Create QGraphicsItems on top of QGraphicsView from QMainWindow

hobbyProgrammer

I've rewritten my code to be able to load the image inside of the QGraphicsView, but the canvas stays empty.
Can someone help me find the mistake?

mainWindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "graphicsview.h"

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

    createActions();
    createMenus();
}

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

void MainWindow::createActions()
{
    openAct = new QAction(tr("&Open..."), this);
    openAct->setShortcuts(QKeySequence::Open);
    connect(openAct, SIGNAL(triggered()), this, SLOT(openPhoto()));

    exitAct = new QAction(tr("&Exit"), this);
    exitAct->setShortcuts(QKeySequence::Quit);
    connect(exitAct, SIGNAL(triggered()), this, SLOT(close()));

    drawAct = new QAction (tr("&Draw"), this);
}



void MainWindow::createMenus()
{

}

void MainWindow::openPhoto()
{
    QString fileName;
    fileName = QFileDialog::getOpenFileName(this, tr("Open File"), QDir::currentPath());

    graphicsView *gv = new graphicsView();
    gv->openImage(fileName);
}

void MainWindow::on_pushButton_clicked()
{
    openPhoto();
}

graphicsView.cpp

#include "graphicsview.h"

graphicsView::graphicsView(QWidget *parent) : QGraphicsView(parent)
{
    scene = new QGraphicsScene();
    this->setScene(scene);
}

void graphicsView::wheelEvent(QWheelEvent *event)
{
    setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    double scaleFactor = 1.2;

    if(event->delta() > 0)
    {
        scale(scaleFactor, scaleFactor);
    }
    else
    {
        scale(1/scaleFactor, 1/scaleFactor);
    }
}

void graphicsView::mousePressEvent(QMouseEvent *event)
{
    if(event->button() == Qt::LeftButton)
    {
    }
    else if(event->button() == Qt::RightButton)
    {

    }

}

void graphicsView::openImage(QString filename)
{
    QImage image = QImage(filename);
    item = new QGraphicsPixmapItem(QPixmap::fromImage(image));
    scene->addItem(item);
}

Asperamanca

@hobbyProgrammer said in Create QGraphicsItems on top of QGraphicsView from QMainWindow:

graphicsView *gv = new graphicsView();
gv->openImage(fileName);

You create a new view, but you do nothing with it.
Do you want to pop up a new window every time you open an image? Or do you want to have the GraphicsView as part of the main window?

hobbyProgrammer

@Asperamanca since I promoted my widget in the designer, I changed the code to this:

    ui->graphicsView->openImage(fileName);

now it works perfectly! Thank you so much!

hobbyProgrammer

Just added this:

void graphicsView::mousePressEvent(QMouseEvent *event)
{
    if(event->button() == Qt::LeftButton)
    {
        QGraphicsEllipseItem *newEllipse = new QGraphicsEllipseItem();

        int x,y;
        x = event->pos().x();
        y = event->pos().y();

        newEllipse->setRect(QRectF(x,y,5,5));
        scene->addItem(newEllipse);
    }
}

but it results into this:

I would really like it if the item showed up on the exact same position where I pressed the mouse.
I have no clue why it doesn't do that.
Any ideas?

hobbyProgrammer

it seems to be the problem where it wants to draw from the middle instead of from the top left corner. I know that this problem is solvable, but I can't find the solution anywhere.

Asperamanca

@hobbyProgrammer
You should really read https://doc.qt.io/qt-5/graphicsview.html# and for this case especially the section "The Graphics View Coordinate System".

You get your coordinates in view coordinates, but add an item in scene coordinates. The scene auto-sizes to the items you already have in there, unless you override the sceneRect (see QGraphicsScene::sceneRect() for a description of that behavior). So depending on the size of your pixmap, the scene may be zoomed out or zoomed in. It's like watching a soccer game on a TV, and measuring the distance to the goal using a ruler on the TV set.

If you want the code inside the QGraphicsView, you'll have to use QGraphicsView::mapToScene

hobbyProgrammer

This post is deleted!

hobbyProgrammer

@Asperamanca
Hi,
Thank you so much for the explaination.
It made me feel like an absolute fool, but that's okay. It's really good to learn and now what everything stands for and you clarified that so much for me using that example.

Asperamanca

@hobbyProgrammer
Not sure whether your last question is still open. You talk about mapToScene, but you don't use it in your code.

You don't sound like a fool to me...to quote John Cleese: "One of the big problems of the world is that the fools are so sure of themselves, and the wise people so full of doubt".
If you were a fool, you wouldn't be asking questions, but instead insisting that Qt is stupid, and you didn't need it anyway :-)

hobbyProgrammer

@Asperamanca No it's solved (that's why I removed it)

"If you were a fool, you wouldn't be asking questions, but instead insisting that Qt is stupid, and you didn't need it anyway :-)"
this is litteraly every one of my co-students. They all hate Qt so much, but it needs a lot of time and patience to get right in. I'm gonna keep trying, because I think you can create a lot of different things with it.