Qt畫箭頭

下面是畫箭頭的程式碼：

void drawArrow(QPoint startPoint, QPoint endPoint, QPainter &p)
{
    double par = 15.0;//箭頭部分三角形的腰長
    double slopy = atan2((endPoint.y() - startPoint.y()), (endPoint.x() - startPoint.x()));
    double cosy = cos(slopy);
    double siny = sin(slopy);
    QPoint point1 = QPoint(endPoint.x() + int(-par*cosy - (par / 2.0*siny)), endPoint.y() + int(-par*siny + (par / 2.0*cosy)));
    QPoint point2 = QPoint(endPoint.x() + int(-par*cosy + (par / 2.0*siny)), endPoint.y() - int(par / 2.0*cosy + par*siny));
    QPoint points[3] = { endPoint, point1, point2 };
    p.setRenderHint(QPainter::Antialiasing, true);//消鋸齒
    QPen drawTrianglePen;//建立畫筆
    drawTrianglePen.setColor(Qt::red);
    drawTrianglePen.setWidth(1);
    p.setPen(drawTrianglePen);
    p.drawPolygon(points, 3);//繪製箭頭部分
    int offsetX = int(par*siny / 3);
    int offsetY = int(par*cosy / 3);
    QPoint point3, point4;
    point3 = QPoint(endPoint.x() + int(-par*cosy - (par / 2.0*siny)) + offsetX, endPoint.y() + int(-par*siny + (par / 2.0*cosy)) - offsetY);
    point4 = QPoint(endPoint.x() + int(-par*cosy + (par / 2.0*siny) - offsetX), endPoint.y() - int(par / 2.0*cosy + par*siny) + offsetY);
    QPoint arrowBodyPoints[3] = { startPoint, point3, point4 };
    p.drawPolygon(arrowBodyPoints, 3);//繪製箭身部分
}