How to solve this problem about drawImage() and paintEvent()?

[Nicho]

Hi, all
I am working under linux with QT4.7, and my test code is like below:

12 void MyWidget :: paintEvent ( QPaintEvent * event )
 13 {
 14     init();
 15 }
 16
 17 void MyWidget :: init()
 18 {
 19     QImage bg( width(), height(), QImage::Format_ARGB32_Premultiplied );
 20     QImage a( width(), height(), QImage::Format_ARGB32_Premultiplied );
 21     QImage b( width(), height(), QImage::Format_ARGB32_Premultiplied );
 22
 23     QPainter p;
 24
 25     p.begin( &bg );
 26     QRect r = rect();
 27     p.fillRect( r, Qt::white );
 28     p.end();
 29
 30     p.begin( &a );
 31     p.setPen( QPen( QColor( Qt::red ), 5 ) );
 32     p.drawLine( 10, 10, 50, 50 );
 33     p.end();
 34
 35     p.begin( &b );
 36     p.setPen( QPen( QColor( Qt::green ), 5 ) );
 37     p.drawLine( 10, 70, 50, 10 );
 38     p.end();
 39
 40     p.begin( this );
 41     p.drawImage( QPoint(0,0), bg );
 42     p.drawImage( QPoint(0,0), a );
 43     p.drawImage( QPoint(0,0), b );
 44     p.end();
 45 }

12 void MyWidget :: paintEvent ( QPaintEvent * event )
 13 {
 14     init();
 15 }
 16
 17 void MyWidget :: init()
 18 {
 19     QImage bg( width(), height(), QImage::Format_ARGB32_Premultiplied );
 20     QImage a( width(), height(), QImage::Format_ARGB32_Premultiplied );
 21     QImage b( width(), height(), QImage::Format_ARGB32_Premultiplied );
 22
 23     QPainter p;
 24
 25     p.begin( &bg );
 26     QRect r = rect();
 27     p.fillRect( r, Qt::white );
 28     p.end();
 29
 30     p.begin( &a );
 31     p.setPen( QPen( QColor( Qt::red ), 5 ) );
 32     p.drawLine( 10, 10, 50, 50 );
 33     p.end();
 34
 35     p.begin( &b );
 36     p.setPen( QPen( QColor( Qt::green ), 5 ) );
 37     p.drawLine( 10, 70, 50, 10 );
 38     p.end();
 39
 40     p.begin( this );
 41     p.drawImage( QPoint(0,0), bg );
 42     p.drawImage( QPoint(0,0), a );
 43     p.drawImage( QPoint(0,0), b );
 44     p.end();
 45 }

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Then, at the beginning the widget is shown correctly - there are two lines cross with each other at the Top-Left corner, one is green and another is red. But when I drag the Bottom-Right corner of this widget to resize it, the two lines will be repainted in a wrong way. Why and how to solve it?

Any suggestion is appreciated.

Nicho