coordinate mapping in Qt - c++

Question

I've been working on a physics simulation, and I've been having some trouble with coordinates.
I haven't been able to find how the scenes are mapped, and to what objects are mapped to 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.

Answer 1

I think you have the right idea about the coordinate system, but have confused yourself with an oversight in the code.
When a Ball item is created, in the constructor, it creates random coordinates and then calls this:
setPos(mapToScene(StartX,StartY));
As the documentation for mapToScene states: -
Maps the point point, which is in this item's coordinate system, to the scene's coordinate system, and returns the mapped coordinate.
However, the code is executed in the Ball's constructor and has not yet been added to the scene, so it can't successfully map the coordinates as it has no parent item and no scene with which to map to.
You can call setPos in the constructor, but without mapToScene
setPos(StartX,StartY);
With no parent, when the item is added to the scene, it will match its scene coordinates (item->pos() == item->scenePos())
Alternatively, create a Ball item, add it to the scene, and then set its position.
for(int i = 0; i <ItemCount; i++)
{
Ball *item = new Ball();
scene->addItem(item);
// note that mapToScene is irrelevant, since there is no parent item for the Ball.
item->setPos(x, y);
}

Answer 2

Just for an experiment you can do the following trick:
//random start position
int StartX = 100;
int StartY = 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();
You will see that coordinates are still random. Basically, you are randomizing coordinates 3 times and no wonder there is no correlation.
Also try this:
double sceneULx = 0;
double sceneULy = 0;
double scenewidth = 800;
double sceneheight = 400;