QGraphicsScene item is drawn at twice(x2) position

[bliako]

In the following code I am creating a custom widget which has a subclass of QGraphicsScene embeded in it. My aim is to click and add a point to the scene. A point is my subclass of QGraphicsItem. I want to move that point around and later connect it to other points and make a spline path. The problem I am facing is that the point is not drawn at where I click but at a place which is form by doubling the mouse-event's scenePos() coordinates. So if I click at (100,100) the point is drawn at (200,200). I suspect that I have misunderstood the coordinate system despite reading the documentation. So I am asking additionally to point me to some simple-to-digest advice on how the widgets placement works. My example code follows:

/* use the following pro:
QT += widgets core gui

CONFIG += debug console
SOURCES = example.cpp
TARGET = example
*/
#include <QGraphicsItem>
#include <QPainter>
#include <QWidget>
#include <QRectF>
#include <QPointF>
#include <QGraphicsScene>
#include <QStyleOptionGraphicsItem>
#include <QGraphicsSceneMouseEvent>
#include <QKeyEvent>
#include <QGraphicsView>
#include <QApplication>
#include <QOpenGLWidget>
#include <QMainWindow>
#include <QGridLayout>
#include <QDebug>

class GIPoint : public QGraphicsItem{
public:
	GIPoint(QGraphicsItem * parent, const QPointF &position);
protected:
	QVariant itemChange(QGraphicsItem::GraphicsItemChange change, const QVariant &value) override;
	QRectF boundingRect() const override;
	void paint(
		QPainter *painter,
		const QStyleOptionGraphicsItem *option,
		QWidget *widget
	);
};
class GraphicsSceneWidget : public QGraphicsScene {

public:
	explicit GraphicsSceneWidget(QObject *parent);
	~GraphicsSceneWidget();
	virtual void mousePressEvent(QGraphicsSceneMouseEvent *event);
};

class VectorGraphicsWidget : public QWidget {
public:
	VectorGraphicsWidget(QWidget *parent);
	~VectorGraphicsWidget();
private:
	GraphicsSceneWidget *myGraphicsSceneWidget;
};

// implementation
GIPoint::GIPoint(
	QGraphicsItem *parent,
	const QPointF &position
) : QGraphicsItem(parent) {
	setFlag(QGraphicsItem::ItemIsMovable, true);
	setFlag(QGraphicsItem::ItemIsSelectable, true);
	setPos(position);
	qWarning() << "GIPoint::GIPoint() : init at " << position;
}
QVariant GIPoint::itemChange(
	GraphicsItemChange change,
	const QVariant &value
){
	if (change == QGraphicsItem::ItemPositionChange) {
		qWarning("position changed");
	}
	return value;
}
QRectF GIPoint::boundingRect() const {
	return QRectF(pos().x(), pos().y(), 5, 5);
//	return QRectF(0,0, 5, 5);
}
void GIPoint::paint(
	QPainter *painter,
	const QStyleOptionGraphicsItem *option,
	QWidget *widget
){
	(void )option;
	(void )widget;
	QPointF xx = scenePos();
	QRectF rect = QRectF(xx.x(), xx.y(), 10, 10);
	qWarning() << "painting: scenePos " << scenePos() << ", rect " << rect;
	QBrush brush = QBrush(Qt::black, Qt::SolidPattern);
	//painter->fillRect(rect, brush);
	painter->drawRect(rect);
}

GraphicsSceneWidget::GraphicsSceneWidget(QObject *parent)
	: QGraphicsScene(parent)
{}
GraphicsSceneWidget::~GraphicsSceneWidget(){}
void GraphicsSceneWidget::mousePressEvent(
	QGraphicsSceneMouseEvent *event
){
	GIPoint *gip = new GIPoint(Q_NULLPTR, event->scenePos());
	addItem(gip);
	QGraphicsScene::mousePressEvent(event);
}
	
VectorGraphicsWidget::VectorGraphicsWidget(QWidget *parent) :
	QWidget(parent)
{
	myGraphicsSceneWidget = new GraphicsSceneWidget(this);
	QGraphicsView *view = new QGraphicsView(myGraphicsSceneWidget);
	myGraphicsSceneWidget->setSceneRect(QRectF(0, 0, 500, 500));
	QGridLayout *centralLayout = new QGridLayout;
	centralLayout->addWidget(view);
	setLayout(centralLayout);
	myGraphicsSceneWidget->addRect(
		QRectF(0, 0, 100, 100),
		QPen(Qt::black),
		QBrush(Qt::green)
	);
	view->show();
}
VectorGraphicsWidget::~VectorGraphicsWidget() {
	delete myGraphicsSceneWidget;
}
int main(int argc, char **argv){
	QApplication app(argc, argv);
	app.setApplicationName("test");
	app.setOrganizationName("myorg");
	app.setOrganizationDomain("myorg.com");

	QMainWindow *w = new QMainWindow();
	w->resize(500, 500);
	w->setCentralWidget(new VectorGraphicsWidget(Q_NULLPTR));
	w->show();
 
	return app.exec();
}

[bliako]

I have just figured it out.

My first misunderstanding was that the boundingRect()'s coordinates must be (-0.5,-0.5) which means they are relative to the current item's position and not absolute on the Scene. and so this is the correct one:

QRectF GIPoint::boundingRect() const {
//	return QRectF(pos().x(), pos().y(), 5, 5);
	return QRectF(-0.5, -0.5, 5, 5);
}

My second misunderstanding was in paint(). Do not use the pos() or scenePos() coordinates in order to draw the item. Instead draw it at (0,0). So the corrected paint() is this:

void GIPoint::paint(
        QPainter *painter,
        const QStyleOptionGraphicsItem *option,
        QWidget *widget
){
        (void )option;
        (void )widget;
        QRectF rect = boundingRect();
        qWarning() << "painting: scenePos " << scenePos() << ", rect " << rect;
        QBrush brush = QBrush(Qt::black, Qt::SolidPattern);
        painter->fillRect(rect, brush);  
        //painter->drawRect(rect); 
}

The original program in my question still needs to be modified. Or shall I edit the changes in?

[bliako]

I have also crossposted this same question at https://stackoverflow.com/questions/57118393/qgraphicsscene-item-is-drawn-at-twicex2-position

[bliako]

I have just figured it out.

My first misunderstanding was that the boundingRect()'s coordinates must be (-0.5,-0.5) which means they are relative to the current item's position and not absolute on the Scene. and so this is the correct one:

QRectF GIPoint::boundingRect() const {
//	return QRectF(pos().x(), pos().y(), 5, 5);
	return QRectF(-0.5, -0.5, 5, 5);
}

My second misunderstanding was in paint(). Do not use the pos() or scenePos() coordinates in order to draw the item. Instead draw it at (0,0). So the corrected paint() is this:

void GIPoint::paint(
        QPainter *painter,
        const QStyleOptionGraphicsItem *option,
        QWidget *widget
){
        (void )option;
        (void )widget;
        QRectF rect = boundingRect();
        qWarning() << "painting: scenePos " << scenePos() << ", rect " << rect;
        QBrush brush = QBrush(Qt::black, Qt::SolidPattern);
        painter->fillRect(rect, brush);  
        //painter->drawRect(rect); 
}

The original program in my question still needs to be modified. Or shall I edit the changes in?