void Widget::paintEvent(QPaintEvent *event)
{
    QPainter painter(this);
    painter.setPen(QPen(Qt::red,2));          //设置画笔
    painter.translate( rect().center() );     //将坐标原点移动到屏幕中心
    painter.drawLine(QPoint(100, 0), QPoint(120, 0));   //画线
    painter.drawEllipse(-100,-100,200,200);   //绘制椭圆
    for(int i = 1;i<=36;i++)
    {
        painter.rotate(10);  //旋转坐标系
        if(i%3 == 0)
        {
            painter.drawLine(QPoint(100, 0), QPoint(120, 0)); //画线
        }
        else
        {
            painter.drawLine(QPoint(100, 0), QPoint(110, 0)); //画线
        }
    }
}