Zoom functionality using Qt

Question

Current implementation, zooms towards the center of View so items present in the top left corner or the current mouse pointer is not visible when we Zoom it.

I want

Answer 1

To zoom always centered at mouse pointer – the position of mouse pointer just has to become the origin of scaling.

It sounds that simple but I struggled a bit to prepare a demonstration. (I'm not that good in linear algebra, sorry.) However, I finally got it running.

My sample code testQWidget-Zoom.cc:

#include <vector>
#include <QtWidgets>

// class for widget to demonstrate zooming
class Canvas: public QWidget {
  // types:
  private:
    struct Geo {
      QRectF rect; QColor color;
      Geo(const QRectF &rect, const QColor &color):
        rect(rect), color(color)
      { }
    };
  // variables:
  private:
    bool _initDone : 1; // flag: true ... sample geo created
    std::vector<Geo> _scene; // contents to render
    QMatrix _mat; // view matrix
  // methods:
  public: 
    // constructor.
    Canvas(): QWidget(), _initDone(false) { }
    // destructor.
    virtual ~Canvas() = default;
    // disabled:
    Canvas(const Canvas&) = delete;
    Canvas& operator=(const Canvas&) = delete;
  private:
    // initializes sample geo
    void init()
    {
      if (_initDone) return;
      _initDone = true;
      // build scene (with NDC i.e. view x/y range: [-1, 1])
      _scene.emplace_back(Geo(QRectF(-1.0f, -1.0f, 2.0f, 2.0f), QColor(0x000000u)));
      _scene.emplace_back(Geo(QRectF(-0.2f, -0.2f, 0.4f, 0.4f), QColor(0x00ff00u)));
      _scene.emplace_back(Geo(QRectF(-0.8f, -0.8f, 0.4f, 0.4f), QColor(0xff0000u)));
      _scene.emplace_back(Geo(QRectF(-0.8f, 0.4f, 0.4f, 0.4f), QColor(0x0000ffu)));
      _scene.emplace_back(Geo(QRectF(0.4f, 0.4f, 0.4f, 0.4f), QColor(0xff00ffu)));
      _scene.emplace_back(Geo(QRectF(0.4f, -0.8f, 0.4f, 0.4f), QColor(0xffff00u)));
      // get initial scaling
      const int wView = width(), hView = height();
      _mat.scale(wView / 2, hView / 2);
      _mat.translate(1, 1);
    }
  protected:
    virtual void paintEvent(QPaintEvent *pQEvent) override
    {
      init();
      // render
      QPainter qPainter(this);
#if 0 // This scales line width as well:
      qPainter.setMatrix(_mat);
      for (const Geo &geo : _scene) {
        qPainter.setPen(geo.color);
        qPainter.drawRect(geo.rect);
      }
#else // This transforms only coordinates:
      for (const Geo &geo : _scene) {
        qPainter.setPen(geo.color);
        QRectF rect(geo.rect.topLeft() * _mat, geo.rect.bottomRight() * _mat);
        qPainter.drawRect(rect);
      }
#endif // 0
    }
    virtual void wheelEvent(QWheelEvent *pQEvent) override
    {
      //qDebug() << "Wheel Event:"
      //qDebug() << "mouse pos:" << pQEvent->pos();
      // pos() -> virtual canvas
      bool matInvOK = false;
      QMatrix matInv = _mat.inverted(&matInvOK);
      if (!matInvOK) {
        qDebug() << "View matrix not invertible!";
        return;
      }
      QPointF posNDC
        = QPointF(pQEvent->pos().x(), pQEvent->pos().y()) * matInv;
      //qDebug() << "mouse pos (NDC):" << posNDC;
      float delta = 1.0f + pQEvent->angleDelta().y() / 1200.0f;
      //qDebug() << "angleDelta:" << pQEvent->angleDelta().y();
      //qDebug() << "scale factor:" << delta;
      _mat.translate(posNDC.x(), posNDC.y()); // origin to spot
      _mat.scale(delta, delta); // scale
      _mat.translate(-posNDC.x(), -posNDC.y()); // spot to origin
      update();
      pQEvent->accept();
    }
};


int main(int argc, char **argv)
{
  QApplication app(argc, argv);
  Canvas canvas;
  canvas.resize(512, 512);
  canvas.show();
  // runtime loop
  return app.exec();
}

and these three lines are the actual interesting ones (in Canvas::wheelEvent()):

      _mat.translate(posNDC.x(), posNDC.y()); // origin to spot
      _mat.scale(delta, delta); // scale
      _mat.translate(-posNDC.x(), -posNDC.y()); // spot to origin

And this is how it looks:

The first image is a snapshot of the application just after starting it.

Then I pointed into the center of the red rectangle and turned the wheel slightly. The red rectangle grew around the mouse pointer as intended.

---

1^st Update:

And, this is the updated version which uses screen coordinates directly (instead of converting everything to NDCs):

#include <vector>
#include <QtWidgets>

// class for widget to demonstrate zooming
class Canvas: public QWidget {
  // types:
  private:
    struct Geo {
      QRectF rect; QColor color;
      Geo(const QRectF &rect, const QColor &color):
        rect(rect), color(color)
      { }
    };
  // variables:
  private:
    bool _initDone : 1; // flag: true ... sample geo created
    std::vector<Geo> _scene; // contents to render
    QMatrix _mat; // view matrix
  // methods:
  public: 
    // constructor.
    Canvas(): QWidget(), _initDone(false) { }
    // destructor.
    virtual ~Canvas() = default;
    // disabled:
    Canvas(const Canvas&) = delete;
    Canvas& operator=(const Canvas&) = delete;
  private:
    // initializes sample geo
    void init()
    {
      if (_initDone) return;
      _initDone = true;
      const int wView = width(), hView = height();
      // build scene (with NDC i.e. view x/y range: [-1, 1])
      _scene.emplace_back(Geo(QRectF(-1.0f, -1.0f, 2.0f, 2.0f), QColor(0x000000u)));
      _scene.emplace_back(Geo(QRectF(-0.2f, -0.2f, 0.4f, 0.4f), QColor(0x00ff00u)));
      _scene.emplace_back(Geo(QRectF(-0.8f, -0.8f, 0.4f, 0.4f), QColor(0xff0000u)));
      _scene.emplace_back(Geo(QRectF(-0.8f, 0.4f, 0.4f, 0.4f), QColor(0x0000ffu)));
      _scene.emplace_back(Geo(QRectF(0.4f, 0.4f, 0.4f, 0.4f), QColor(0xff00ffu)));
      _scene.emplace_back(Geo(QRectF(0.4f, -0.8f, 0.4f, 0.4f), QColor(0xffff00u)));
      // scale geometry to screen coordinates
      QMatrix mat;
      mat.scale(wView / 2, hView / 2);
      mat.translate(1, 1);
      for (Geo &geo : _scene) {
        geo.rect = QRectF(geo.rect.topLeft() * mat, geo.rect.bottomRight() * mat);
      }
    }
  protected:
    virtual void paintEvent(QPaintEvent *pQEvent) override
    {
      init();
      // render
      QPainter qPainter(this);
      qPainter.setMatrix(_mat);
      for (const Geo &geo : _scene) {
        qPainter.setPen(geo.color);
        qPainter.drawRect(geo.rect);
      }
    }
    virtual void wheelEvent(QWheelEvent *pQEvent) override
    {
      //qDebug() << "Wheel Event:";
      //qDebug() << "mouse pos:" << pQEvent->pos();
      float delta = 1.0f + pQEvent->angleDelta().y() / 1200.0f;
      //qDebug() << "angleDelta:" << pQEvent->angleDelta().y();
      //qDebug() << "scale factor:" << delta;
      _mat.translate(pQEvent->pos().x(), pQEvent->pos().y()); // origin to spot
      _mat.scale(delta, delta); // scale
      _mat.translate(-pQEvent->pos().x(), -pQEvent->pos().y()); // spot to origin
      update();
      pQEvent->accept();
    }
};

int main(int argc, char **argv)
{
  QApplication app(argc, argv);
  Canvas canvas;
  canvas.resize(256, 256);
  canvas.show();
  // runtime loop
  return app.exec();
}

The relevant three lines didn't change much – the mouse coordinates are applied directly to transformation.

Btw. I changed the rendering – it now scales line width as well as I used

      qPainter.setMatrix(_mat);

in Canvas::paintEvent() instead of transforming all points "manually".

The snapshot shows the application after I pointed into the center of the blue rectangle and turned the mouse wheel:

---

2^nd Update:

The suggested matrix manipulation works in a QGraphicsView as well:

#include <QtWidgets>

// class for widget to demonstrate zooming
class Canvas: public QGraphicsView {
  // methods:
  public: 
    // constructor.
    Canvas() = default;
    // destructor.
    virtual ~Canvas() = default;
    // disabled:
    Canvas(const Canvas&) = delete;
    Canvas& operator=(const Canvas&) = delete;

  protected:

    virtual void wheelEvent(QWheelEvent *pQEvent) override
    {
      //qDebug() << "Wheel Event:";
      // pos() -> virtual canvas
      QPointF pos = mapToScene(pQEvent->pos());
      //qDebug() << "mouse pos:" << pos;
      // scale from wheel angle
      float delta = 1.0f + pQEvent->angleDelta().y() / 1200.0f;
      //qDebug() << "angleDelta:" << pQEvent->angleDelta().y();
      //qDebug() << "scale factor:" << delta;
      // modify transform matrix
      QTransform xform = transform();
      xform.translate(pos.x(), pos.y()); // origin to spot
      xform.scale(delta, delta); // scale
      xform.translate(-pos.x(), -pos.y()); // spot to origin
      setTransform(xform);
      //qDebug() << "transform:" << xform;
      // force update
      update();
      pQEvent->accept();
    }
};

QRectF toScr(QWidget *pQWidget, float x, float y, float w, float h)
{
  const int wView = pQWidget->width(), hView = pQWidget->height();
  const int s = wView < hView ? wView : hView;
  return QRectF(
    (0.5f * x + 0.5f) * s, (0.5f * y + 0.5f) * s,
    0.5f * w * s, 0.5f * h * s);
}

int main(int argc, char **argv)
{
  QApplication app(argc, argv);
  // setup GUI
  Canvas canvas;
  canvas.setTransformationAnchor(QGraphicsView::NoAnchor);
  canvas.resize(256, 256);
  canvas.show();
  // prepare scene
  QGraphicsScene qGScene;
  qGScene.addRect(toScr(canvas.viewport(), -1.0f, -1.0f, 2.0f, 2.0f), QColor(0x000000u));
  qGScene.addRect(toScr(canvas.viewport(), -0.2f, -0.2f, 0.4f, 0.4f), QColor(0x00ff00u));
  qGScene.addRect(toScr(canvas.viewport(), -0.8f, -0.8f, 0.4f, 0.4f), QColor(0xff0000u));
  qGScene.addRect(toScr(canvas.viewport(), -0.8f, 0.4f, 0.4f, 0.4f), QColor(0x0000ffu));
  qGScene.addRect(toScr(canvas.viewport(), 0.4f, 0.4f, 0.4f, 0.4f), QColor(0xff00ffu));
  qGScene.addRect(toScr(canvas.viewport(), 0.4f, -0.8f, 0.4f, 0.4f), QColor(0xffff00u));
  canvas.setScene(&qGScene);
  // runtime loop
  return app.exec();
}

Using a QGraphicsView simplifies code as no rendering code is needed – it's already built-in.

As I have not (yet) much experience with QGraphicsView, another issue hit me quite hard: The QGraphicsView is able to fix the view position automati[cg]ally after a transformation has been applied. In my case, this was rather counter-productive as obviously my transformation and the QGraphicsView seemed to "pull" in opposite directions.

Hence, I've learnt my lesson of the day: QGrapicsView::setTransformationAnchor(QGraphicsView::NoAnchor) is necessary to switch off this (in my case not-intended) auto-centering.

The other detail I find worth to notice is QGraphicsView::mapToScene() which can be used to conveniently convert widget coordinates (e.g. mouse coordinates) to scene space.