c++ - how to pan images in QGraphicsView

Question

I am currently able to load my image into a grahpics scene, and then again into a QGraphicsViewer.

I am able to implement a zoom feature by dtecting a QEvent::Wheel and then calling the graphicsViews's scale() function.

However, I can't seem to figure out how to get the pan functionality working. I basically want to detect when a mouse has clicked down on the image, and then move the image left, right, up or down along with the mouse.

As of right now, I basically have a MouseFilter class that is detecting events, and doing different things depending on the event type. I attached that listener to the QGraphicsView object

Answer 1

In case someone is wondering how to do it on their own, it's actually quite simple. Here's the code from my app:

class ImageView : public QGraphicsView
{
public:
    ImageView(QWidget *parent);
    ~ImageView();

private:
    virtual void mouseMoveEvent(QMouseEvent *event);
    virtual void mousePressEvent(QMouseEvent *event);
    virtual void mouseReleaseEvent(QMouseEvent *event);

    bool _pan;
    int _panStartX, _panStartY;
};

You need to store the start position of the drag, for example like this (I used the right button):

void ImageView::mousePressEvent(QMouseEvent *event)
{
    if (event->button() == Qt::RightButton)
    {
        _pan = true;
        _panStartX = event->x();
        _panStartY = event->y();
        setCursor(Qt::ClosedHandCursor);
        event->accept();
        return;
    }
    event->ignore();
}

Also, you need to clear the flag and restore the cursor once the button is released:

void ImageView::mouseReleaseEvent(QMouseEvent *event)
{
    if (event->button() == Qt::RightButton)
    {
        _pan = false;
        setCursor(Qt::ArrowCursor);
        event->accept();
        return;
    }
    event->ignore();
}

To actually manage the drag, you need to override the mouse move event. QGraphicsView inherits a QAbstractScrollArea, and its scrollbars are easily accessible. You also need to update the pan position:

void ImageView::mouseMoveEvent(QMouseEvent *event)
{
    if (_pan)
    {
        horizontalScrollBar()->setValue(horizontalScrollBar()->value() - (event->x() - _panStartX));
        verticalScrollBar()->setValue(verticalScrollBar()->value() - (event->y() - _panStartY));
        _panStartX = event->x();
        _panStartY = event->y();
        event->accept();
        return;
    }
    event->ignore();

}