Set zoom out limit for a picture in QGraphicsView

Question

I am zooming an image on a label using QGraphicsView. But when I zoom out I want to set a specific limit for the zoom out. I am using the following code

scene = new QGraphicsScene(this);
view = new QGraphicsView(label);
QPixmap pix("/root/Image);
Scene->addPixmap(pixmap);
view->setScene(scene);
view->setDragMode(QGraphicsView::scrollHandDrag);

In the slot of wheel event

void MainWindow::wheelEvent(QWheelEvent *ZoomEvent)
{
    view->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    double scaleFactor = 1.15;
    if(ZoomEvent->delta() >0)
    {
        view->scale(scaleFactor,scaleFactor);
    }
    else
    {
        view->scale(1/scaleFactor,1/scaleFactor);
    }
}

I want that the image should not zoom out after a certain extent. What should I do? I tried setting the minimum size for QGraphicsView but that didn't help.

Thank You :)

Answer 1

Maybe something like this would help:

void MainWindow::wheelEvent(QWheelEvent *ZoomEvent)
{
    view->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    static const double scaleFactor = 1.15;
    static double currentScale = 1.0;  // stores the current scale value.
    static const double scaleMin = .1; // defines the min scale limit.

    if(ZoomEvent->delta() > 0) {
        view->scale(scaleFactor, scaleFactor);
        currentScale *= scaleFactor;
    } else if (currentScale > scaleMin) {
        view->scale(1 / scaleFactor, 1 / scaleFactor);
        currentScale /= scaleFactor;
    }
}

The idea, as you can see, is caching the current scale factor and do not zoom out in case of it smaller than some limit.

Answer 2

Here is one more implementation idea in which the current zoom factor is obtained from the transformation matrix for the view:

void View::wheelEvent(QWheelEvent *event) {
   const qreal detail = QStyleOptionGraphicsItem::levelOfDetailFromTransform(transform());
   const qreal factor = 1.1;

   if ((detail < 10) && (event->angleDelta().y() > 0)) scale(factor, factor);
   if ((detail > 0.1) && (event->angleDelta().y() < 0)) scale((1 / factor), (1 / factor));
}

Answer 3

Another way to solve the problem:

void GraphWidget::wheelEvent(QWheelEvent *event)
{
    int scaleFactor = pow((double)2, - event->delta() / 240.0);

    qreal factor = transform().scale(scaleFactor, scaleFactor).mapRect(QRectF(0, 0, 1, 1)).width();
    if (factor < 0.5 || factor > 20)
        return;
    scale(scaleFactor, scaleFactor);
}