Incorrect behaviour of scroll bars upon zooming an image in qt

Question

I'm trying to implement zooming feature to my image-viewer-like appliction. I'm uploading an image like so:

void MeasuresWidget::on_actionOpen_triggered()
{
    QString fileName = QFileDialog::getOpenFileName(this,
                                         tr("Open File"), QDir::currentPath());
    if (!fileName.isEmpty())
    {
    QImage image(fileName);
    if (image.isNull()) {
        QMessageBox::information(this, tr("Image Viewer"),
                                   tr("Cannot load %1.").arg(fileName));
        return;
    }

    ui->imageLabel->setPixmap(QPixmap::fromImage(image));
    }

    scaleFactor = 1.0;
//    ui->imageLabel->adjustSize();

}

And zoom in/out like so:

void MeasuresWidget::on_actionZoom_in_triggered()
{
    scaleImage(1.25);
}

void MeasuresWidget::on_actionZoom_out_triggered()
{
    scaleImage(0.8);
}

void MeasuresWidget::scaleImage(double factor)
{
    Q_ASSERT(ui->imageLabel->pixmap());
    scaleFactor *= factor;
    ui->imageLabel->resize(scaleFactor * ui->imageLabel->pixmap()->size());

    adjustScrollBar(ui->scrollArea->horizontalScrollBar(), factor);
    adjustScrollBar(ui->scrollArea->verticalScrollBar(), factor);
}

void MeasuresWidget::adjustScrollBar(QScrollBar *scrollBar, double factor)
{
    scrollBar->setValue(int(factor * scrollBar->value()
                            + ((factor - 1) * scrollBar->pageStep()/2)));
}

The problem is that when i zoom in/out, scroll bars do not change their size, i.e. the size of scroll area is always equal to the size of uploaded image, no matter whether i zoom in or out. Pictures illustrate my problem (1 - no zoom, 2 - zoomed out once) Any idea on possible solution?

Could the property widgetResizable help? See doc.qt.io/archives/qt-4.8/qscrollarea.html. — Benjamin Bihler

George George · Accepted Answer · 2019-05-24T09:12:47

I've come to a solution and i still don't know why is this working now, but creating scroll area and label through code and then using setWidget method solved my problem. Declaration in .h file:

QLabel *imageLabel;
QScrollArea *scrollArea;

And constructor in .cpp file:

imageLabel = new QLabel;
imageLabel->setBackgroundRole(QPalette::Base);
imageLabel->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
imageLabel->setScaledContents(true);


scrollArea = new QScrollArea;
ui->verticalLayout_4->addWidget(scrollArea);
scrollArea->setWidget(imageLabel);

Incorrect behaviour of scroll bars upon zooming an image in qt

1 Answers