Calculation of the offset of the view to increase in the position of the mouse cursor

Question

I have a "canvas" on which the user can draw pixels, etc. It works well, but my zoom function currently uses the same origin regardless of mouse position. I would like to implement features such as the behavior of Google Maps:

That is, the start of the zoom should always be the position of the mouse cursor.

What I have now is not quite right ...

My attempts were mostly blows in the dark, but I also tried unsuccessfully to use the code for this answer .

main.cpp:

#include <QGuiApplication>
#include <QtQuick>

class Canvas : public QQuickPaintedItem
{
    Q_OBJECT

public:
    Canvas() :
        mTileWidth(25),
        mTileHeight(25),
        mTilesAcross(10),
        mTilesDown(10),
        mOffset(QPoint(400, 400)),
        mZoomLevel(1)
    {
    }

    void paint(QPainter *painter) override {
        painter->translate(mOffset);

        const int zoomedTileWidth =  mTilesAcross * mZoomLevel;
        const int zoomedTileHeight =  mTilesDown * mZoomLevel;
        const int zoomedMapWidth = qMin(mTilesAcross * zoomedTileWidth, qFloor(width()));
        const int zoomedMapHeight = qMin(mTilesDown * zoomedTileHeight, qFloor(height()));
        painter->fillRect(0, 0, zoomedMapWidth, zoomedMapHeight, QColor(Qt::gray));

        for (int y = 0; y < mTilesDown; ++y) {
            for (int x = 0; x < mTilesAcross; ++x) {
                const QRect rect(x * zoomedTileWidth, y * zoomedTileHeight, zoomedTileWidth, zoomedTileHeight);
                painter->drawText(rect, QString::fromLatin1("%1, %2").arg(x).arg(y));
            }
        }
    }

protected:
    void wheelEvent(QWheelEvent *event) override {
        const int oldZoomLevel = mZoomLevel;
        mZoomLevel = qMax(1, qMin(mZoomLevel + (event->angleDelta().y() > 0 ? 1 : -1), 30));

        const QPoint cursorPosRelativeToOffset = event->pos() - mOffset;

        if (mZoomLevel != oldZoomLevel) {
            mOffset.rx() -= cursorPosRelativeToOffset.x();
            mOffset.ry() -= cursorPosRelativeToOffset.y();

            // Attempts based on /questions/278471/jscrollpane-zoom-relative-to-mouse-position/1392303#1392303
//            mOffset.setX((event->pos().x() * (mZoomLevel - oldZoomLevel)) + (mZoomLevel * -mOffset.x()));
//            mOffset.setY((event->pos().y() * (mZoomLevel - oldZoomLevel)) + (mZoomLevel * -mOffset.y()));

//            mOffset.setX((cursorPosRelativeToOffset.x() * (mZoomLevel - oldZoomLevel)) + (mZoomLevel * -mOffset.x()));
//            mOffset.setY((cursorPosRelativeToOffset.y() * (mZoomLevel - oldZoomLevel)) + (mZoomLevel * -mOffset.y()));

            update();
        }
    }

    void keyReleaseEvent(QKeyEvent *event) override {
        static const int panDistance = 50;
        switch (event->key()) {
        case Qt::Key_Left:
            mOffset.rx() -= panDistance;
            update();
            break;
        case Qt::Key_Right:
            mOffset.rx() += panDistance;
            update();
            break;
        case Qt::Key_Up:
            mOffset.ry() -= panDistance;
            update();
            break;
        case Qt::Key_Down:
            mOffset.ry() += panDistance;
            update();
            break;
        }
    }

private:
    const int mTileWidth;
    const int mTileHeight;
    const int mTilesAcross;
    const int mTilesDown;
    QPoint mOffset;
    int mZoomLevel;
};

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    qmlRegisterType<Canvas>("App", 1, 0, "Canvas");

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

#include "main.moc"

main.qml:

import QtQuick 2.5
import QtQuick.Window 2.2

import App 1.0 as App

Window {
    visible: true
    width: 1200
    height: 900
    title: qsTr("Hello World")

    Shortcut {
        sequence: "Ctrl+Q"
        onActivated: Qt.quit()
    }

    App.Canvas {
        focus: true
        anchors.fill: parent
    }
}

What am I doing wrong in a function wheelEvent()?

Answer 1

R = [x_0, x_0 + w] x [y_0, y_0 + h] . ( ), - T W R. :

a_x, b_x, a_y, b_y , .

(x_c, y_c) R. W T(x_c, y_c). ,

a_x, a_y a_x', a_y' : , (x_c, y_c) R. T'(x_c, y_c) = T(x_c, y_c) - . b_x', b_y' .

- (x_c, y_c) (x_p, y_p) = T(x_c, y_c):

:

mOffset = event->pos() - float(mZoomLevel) / float(oldZoomLevel) *
     (event->pos() - mOffset);