Implementing freely dragging the scene in graphics-view

Question

I would like to implement a free dragging functionality based on C++ Qt's graphics-view.

However, I need to achieve this without changing the sceneRect and without using view->setDragMode(QGraphicsView::ScrollHandDrag) , as some of my business logic relies on the sceneRect for calculations.

Below is the code that can reproduce the issue I am currently facing while still running successfully.

view.h:

#include <QGraphicsView>
#include <QMouseEvent>
#include <QWheelEvent>
#include <qscrollbar.h>
class CView : public QGraphicsView
{
    Q_OBJECT
public:
    CView(QWidget* parent = nullptr) : QGraphicsView(parent) {
        setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
        setResizeAnchor(QGraphicsView::AnchorUnderMouse);
    }

protected:
    void mousePressEvent(QMouseEvent* event) override {
        if (event->button() == Qt::LeftButton) {
            m_lastPos = event->pos();
        }

        QGraphicsView::mousePressEvent(event);
    }

    void mouseMoveEvent(QMouseEvent* event) override {
        if (event->buttons() & Qt::LeftButton) {
            QPoint delta = event->pos() - m_lastPos;

            QRectF sceneRect = this->sceneRect();
            sceneRect.translate(-delta);
            this->setSceneRect(sceneRect);

            ensureVisible(sceneRect);

            m_lastPos = event->pos();
        }

        QGraphicsView::mouseMoveEvent(event);
    }


    void wheelEvent(QWheelEvent* event) override {
        QPoint scrollAmount = event->angleDelta();

        double scaleFactor = 1.15;
        if (scrollAmount.y() > 0) {
            scale(scaleFactor, scaleFactor);
        }
        else if (scrollAmount.y() < 0) {
            scale(1.0 / scaleFactor, 1.0 / scaleFactor);
        }

        // QRectF sceneRect = this->sceneRect();
        // QPointF viewCenter = mapToScene(viewport()->rect().center());
        // sceneRect.moveCenter(viewCenter);
        // this->setSceneRect(sceneRect);

        event->accept();
    }

private:
    QPoint m_lastPos;
};

scene.h:

#include <QObject>
#include <QGraphicsScene>

class CScene : public QGraphicsScene
{
    Q_OBJECT
public:
    CScene() {}

protected:
    void drawBackground(QPainter *painter, const QRectF &rect) override {
        QGraphicsScene::drawBackground(painter, rect);
        painter->fillRect(sceneRect(), Qt::black);
    }
};

main.cpp:

#include "View.h"
#include "Scene.h"
#include <QApplication>

int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    CView* view = new CView;
    view->resize(600, 500);
    CScene* scene = new CScene;
    scene->setSceneRect(0, 0, 300, 200);
    view->setScene(scene);
    view->show();
    return a.exec();
}

I am encountering two issues:

When I start dragging the mouse, the view jumps (I noticed that if the scrollbars are not hidden, their values become the maximum. I'm not sure if this is due to a coordinate problem or because I added ensureVisible(sceneRect) in the mouseMoveEvent).
If I embed the view into a widget and place a QPushButton inside the widget, when I click the button and call view->fitInView(sceneRect, Qt::KeepAspectRatio), the scene's position is not centered within the view.

Could you also add the desired behavior? I cannot figure it out, but it sounds like you're trying to replicate something and add your own tweaks to it. So it would be helpful if you could show us an example of the standard `QGraphicsView` behavior. (For both issues) — Abderrahmene Rayene Mihoub, Jun 27 '23 at 18:12

score 0 · Accepted Answer · answered Jun 28 '23 at 08:35

I have identified the source of the problem.

The first issue indeed occurred due to the ensureVisible(sceneRect) function, which aims to ensure that the given rectangle is visible within the view.

Regarding the second issue, by calling qDebug() << viewportTransform(), I discovered that m31 and m32 (the translation components) have changed, resulting in the offset problem. To resolve this issue, all I need to do is set m31 and m32 to 0. This can be achieved by using QGraphicsView::setSceneRect(0, 0, width, height).

Implementing freely dragging the scene in graphics-view

1 Answers1