I have some items lines in a scene and now I would like to draw a head arrow in each line.
I have tried this with head arrows as Poligon and then add to the scene like this:

    QPolygonF head_arrow;

for (.....)
{
..................
                   head_arrow << QPoint((lines_vector[i].x_end)-min_x,max_y-(lines_vector[i].y_end));
                    head_arrow << QPoint((lines_vector[i].x_end)-min_x-0.01,max_y-lines_vector[i].y_end+0.01*tan(30*3.14/180));
                    head_arrow << QPoint((lines_vector[i].x_end)-min_x-0.01,max_y-lines_vector[i].y_end-0.01*tan(30*3.14/180));
                    head_arrow << QPoint((lines_vector[i].x_end)-min_x,max_y-(lines_vector[i].y_end));

                    scene->addPolygon(head_arrow,black);
................

It compiles fine but the head arrows does not fit like my code...
is there any other way to do that or any ideas to fit my problem?

thanks a lot.

EDIT:

I have tried with Qpaintevent as well like this:

void MainWindow::paintEvent(QPaintEvent *e)
{
    QPainter painter(ui->graphicsview);
    QPolygonF head_arrow;
    QPen linepen;
    linepen.setWidthF(1);
    linepen.setColor(Qt::red);
    painter.setPen(linepen);

    for (int i=0;i<lines_vector.size();i++)
    {
        head_arrow << QPoint((lines_vector[i].x_end)-min_x,max_y-(lines_vector[i].y_end));
        head_arrow << QPoint((lines_vector[i].x_end)-min_x-0.01,max_y-lines_vector[i].y_end+0.01*tan(30*3.14/180));
        head_arrow << QPoint((lines_vector[i].x_end)-min_x-0.01,max_y-lines_vector[i].y_end-0.01*tan(30*3.14/180));
        head_arrow << QPoint((lines_vector[i].x_end)-min_x,max_y-(lines_vector[i].y_end));
        painter.drawPolygon(head_arrow);
    }
}

It compiles fine but it says errors:
QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::setPen: Painter not active

And if I change this line QPainter painter(ui->graphicsview); to QPainter painter(this);
it has not errors but it does not paint anything...