mapToScene returns position with offset

Peter_Dev

Hello everyone!
I'm making an app for resizing bmp images. I want to be able to choose area for resizing, so i implemented QRubberBand for this aim. Everything is fine except mapToScene.
When i'm trying to get point coordinates according to original image, i get them with offset. Moreover, offset is different from image to image, but around 50 pixels on x axis and 70 on y axis.
cpp file

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "QGraphicsPixmapItem"
#include "QPixmap"
#include "QGraphicsScene"
#include "QFileDialog"
#include "QDir"
#include "QStandardItemModel"
#include "QLineEdit"
#include "QMouseEvent"
#include "QRubberBand"
#include "QDebug"

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

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPoint origin = event->pos();
    QPointF pic_pos = ui->graphicsView->mapToScene(event->pos());
    QPoint p = pic_pos.toPoint();
    qDebug() << p.rx(); //prints position with some offset
    qDebug()<<p.ry();
    origin.setX(origin.rx()-50); //i want cursor to be at center of square
    origin.setY(origin.ry()-50);
    QSize pos(100,100); //chosing area is a square with 100 by 100 size
    rubberBand->setGeometry(QRect(origin, pos));
    QPalette pal;
    pal.setBrush(QPalette::Highlight, QBrush(Qt::green));
    rubberBand->setPalette(pal);
    rubberBand->show();
}

void MainWindow::on_actionOpen_file_triggered()
{
    QString filter = "BMP image (*.bmp)";
    QString file_name = QFileDialog::getOpenFileName(this, "Choose image",QDir::homePath(), filter);
    QGraphicsScene *scene = new QGraphicsScene(this);
    QPixmap pixmap(file_name);
    scene->addPixmap(pixmap);

    ui->graphicsView->setScene(scene);
    ui->graphicsView->fitInView(scene->sceneRect(), Qt::KeepAspectRatio);
    ui->graphicsView->show();
}

main

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

header

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QRubberBand>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void on_actionOpen_file_triggered();
    void mousePressEvent(QMouseEvent *event) override;

private:
    Ui::MainWindow *ui;
    QRubberBand *rubberBand = new QRubberBand(QRubberBand::Rectangle, this);
};
#endif // MAINWINDOW_H

Christian Ehrlicher

See QMouseEvent::pos() - you have a position relative to your mainmwindow, not to your view here.

Peter_Dev

@Christian-Ehrlicher Thanks for your help! But could you please explain how to do it?

Christian Ehrlicher

You have to map the coordinates from your mainwindow to your scene -> QWidget::mapTo()

Peter_Dev

@Christian-Ehrlicher I got the idea, but still don't know how to implement it. Could you please provide some code?

Christian Ehrlicher

@Peter_Dev said in mapToScene returns position with offset:

Could you please provide some code?

So hard to call mapTo(ui->graphicsView, event->pos()) ?

Peter_Dev

@Christian-Ehrlicher when i make it like this, program finishes unexpectedly if i click anywhere on picture

Christian Ehrlicher

Use a debugger and take a look at the backtrace to see where exactly t crashes.

Peter_Dev

@Christian-Ehrlicher Segmentation fault. Seems to happen at mapTo

Christian Ehrlicher

Then make sure all your pointers you're using are valid. And please show some code...

Peter_Dev

@Christian-Ehrlicher Only change i made was in this function:

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPoint origin = event->pos();
    QPointF res = ui->graphicsView->mapToScene(mapTo(ui->graphicsView,event->pos()));//here
    QPoint p = res.toPoint(); 
    qDebug() << p.rx();
    qDebug()<<p.ry();
    origin.setX(origin.rx()-50);
    origin.setY(origin.ry()-50);
    QSize pos(100,100);
    rubberBand->setGeometry(QRect(origin, pos));
    QPalette pal;
    pal.setBrush(QPalette::Highlight, QBrush(Qt::green));
    rubberBand->setPalette(pal);
    rubberBand->show();
}

jsulm

@Peter_Dev said in mapToScene returns position with offset:

PointF res = ui->graphicsView->mapToScene(mapTo(ui->graphicsView,event->pos()));//here

What is "mapTo" here?
Did you check all the pointers as already suggested?

Peter_Dev

@jsulm Checking now. mapTo maps click position relative to graphicsView, i guess. Am i wrong?

jsulm

@Peter_Dev Did you debug and/or check the pointers? This is actually the first thing to do if your app is crashing...

Peter_Dev

@jsulm it crushes at mapTo, when it tries to call mapToParent inside itself. I think problem is in passing ui->graphicsView as first argument in mapTo

jsulm

@Peter_Dev Again: did you check the pointers? Are those valid?

Peter_Dev

@jsulm Sorry, i'm new here. I don't quite understand what you mean by checking pointers, but as i understood you, yes, all pointers are valid. Before i put mapTo here, everything was working correctly. Sorry for wasting your time, but it's really important project for me, so i need your help very much.

JonB

@Peter_Dev
Your code shows

mapTo(ui->graphicsView,event->pos())

And you wrote

@jsulm it crushes at mapTo, when it tries to call mapToParent inside itself. I think problem is in passing ui->graphicsView as first argument in mapTo

If you're saying that crashes, you need to look at its code to determine why. If you're asking for our help, how can we help if you don't show mapTo()'s code?

jsulm

@JonB I think it's the mapTo from QMainWindow from QWidget :-)
This was my mistake also when reading this code :-)

Peter_Dev

@JonB

QPoint QWidget::mapTo(const QWidget * parent, const QPoint & pos) const
{
    QPoint p = pos;
    if (parent) {
        const QWidget * w = this;
        while (w != parent) {
            Q_ASSERT_X(w, "QWidget::mapTo(const QWidget *parent, const QPoint &pos)",
                       "parent must be in parent hierarchy");
            p = w->mapToParent(p); 
            w = w->parentWidget();
        }
    }
    return p;
}

QPoint QWidget::mapToParent(const QPoint &pos) const
{
    return pos + data->crect.topLeft(); //crushes here
}