How to draw over an old picture in QGraphicsScene

[Mikeeeeee]

I append 2 images on QLabel

    QImage mapImage(":/Images/Images/mapMain.png");
    myImageViewer->setImage(mapImage);
    QImage base(":/Images/Images/mapMain.png"); // set to some file/size
    QImage overlay(":/Images/Images/mapTop.png");  // set to some file/size
    QPainter paint(&base);
    paint.drawImage(0,0,overlay);
    myImageViewer->imageLabel->setPixmap(QPixmap::fromImage(base));
    ui->verticalLayout->addWidget(myImageViewer);

    //Pen
    myPen.setWidth(20);
    myPen.setColor(Qt::green);
    //myPen.setColor(QColor(255, 0, 0, 10));
    myPen.setCapStyle(Qt::RoundCap);
    myPen.setStyle(Qt::SolidLine);
    myImageViewer->installEventFilter(this);

In the Mainwindow constructor, I wrote:

    QPixmap myPixmap;
    QPen myPen;
    QPoint lastPoint;
    bool eventFilter(QObject *obj, QEvent *event);

And wrote such eventFilter:

bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
    qDebug()<<"eventFilter";
    if(obj == myImageViewer and event->type() == QEvent::MouseButtonPress)
    {
        qDebug()<<" QEvent::MouseButtonPress";
        QMouseEvent *mous = (QMouseEvent*)event;
        QPainter p(&myPixmap);
        p.setPen(myPen);
        p.drawPoint(mous->pos());
        p.end();
        lastPoint = mous->pos();
        myImageViewer->imageLabel->setPixmap(myPixmap);
    }
    if(obj == myImageViewer and event->type() == QEvent::MouseMove)
    {
        QMouseEvent *mous = (QMouseEvent*)event;
        QPainter p(&myPixmap);
        p.setPen(myPen);
        p.drawLine(lastPoint,mous->pos());
        p.end();
        lastPoint = mous->pos();
        myImageViewer->imageLabel->setPixmap(myPixmap);
    }
}

When eventfilter is triggered, the picture disappears and a white screen appears. And Qt gives an error message:

QEvent::MouseButtonPress
QPainter::begin: Paint device returned engine == 0, type: 2
QPainter::setPen: Painter not active
QPainter::drawPoints: Painter not active
QPainter::end: Painter not active, aborted

[jsulm]

@Mikeeeeee Why do you paint in eventFilter?!
You already were told to paint in paintEvent.
In eventFilter you only store needed information (like x/y).

[Mikeeeeee]

But in the example that I showed you drawing eventfilter occurs, and if you do so, then paintEvent works only a couple of times and then for some reason no longer works.

void MainWindow::paintEvent(QPaintEvent *event)
{
    qDebug()<<"paintEvent";
}

[jsulm]

@Mikeeeeee After getting new coordinates you can call https://doc.qt.io/qt-5/qwidget.html#update to trigger paint event.

[Mikeeeeee]

It turns out to draw in eventFilter, but it was necessary to add this line to the constructor.

myPixmap  = QPixmap(myImageViewer->image.width(),myImageViewer->image.height());

I draw so:

bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
    if(obj == myImageViewer and event->type() == QEvent::MouseButtonPress)
    {
        qDebug()<<" QEvent::MouseButtonPress";
        QMouseEvent *mous = (QMouseEvent*)event;
        QPainter p(&myPixmap);
        p.setPen(myPen);
        p.drawPoint(mous->pos());
        p.end();
        lastPoint = mous->pos();
        myImageViewer->imageLabel->setPixmap(myPixmap);
    }
}

But when drawing the picture disappears and the screen turns black. How to fix? Can I set the drawing on the QImage overlay(":/Images/Images/mapTop.png");?

[Mikeeeeee]

And if I do so, the screen becomes white.

myPixmap.fill();

How to make myPixmap transparent?

[mrjj]

myPixmap.fill(Qt::transparent);

[Mikeeeeee]

Thanks. Do so:

    myPixmap  = QPixmap(myImageViewer->image.width(),myImageViewer->image.height());
    myPixmap.fill(Qt::transparent);
    myPixmap = QPixmap(":/Images/Images/mapTop.png");

But after drawing the picture disappears :

myImageViewer->setImage(mapImage);

How to get myPixmap the images:":/Images/Images/mapMain.png" and ":/Images/Images/mapTop.png" , but draw only on:
":/Images/Images/mapTop.png"?

[mrjj]

@Mikeeeeee
You mean draw myPixmap on mapImage and then
show the combined image with myImageViewer->setImage(mapImage); ?
Thats like before where we combined them.

[Mikeeeeee]

This post is deleted!

[Mikeeeeee]

Yes.The background will be mapImage, and I want to draw on it, with the ability to erase and make transparent Qpixmap. You may need to use: void QPixmap::setMask(const Bitmap &mask)

[Mikeeeeee]

Tried to do so, but got an error: reference to type 'const QBitmap' could not bind to an lvalue of type 'QBitmap *'

    QBitmap *myQBitmap = new QBitmap();
    myQBitmap->fromImage(mapImage);
    myPixmap.setMask(myQBitmap);

[SGaist]

@Mikeeeeee Please, read the documentation.

QBitmap::fromImage is a static method. You're using it wrong.
QPixmap::setMask takes a const reference to a QBitmap, not a pointer.

[Mikeeeeee]

But how to set 2 images and draw on top image?