Offset in QMouseEvent based line drawing

[Vyata]

Hi all,

I am working on an application that draws lines based on mouse clicks. I made a sub class of QMainWindow called MainWindow. MainWindow has QGraphicsScene, QGraphicsView and QList<QPointF> as members.

mainwindow.h

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = 0);
    ~MainWindow();

protected:
    void mousePressEvent(QMouseEvent * p_event) Q_DECL_OVERRIDE;

private:
    QList<QPointF> PtList;
    QGraphicsScene * p_scene;
    QGraphicsView * p_view;
    QTextEdit * p_textEdit;
    QPixmap pix;

    void redrawScene();
    void redrawPersistentItems();
};

I am collecting the coordinates in the mousePressEvent, inserting them into QList and drawing lineItems on the Scene.

---

mainwindow.cpp

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QWidget *widget = new QWidget;
    setCentralWidget(widget);

    // add pixmap item
    p_scene = new QGraphicsScene;
    p_scene->setSceneRect(0,0,400,400);
    pix = QPixmap("/Users/Vyata/qt/images/pic100x100.gif");
    p_scene->addPixmap(pix);

    p_view = new QGraphicsView(p_scene);
    p_view->setFrameShape(QGraphicsView::NoFrame);
    p_scene->setBackgroundBrush(Qt::yellow);

    p_textEdit = new QTextEdit();
    p_textEdit->setFrameShape(QFrame::NoFrame);

    // add view and textedit to layout
    QVBoxLayout *layout = new QVBoxLayout(widget);
    layout->addWidget(p_view);
    layout->addWidget(p_textEdit);

    widget->show();

    setWindowTitle("Draw with Mouse");
    resize(640,480);

}

MainWindow::~MainWindow()
{

}


void MainWindow::mousePressEvent(QMouseEvent *p_event)
{
    // collect mouse click coordinates using different methods provided by QMouseEvent
    QPointF ptL = p_event->localPos();
    QPointF ptS = p_event->screenPos();
    QPointF ptW = p_event->windowPos();
    QPoint ptG = p_event->globalPos();
    QPoint ptV = p_event->pos();

    // Print coordinates to QTextEdit
    QString strL = "LocalPos( " + QString::number(ptL.x()) +", "+ QString::number(ptL.y()) + " ); ";
    QString strS = "ScreenPos( " + QString::number(ptS.x()) +", "+ QString::number(ptS.y()) + " ); ";
    QString strW = "WindowPos( " + QString::number(ptW.x()) +", "+ QString::number(ptW.y()) + " ); ";
    QString strG = "GlobalPos( " + QString::number(ptG.x()) +", "+ QString::number(ptG.y()) + " ); ";
    QString strV = "ViewPos( " + QString::number(ptV.x()) + ", "+ QString::number(ptV.y()) + "); ";

    QString strScene = "Scene Width = " + QString::number(p_scene->sceneRect().width())
            +" Ht = " + QString::number(p_scene->sceneRect().height());

    p_textEdit->append(strL + strS + strW + strG + strV + "\n"+ strScene + "\n");

    // Add the Point to QList
    QPointF pt1 = p_view->mapToScene(ptV);
    PtList.append(pt1);
    MainWindow::redrawScene();
}

void MainWindow::redrawScene()
{
    // clear scene and insert imiage and boundary rect
    p_scene->clear();
    redrawPersistentItems();

    // draw the ROI polygon from the QList
    int ListSize = PtList.size();
    QPointF ptVar1, ptVar2;

    if(ListSize == 2)
    {
        ptVar1 = PtList.last();
        ptVar2 = PtList.first();
        p_scene->addLine(ptVar1.x(),ptVar1.y(),ptVar2.x(),ptVar2.y());
    }
    else if (ListSize > 2)
    {
        for(int i = 0; i< ListSize-1; i++)
        {
            ptVar1 = PtList[i];
            ptVar2 = PtList[i+1];
            p_scene->addLine(ptVar1.x(),ptVar1.y(),ptVar2.x(),ptVar2.y());
        }

        ptVar1 = PtList.last();
        ptVar2 = PtList.first();
        p_scene->addLine(ptVar1.x(),ptVar1.y(),ptVar2.x(),ptVar2.y());
    }
}

// add pixmap and rect to scene
void MainWindow::redrawPersistentItems()
{
    p_scene->addPixmap(pix);
    QRectF rect = p_scene->sceneRect();
    p_scene->addRect(rect);

}

---

main.cpp

#include "mainwindow.h"
#include <QApplication>

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

    return a.exec();
}

---

I want my application to join all the lines I click on, creating a polygon. But I see an offset between where I click and where the actual coordinates are drawn on the screen.

current output:

screenshot

In the screenshot I clicked along the corners of the 100 x 100 image, but my polygon is drawn with an offset.

please help.