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Offline Mapping



  • Hi everyone. Im trying to do offline map. There is an example called lightmaps. I did not understand one thing. There are too many tiles according to zoom level. Even we have zoom level 2, there were 16 tiles. How can we declare all of them ?

    QString path = "http://tile.openstreetmap.org/%1/%2/%3.png";
    

    In the example, it was doing in that way. What refers %1, %2 and %3 i didnt understand it.
    Can anybody help me in this issue?

    https://doc.qt.io/archives/qt-4.8/qt-demos-embedded-lightmaps-example.html


  • Lifetime Qt Champion

    @suslucoder In another thread I told you what it does and gave you a link - did you check that?
    I post the link once more: https://doc.qt.io/qt-5/qstring.html#arg



  • @suslucoder
    And in addition to @jsulm, if you are taking this from that example code look at how they use the string in their code?



  • @jsulm I've checked. I couldnt find the same thing on that example.
    It gives an osm link, I dont think they are used QString i which refers to %1, or anything like that.

    There is just one line which include QString. It is the one I asked.



  • @suslucoder
    They are defining that string but never using it? You could always have told us in which file in the example code...



  • @JonB on slippymap.cpp

    #include <QtWidgets>
    #include <QtNetwork>
    #include "slippymap.h"
    #include "qmath.h"
    
    uint qHash(const QPoint& p)
    {
        return p.x() * 17 ^ p.y();
    }
    
    // tile size in pixels
    const int tdim = 256;
    
    QPointF tileForCoordinate(qreal lat, qreal lng, int zoom)
    {
        qreal radianLat = qDegreesToRadians(lat);
        qreal zn = static_cast<qreal>(1 << zoom);
        qreal tx = (lng + 180.0) / 360.0;
        qreal ty = 0.5 - log(tan(radianLat) + 1.0 / cos(radianLat)) / M_PI / 2.0;
        return QPointF(tx * zn, ty * zn);
    }
    
    qreal longitudeFromTile(qreal tx, int zoom)
    {
        qreal zn = static_cast<qreal>(1 << zoom);
        qreal lat = tx / zn * 360.0 - 180.0;
        return lat;
    }
    
    qreal latitudeFromTile(qreal ty, int zoom)
    {
        qreal zn = static_cast<qreal>(1 << zoom);
        qreal n = M_PI - 2 * M_PI * ty / zn;
        return qRadiansToDegrees(atan(sinh(n)));
    }
    
    
    SlippyMap::SlippyMap(QObject *parent)
        : QObject(parent), width(400), height(300), zoom(15),
          latitude(59.9138204), longitude(10.7387413)
    {
        m_emptyTile = QPixmap(tdim, tdim);
        m_emptyTile.fill(Qt::lightGray);
    
        QNetworkDiskCache *cache = new QNetworkDiskCache;
        cache->setCacheDirectory(QStandardPaths::writableLocation(QStandardPaths::CacheLocation));
        m_manager.setCache(cache);
        connect(&m_manager, SIGNAL(finished(QNetworkReply*)),
                this, SLOT(handleNetworkData(QNetworkReply*)));
    }
    
    void SlippyMap::invalidate()
    {
        if (width <= 0 || height <= 0)
            return;
    
        QPointF ct = tileForCoordinate(latitude, longitude, zoom);
        qreal tx = ct.x();
        qreal ty = ct.y();
    
        // top-left corner of the center tile
        int xp = width / 2 - (tx - floor(tx)) * tdim;
        int yp = height / 2 - (ty - floor(ty)) * tdim;
    
        // first tile vertical and horizontal
        int xa = (xp + tdim - 1) / tdim;
        int ya = (yp + tdim - 1) / tdim;
        int xs = static_cast<int>(tx) - xa;
        int ys = static_cast<int>(ty) - ya;
    
        // offset for top-left tile
        m_offset = QPoint(xp - xa * tdim, yp - ya * tdim);
    
        // last tile vertical and horizontal
        int xe = static_cast<int>(tx) + (width - xp - 1) / tdim;
        int ye = static_cast<int>(ty) + (height - yp - 1) / tdim;
    
        // build a rect
        m_tilesRect = QRect(xs, ys, xe - xs + 1, ye - ys + 1);
    
        if (m_url.isEmpty())
            download();
    
        emit updated(QRect(0, 0, width, height));
    }
    
    void SlippyMap::render(QPainter *p, const QRect &rect)
    {
        for (int x = 0; x <= m_tilesRect.width(); ++x)
            for (int y = 0; y <= m_tilesRect.height(); ++y) {
                QPoint tp(x + m_tilesRect.left(), y + m_tilesRect.top());
                QRect box = tileRect(tp);
                if (rect.intersects(box)) {
                    if (m_tilePixmaps.contains(tp))
                        p->drawPixmap(box, m_tilePixmaps.value(tp));
                    else
                        p->drawPixmap(box, m_emptyTile);
                }
            }
    }
    
    void SlippyMap::pan(const QPoint &delta)
    {
        QPointF dx = QPointF(delta) / qreal(tdim);
        QPointF center = tileForCoordinate(latitude, longitude, zoom) - dx;
        latitude = latitudeFromTile(center.y(), zoom);
        longitude = longitudeFromTile(center.x(), zoom);
        invalidate();
    }
    
    void SlippyMap::handleNetworkData(QNetworkReply *reply)
    {
        QImage img;
        QPoint tp = reply->request().attribute(QNetworkRequest::User).toPoint();
        if (!reply->error())
            if (!img.load(reply, 0))
                img = QImage();
        reply->deleteLater();
        m_tilePixmaps[tp] = QPixmap::fromImage(img);
        if (img.isNull())
            m_tilePixmaps[tp] = m_emptyTile;
        emit updated(tileRect(tp));
    
        // purge unused spaces
        const QRect bound = m_tilesRect.adjusted(-2, -2, 2, 2);
        for (auto it = m_tilePixmaps.keyBegin(); it != m_tilePixmaps.keyEnd(); ++it) {
            const QPoint &tp = *it;
            if (!bound.contains(tp))
                m_tilePixmaps.remove(tp);
        }
    
        download();
    }
    
    void SlippyMap::download()
    {
        QPoint grab(0, 0);
        for (int x = 0; x <= m_tilesRect.width(); ++x)
            for (int y = 0; y <= m_tilesRect.height(); ++y) {
                QPoint tp = m_tilesRect.topLeft() + QPoint(x, y);
                if (!m_tilePixmaps.contains(tp)) {
                    grab = tp;
                    break;
                }
            }
        if (grab == QPoint(0, 0)) {
            m_url = QUrl();
            return;
        }
    
        QString path = "http://tile.openstreetmap.org/%1/%2/%3.png";
        m_url = QUrl(path.arg(zoom).arg(grab.x()).arg(grab.y()));
        QNetworkRequest request;
        request.setUrl(m_url);
        request.setRawHeader("User-Agent", "The Qt Company (Qt) Graphics Dojo 1.0");
        request.setAttribute(QNetworkRequest::User, QVariant(grab));
        m_manager.get(request);
    }
    
    QRect SlippyMap::tileRect(const QPoint &tp)
    {
        QPoint t = tp - m_tilesRect.topLeft();
        int x = t.x() * tdim + m_offset.x();
        int y = t.y() * tdim + m_offset.y();
        return QRect(x, y, tdim, tdim);
    }
    


  • @suslucoder
    So why do you say it does not use it, or it's doing something else??

        QString path = "http://tile.openstreetmap.org/%1/%2/%3.png";
        m_url = QUrl(path.arg(zoom).arg(grab.x()).arg(grab.y()));
    

    It's using it on the following line!



  • @JonB you are so clear!
    I wouldn't ask such a question if I understood what they were doing



  • @suslucoder
    OK, forget the QUrl part on that second line. path is a QString. And it's using path.arg().arg().... Look up exactly what @jsulm referred you to:

    I post the link once more: https://doc.qt.io/qt-5/qstring.html#arg

    That explains exactly why there are %1, %2s etc. in the string and what QString::arg() does with them. He & I don't know how to explain any better!


  • Lifetime Qt Champion

    @suslucoder Maybe one example:

    QString path = "http://tile.openstreetmap.org/%1/%2/%3.png";
    qDebug() << path.arg("ONE").arg("TWO").arg("THREE");
    

    Will output: http://tile.openstreetmap.org/ONE/TWO/THREE.png



  • @jsulm understand now. Thank you



  • @jsulm I understand it. You locked my question, but it was another question.
    Im trying to find a way for tile a map.


  • Lifetime Qt Champion

    @suslucoder said in Offline Mapping:

    You locked my question

    Which one?


  • Lifetime Qt Champion

    @jsulm said in Offline Mapping:

    Which one?

    It was me. The question was to similar.