Рисование прямой на картинке

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
 
#include <QMainWindow>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QTimer>
#include <QGraphicsEllipseItem>
 
namespace Ui {
class MainWindow;
}
 
class MainWindow : public QMainWindow
{
    Q_OBJECT
 
public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
 
protected slots:
    void onTimeout();
 
private:
    QGraphicsScene* _scene;
    QGraphicsView* _view;
    QTimer* _timer;
    QGraphicsEllipseItem* _point;
    Ui::MainWindow *ui;
 
};
#endif // MAINWINDOW_H

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QVBoxLayout>
#include <QPixmap>
 
MainWindow::MainWindow(QWidget *parent)
   : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    QPixmap menu(":/img/image1.png");
 
 
    _scene = new QGraphicsScene(0, 0, 800, 600, this);
    _scene->addRect(_scene->sceneRect());
 
    _view = new QGraphicsView;
    _view->setScene(_scene);
 
    QVBoxLayout* mainLayout = new QVBoxLayout;
    mainLayout->addWidget(_view);
 
    QWidget* mainWidget = new QWidget;
    mainWidget->setLayout(mainLayout);
 
    this->setCentralWidget(mainWidget);
 
    _point = new QGraphicsEllipseItem(0, 0, 10, 10);
    _scene->addItem(_point);
    
 
   _timer = new QTimer(this);
   connect(_timer, &QTimer::timeout, this, &MainWindow::onTimeout);
   _timer->start(100);
 
}
 
MainWindow::~MainWindow()
{
    delete ui;
}
 
void MainWindow::onTimeout()
{
    QPointF oldPos = _point->pos();
    QPointF newPos = QPointF(_point->pos().x() + 10, _point->pos().y() + 10);
 
    _point->setPos(newPos);
 
    QGraphicsLineItem* line = new QGraphicsLineItem (QLineF(oldPos, newPos));
 
    _scene->addItem(line);
}

void MainWindow::onTimeout()
{
...
    QGraphicsLineItem* line = new QGraphicsLineItem (QLineF(oldPos, newPos));
 
    _scene->addItem(line);
}