Qgraphicsview Zooming in and Out Under Mouse Position Using Mouse Wheel

QGraphicsView Zooming in and out under mouse position using mouse wheel

Such zooming is a bit tricky. Let me share my own class for doing that.

Header:

#include <QObject>
#include <QGraphicsView>

/*!
 * This class adds ability to zoom QGraphicsView using mouse wheel. The point under cursor
 * remains motionless while it's possible.
 *
 * Note that it becomes not possible when the scene's
 * size is not large enough comparing to the viewport size. QGraphicsView centers the picture
 * when it's smaller than the view. And QGraphicsView's scrolls boundaries don't allow to
 * put any picture point at any viewport position.
 *
 * When the user starts scrolling, this class remembers original scene position and
 * keeps it until scrolling is completed. It's better than getting original scene position at
 * each scrolling step because that approach leads to position errors due to before-mentioned
 * positioning restrictions.
 *
 * When zommed using scroll, this class emits zoomed() signal.
 *
 * Usage:
 *
 *   new Graphics_view_zoom(view);
 *
 * The object will be deleted automatically when the view is deleted.
 *
 * You can set keyboard modifiers used for zooming using set_modified(). Zooming will be
 * performed only on exact match of modifiers combination. The default modifier is Ctrl.
 *
 * You can change zoom velocity by calling set_zoom_factor_base().
 * Zoom coefficient is calculated as zoom_factor_base^angle_delta
 * (see QWheelEvent::angleDelta).
 * The default zoom factor base is 1.0015.
 */
class Graphics_view_zoom : public QObject {
  Q_OBJECT
public:
  Graphics_view_zoom(QGraphicsView* view);
  void gentle_zoom(double factor);
  void set_modifiers(Qt::KeyboardModifiers modifiers);
  void set_zoom_factor_base(double value);

private:
  QGraphicsView* _view;
  Qt::KeyboardModifiers _modifiers;
  double _zoom_factor_base;
  QPointF target_scene_pos, target_viewport_pos;
  bool eventFilter(QObject* object, QEvent* event);

signals:
  void zoomed();
};

Source:

#include "Graphics_view_zoom.h"
#include <QMouseEvent>
#include <QApplication>
#include <QScrollBar>
#include <qmath.h>

Graphics_view_zoom::Graphics_view_zoom(QGraphicsView* view)
  : QObject(view), _view(view)
{
  _view->viewport()->installEventFilter(this);
  _view->setMouseTracking(true);
  _modifiers = Qt::ControlModifier;
  _zoom_factor_base = 1.0015;
}

void Graphics_view_zoom::gentle_zoom(double factor) {
  _view->scale(factor, factor);
  _view->centerOn(target_scene_pos);
  QPointF delta_viewport_pos = target_viewport_pos - QPointF(_view->viewport()->width() / 2.0,
                                                             _view->viewport()->height() / 2.0);
  QPointF viewport_center = _view->mapFromScene(target_scene_pos) - delta_viewport_pos;
  _view->centerOn(_view->mapToScene(viewport_center.toPoint()));
  emit zoomed();
}

void Graphics_view_zoom::set_modifiers(Qt::KeyboardModifiers modifiers) {
  _modifiers = modifiers;

}

void Graphics_view_zoom::set_zoom_factor_base(double value) {
  _zoom_factor_base = value;
}

bool Graphics_view_zoom::eventFilter(QObject *object, QEvent *event) {
  if (event->type() == QEvent::MouseMove) {
    QMouseEvent* mouse_event = static_cast<QMouseEvent*>(event);
    QPointF delta = target_viewport_pos - mouse_event->pos();
    if (qAbs(delta.x()) > 5 || qAbs(delta.y()) > 5) {
      target_viewport_pos = mouse_event->pos();
      target_scene_pos = _view->mapToScene(mouse_event->pos());
    }
  } else if (event->type() == QEvent::Wheel) {
    QWheelEvent* wheel_event = static_cast<QWheelEvent*>(event);
    if (QApplication::keyboardModifiers() == _modifiers) {
      if (wheel_event->orientation() == Qt::Vertical) {
        double angle = wheel_event->angleDelta().y();
        double factor = qPow(_zoom_factor_base, angle);
        gentle_zoom(factor);
        return true;
      }
    }
  }
  Q_UNUSED(object)
  return false;
}

Usage example:

Graphics_view_zoom* z = new Graphics_view_zoom(ui->graphicsView);
z->set_modifiers(Qt::NoModifier);

Zoom on mouse position QGraphicsView

A possible solution is to focus on the point where the mouse is, scale and recalculate the point where the new center should be:

def wheelEvent(self, event):
    factor = 1.1
    if event.delta() < 0:
        factor = 0.9
    view_pos = event.pos()
    scene_pos = self.mapToScene(view_pos)
    self.centerOn(scene_pos)
    self.scale(factor, factor)
    delta = self.mapToScene(view_pos) - self.mapToScene(self.viewport().rect().center())
    self.centerOn(scene_pos - delta)

How to disable scrolling in QGraphicsView?

I was overriding the main method of mouse wheel but it was causing the effect I wrote above

void MainWindow::wheelEvent( QWheelEvent* event )

Solved by using event filter. The code below provides zoom in/out with holding ctrl button.

bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
    if (event->type() == QEvent::GraphicsSceneWheel)
    {
        ui->GV_image->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
        double scaleFactor = 1.15;
        bool ok = QApplication::keyboardModifiers() & Qt::ControlModifier;
        if (ok)
        {
            QGraphicsSceneWheelEvent *scrollevent = static_cast<QGraphicsSceneWheelEvent *>(event);
            if (scrollevent->delta() > 0)
            {
                ui->GV_image->scale(scaleFactor, scaleFactor);
            }
            else
            {
                ui->GV_image->scale(1/scaleFactor, 1/scaleFactor);
            }
        }

        event->accept();
        return true;
    }
    return false;
}

Put this line into your constructor or other your init function

    QGraphicsView *GV_image;

...

    ui->GV_image->scene()->installEventFilter(this);

Qt 5 : Mouse Wheel event behaviour for zooming image

The code shows that you didn't subclass QGraphicsView, but instead use one in your own widget.

The wheel event will be first sent to the actual graphics view widget. There it is handled with Qt's default behaviour, namely scrolling. Only if you scrolled to the bottom, the graphics view cannot handle the wheel event, and it is propagated to its parent, your class. That's why you only can zoom when scrolled to the border.

To fix this, you should install an event filter. That allows you to intercept the wheel event and process it in your class:

// Outline, not tested
viewer::viewer(QWidget *parent) : QWidget(parent),ui2(new Ui::viewer)
{
    ui2->setupUi(this);
    // Let me handle your events
    ui2->graphicsView->installEventFilter(this);
} 

// should be protected
bool viewer::eventFilter(QObject *obj, QEvent *event) {
  if (event->type() == QEvent::GraphicsSceneWheel) {
      // Your implementation. 
      // You can't use QWheelEvent, as Graphicscene works with its own events...
     handleWheelOnGraphicsScene(static_cast<QGraphicsSceneWheelEvent*> (event));

     // Don't propagate
     return true;
  }

  // Other events should propagate
  return false;
}

Update
I just figured out that the event filter will not recieve GraphicsSceneWheel events on the graphics view. Instead, you have to install the filter on the Graphics Scene. Also, you have to call event->accept() so that it will not propagated.

So Updated Code:

// In Constructor, or where appropriate
ui2->graphicsView->scene()->installEventFilter(this);

bool viewer::eventFilter(QObject *obj, QEvent *event) {
    if (event->type() == QEvent::GraphicsSceneWheel) {
        handleWheelOnGraphicsScene(static_cast<QGraphicsSceneWheelEvent*> (event));

        // Don't propagate
        event->accept();
        return true;
    }
    return false;
}

also note that handleWheelOnGraphicsScene or whatever you want to call it, should be a private method, and doesn't have to be a slot.

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