问在Qt中，如果提供负值参数，drawPoint方法不会绘制任何内容。

EN

Bresenham::Bresenham(QWidget*parent):QWidget(parent)  
{}

void  Bresenham::paintEvent(QPaintEvent *e)  
{  
     Q_UNUSED(e);  
     QPainter qp(this);  
     drawPixel(&qp);  
}  
void  Bresenham::drawPixel(QPainter *qp)  
{  
    QPen pen(Qt::red,2,Qt::SolidLine);  
    qp->setPen(pen);  
    int x=0,y,d,r=100;  
    y=r;  
    d=3-2*r;  
    do  
    {  
       qp->drawPoint(x,y);  
       qp->drawPoint(y,x);  
       qp->drawPoint(y,-x);  
       qp->drawPoint(x,-y);  
       qp->drawPoint(-x,-y);  
       qp->drawPoint(-y,-x);  
       qp->drawPoint(-x,y);  
       qp->drawPoint(-y,x);
       if(d<0)  
       {  
          d=d+4*x+6;  
       }  
       else  
       {  
          d=d+(4*x-4*y)+10;  
          y=y-1;  
       }  
       x=x+1;  
      }while(x<y); 
}