Ball bounce off the walls in C++ using Qt - c++

Question

I'm trying to create a simple animation of a ball that is wandering on a screen and simply bounces off on walls. Since I'm fairly new to C++, I've got some problems with this task. I'm also using the Qt-library.
The ball just stays on the same spot and moves back and forth a little which is odd, because I create a SceneRect with the same size as my graphicsView.
mainwindow.cpp:
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QGraphicsScene>
#include <QtWidgets>
float movement_x = 2.5;
float movement_y = 2.0;
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
void MainWindow::showEvent(QShowEvent *event){
scene = new QGraphicsScene(this);
scene -> setSceneRect(ui->graphicsView->geometry().x(), ui->graphicsView->geometry().y(),
ui->graphicsView->geometry().width(), ui->graphicsView->geometry().height());
ui->graphicsView->setScene(scene);
QPen redpen(Qt::red);
QBrush brush(Qt::green);
//QBrush brush2(Qt::black);
QPen mypen(Qt::blue);
mypen.setWidth(7);
ellipse = scene->addEllipse(100.0,100.0,20.0,20.0,mypen,brush);
timer1 = startTimer(50);
timer2 = startTimer(600);
}
void MainWindow::timerEvent(QTimerEvent *event){
// check for radius
if(ellipse->pos().x() >= scene->sceneRect().right() || ellipse->pos().x() <= scene->sceneRect().left()){
movement_x = -movement_x;
}
if(ellipse->pos().y() >= scene->sceneRect().bottom() || ellipse->pos().y() <= scene->sceneRect().top()){
movement_y = -movement_y;
}
ellipse->moveBy(movement_x, movement_y);
}
MainWindow::~MainWindow()
{
delete ui;
}
mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QGraphicsScene>
#include <QMainWindow>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
protected:
void showEvent(QShowEvent* event);
void timerEvent(QTimerEvent* event);
private:
Ui::MainWindow *ui;
QGraphicsScene *scene;
QGraphicsEllipseItem *ellipse;
};
#endif // MAINWINDOW_H
main.cpp:
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.resize(800,600);
w.show();
return a.exec();
}

Answer 1

The initial value of pos is (0,0). Because of this your update oscillate between two values. After addEllipse you have to set an initial position within the scene rect. For example
ellipse->setPos(100,100);

Answer 2

You should map the ellipse position from the scene to find the boundaries :
QPoint posOfEclps = ellipse->mapToScene(ellipse->pos());
if(posOfEclps.x() >= scene->sceneRect().right() || posOfEclps.x() <= scene->sceneRect().left())
{
movement_x = -movement_x;
}
if(posOfEclps.y() >= scene->sceneRect().bottom() || posOfEclps.y() <= scene->sceneRect().top())
{
movement_y = -movement_y;
}
ellipse->moveBy(movement_x, movement_y);