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    void MyWidget::paintEvent(QPaintEvent *evt)
    {
        // any initialization
        QPainter painter(this);
        QRegion unpainted(evt->clipRegion());
        static QPixmap *doubleBuffer = 0;
        if (!doubleBuffer)
            doubleBuffer = new QPixmap;
        QPainter dbPainter(doubleBuffer);

	for (int i = 0; i < lines.count(); i++) {
            Line *line = lines[i];
            doubleBuffer->resize(QMAX(doubleBuffer->width(),
 line->boundingRect().width()),
				 QMAX(doubleBuffer->height(), line->boundingRect().height()));
            doubleBuffer->fill(backgroundColor);
            line->draw(&dbPainter);
            painter.drawPixmap(0, line->boundingRect().y(), *doubleBuffer, 0,
 0,
                               line->boundingRect().width(),
 line->boundingRect().height());
            unpainted -= line->boundingRect();
        }
        painter.setClipRegion(unpainted);
        painter.fillRect(unpainted.boundingRect(), backgroundColor);
    }