https://stackoverflow.com/questions/19113532
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RussianSuch 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);
