coordinate mapping

[Tomathor]

Hello Guys
I've been working on a physics simulation, and I've been having some trouble with cordinates.

I haven't been able to find how the scenes are mapped, and to what objects are mapped ot when I don't use mapToScene();

My Code is as follows:

Ball.cpp

Ball::Ball()
{
    //random start angle

    angle = (qrand() % 360);
    setRotation(angle);

    //set the speed
    speed = 5;


    //random start position
    int StartX = 0;
    int StartY = 0;


    if((qrand() % 1))
    {
        StartX = (qrand() % 200);
        StartY = (qrand() % 200);

    }
    else
    {
        StartX = (qrand() % 100);
        StartY = (qrand() % 100);
    }


    setPos(mapToScene(StartX,StartY));
    qDebug() << "xposition:" << pos().x();
    qDebug() << "yposition:" << pos().y();
    qDebug() << "xposition mapped:"  << mapToScene(pos()).x();
    qDebug() << "yposition mapped:" << mapToScene(pos()).y();

}

mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    scene = new QGraphicsScene (this);
    ui->graphicsView->setScene (scene);

    ui->graphicsView->setRenderHint(QPainter::Antialiasing);
    double sceneULx = -400;
    double sceneULy = 200;
    double scenewidth = 800;
    double sceneheight = 400;

    scene->setSceneRect(sceneULx, sceneULy,scenewidth,sceneheight);

    QPen mypen = QPen(Qt::red);

    QLineF TopLine(scene->sceneRect().topLeft(),scene->sceneRect().topRight());
    QLineF LeftLine(scene->sceneRect().topLeft(),scene->sceneRect().bottomLeft());
    QLineF RightLine(scene->sceneRect().bottomRight(),scene->sceneRect().topRight());
    QLineF BottomLine(scene->sceneRect().bottomLeft(),scene->sceneRect().bottomRight());

    scene->addLine(TopLine,mypen);
    scene->addLine(LeftLine,mypen);
    scene->addLine(RightLine,mypen);
    scene->addLine(BottomLine,mypen);

    int ItemCount = 5;
    for(int i = 0; i <ItemCount; i++)
    {
       Ball *item = new Ball();
       scene->addItem(item);
    }

    timer = new QTimer(this);
    connect(timer,SIGNAL(timeout()), scene,SLOT(advance()));
    timer->start(10);
}

currently all the objects I generate are not in the area of the scene, which doesn't make any sense to me since I thought that the upper left corner of the scene was 0/0 and it went into the positive when you went down or to the right.

also the qDebugs yield following results:

xposition: 25.6601
yposition: 22.306
xposition mapped: 30.392
yposition mapped: 55.9751
xposition: -56.6299
yposition: 79.0004
xposition mapped: -136.607
yposition mapped: 23.7579
xposition: 31.6448
yposition: 32.3822
xposition mapped: 76.5177
yposition mapped: 38.4172
xposition: 50.017
yposition: -97.4695
xposition mapped: -59.4804
yposition mapped: -93.9612
xposition: -44.0378
yposition: 9.25605
xposition mapped: -33.8154
yposition mapped: -34.5675

I can't really find a correlation between the mapped and the unmapped numbers and it is also very strange the the mapped numbers have decimals altough I define them with an int.

I hope somebody can help me clear my confusion with the coordinate system of this programm.

[kumararajas]

@Tomathor Looks like more of your algorithm problem than Qt related problem..