::mousePressEvent( QMouseEvent *e ){  
  2. 	if (e->button() == Qt::LeftButton) {
  3. 		m_firstpt = e->pos();
  4. 		m_lastpt = e->pos();		
  5.      }
  6. }
  7.  
  8. void setRasterOpTest::mouseMoveEvent( QMouseEvent *e ){  
  9. 	if ((e->buttons() & Qt::LeftButton) ){
  10. 		m_lastpt = e->pos();		
  11. 		update();
  12. 	}
  13. }
  14.  
  15. void setRasterOpTest::mouseReleaseEvent(QMouseEvent *e){
  16. 	if (e->button() == Qt::LeftButton ) {
  17. 		m_lastpt = e->pos();     		
  18. 		update();
  19.      }
  20. }
  21.  
  22. void setRasterOpTest::paintEvent(QPaintEvent *e){  
  23. 	 QPainter painter(this);
  24. 	 painter.setPen( Qt::black );	 	 	 
  25. 	 m_vectorLine << QLine( m_firstpt, m_lastpt );
  26. 	 painter.drawLines( m_vectorLine );	 
  27. }
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If you get it before please let me know...
Thanks