QGraphicsView scale issue

nigeroid

Hello.
I try to implement my own graphics item which simply draws the frame around the pixel.
I have already written some code, and it works partially. The problem is that when I scroll down to the right-bottom and scale my view for some particular factors (22-30), frame offsets for up to a half of neighbour pixel.
.
This was made for scale value 30. For scale values 1-20, 32- problem disappears.

Here is a class of my own graphics item (frame around the pixel):

PixelRect::PixelRect(QGraphicsView *view, QGraphicsItem *parent) :
    QGraphicsItem(parent),
    view_(view),
    scenePoint_(15, 15),
    zoom_(view_->matrix().m11())
{
    view_->scene()->addItem(this);
}

void PixelRect::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    qreal zoom = view_->matrix().m11();

    QPen pen = QPen();
    pen.setWidth(1);
    pen.setColor(Qt::black);
    pen.setStyle(Qt::DotLine);
    painter->setPen(pen);

    int x = scenePoint_.x() * zoom;
    int y = scenePoint_.y() * zoom;

    QPoint p = view_->mapFromScene(scenePoint_.x(), scenePoint_.y());
    painter->drawRect(QRectF(p.x() + view_->horizontalScrollBar()->value(), p.y() + view_->verticalScrollBar()->value(), (int)zoom, (int)zoom ));
}

QRectF PixelRect::boundingRect() const
{
    qreal zoom = view_->matrix().m11();

    QRectF rectOut;

    int x = scenePoint_.x() * zoom;
    int y = scenePoint_.y() * zoom;

    QPoint p = view_->mapFromScene(scenePoint_.x(), scenePoint_.y());
    rectOut = QRectF(p.x() + view_->horizontalScrollBar()->value(), p.y() + view_->verticalScrollBar()->value(), (int)zoom, (int)zoom );

    return rectOut;
}


void PixelRect::setPos(const QPoint &pos)
{
    qreal zoom = view_->matrix().m11();

    qreal offsetX = (view_->viewport()->width() - view_->scene()->width() * zoom) / 2;
    if (offsetX < 0) offsetX = 0;
    qreal offsetY = (view_->viewport()->height() - view_->scene()->height() * zoom) / 2;
    if (offsetY < 0) offsetY = 0;

    int x = (pos.x() - offsetX + view_->horizontalScrollBar()->value()) / zoom;
    int y = (pos.y() - offsetY + view_->verticalScrollBar()->value()) / zoom;
    if (x < 0) x = 0;
    if (y < 0) y = 0;
    if (x >= view_->scene()->width()) x = view_->scene()->width() - 1;
    if (y >= view_->scene()->height()) y = view_->scene()->height() - 1;

    scenePoint_.setX(x);
    scenePoint_.setY(y);

    update();
}

Usage:


MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    c_(1),
    oldMousePos_(15, 15)
{
    ui->setupUi(this);

    QPixmap pix;
    pix.load(":images/city.jpeg");

    QGraphicsScene * scene = new QGraphicsScene(ui->widget);
    view_ = new QGraphicsView(scene, this);
    view_->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    view_->setMouseTracking(true);
    view_->viewport()->installEventFilter(this);

    QGridLayout *topLayout = new QGridLayout;
    topLayout->addWidget(view_, 0, 0);
    ui->widget->setLayout(topLayout);

    QGraphicsPixmapItem* pixmapItem = new QGraphicsPixmapItem(pix);
    scene->addItem(pixmapItem);

    pixelRect_ = new PixelRect(view_);
    pixelRect_->setFlag(QGraphicsItem::ItemIgnoresTransformations);
}

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

void MainWindow::on_pushButton_clicked()
{
    QMatrix m;
    m.scale(c_, c_);
    c_++;
    view_->setMatrix(m);

    qDebug() << c_;
}

bool MainWindow::eventFilter(QObject *object, QEvent *event)
{
    if (event->type() == QEvent::MouseMove)
    {
        QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);

        if (pixelRect_) pixelRect_->setPos(mouseEvent->pos());

        oldMousePos_ = mouseEvent->pos();
    }
}

Full example https://drive.google.com/file/d/1iL_TR8p8kEGNzPJKfZCiNP6Y4x8pgpFB/view?usp=sharing
Thanks.