QML Map: Limit tile fetch to specific zoom levels.

imbachb

I'm working on an application containing an offline map using the OpenStreetMap plugin.
My offline tile maps only provide tiles between zoom level 1 - 6 due to disk space restrictions. When I zoom in more than zoom level 6 following warnings appear:

QGeoTileRequestManager: Failed to fetch tile (67,41,7) 5 times, giving up. Last error message was: 'Error opening C:/Path/To/offline_tiles/7/67/41.png: The system cannot find the path specified.'

It is important that the user can zoom in all the way, I do not want to prevent that.
Is there a way to limit the map tile fetcher to only fetch tiles up to a certain zoom level?

In the Qt source code I found the following parameter MaximumZoomLevel :
https://code.qt.io/cgit/qt/qtlocation.git/tree/src/plugins/geoservices/osm/qgeotileproviderosm.cpp#n414

     * MinimumZoomLevel and MaximumZoomLevel are also optional, and allow to prevent invalid tile
     * requests to the providers, if they do not support the specific ZL. Default is 0 and 19,
     * respectively.

This parameter would be the solution, however, I do not know how I to set it... The documentation for the OSM plugin does not list such a parameter (https://doc.qt.io/qt-5/location-plugin-osm.html). Is there another way to set this parameter?

My QML:

Window {
    id: win
    objectName: "window"
    visible: true
    width: 512
    height: 512

    Map {
        id: map
        anchors.fill: parent
        zoomLevel: 1
        plugin: Plugin {
            name: 'osm';
            PluginParameter {
                name: 'osm.mapping.custom.host'
                value: 'file:///C:/Path/To/offline_tiles/'
            }
            PluginParameter {
                name: "osm.mapping.providersrepository.disabled"
                value: true
            }
            PluginParameter {
                name: "osm.mapping.cache.disk.size"
                value: "0"
            }
        }

        Component.onCompleted: {
            for( var i_type in supportedMapTypes) {
                console.log(supportedMapTypes[i_type].name)
                if ( supportedMapTypes[i_type].name.localeCompare( "Custom URL Map" ) === 0) {
                    activeMapType = supportedMapTypes[i_type]
                }
            }
        }
    }
}

imbachb

I looked through the Qt source a bit more closely and saw that the "offline" constructor for the TileProvider sets the minimumZoomLevel and maximumZoomLevel by default:

https://code.qt.io/cgit/qt/qtlocation.git/tree/src/plugins/geoservices/osm/qgeotileproviderosm.h#n67

    // Offline constructor. Doesn't need URLRedirector and networkmanager
    TileProvider(const QString &urlTemplate,
             const QString &format,
             const QString &copyRightMap,
             const QString &copyRightData,
             bool highDpi = false,
             int minimumZoomLevel = 0,
             int maximumZoomLevel = 19);

There does not seem to be a way to set those values. QGeoTiledMappingManagerEngineOsm instantiates the TileProvider without setting minimumZoomLevel or maximumZoomLevel, so they are stuck forever at 0 or 19 respectively...
https://code.qt.io/cgit/qt/qtlocation.git/tree/src/plugins/geoservices/osm/qgeotileproviderosm.h#n67

m_providers.push_back(
            new QGeoTileProviderOsm( nmCached,
                QGeoMapType(QGeoMapType::CustomMap, tr("Custom URL Map"), tr("Custom url map view set via urlprefix parameter"), false, false, 8, pluginName, cameraCaps),
                { new TileProvider(tmsServer + QStringLiteral("%z/%x/%y.png"),
                    QStringLiteral("png"),
                    mapCopyright,
                    dataCopyright) }, cameraCaps
                ));

Would be handy if these could be set for offline maps...

akadirdd

use maximumZoomLevel in Map component

imbachb

@akadirdd
Thanks for the suggestion.
The 'maximumZoomLevel' property limits how much I can zoom on the map. But I do not want to limit that. I display icons that can be close to each other. The user has to zoom in close to differentiate the icons. However, at high zoom levels I do not have offline tiles which results in aforementioned tile fetching errors.

I wish we could tell the tile fetcher what zoom levels we provide, so that it doesn't try to fetch tiles that are not there.

For now I ended up ignoring the error with a custom message handler:

// Default message handler to be called to bypass all other warnings.
static const QtMessageHandler QT_DEFAULT_MESSAGE_HANDLER = qInstallMessageHandler(0);
// A custom message handler to intercept warnings
void
customMessageHandler(QtMsgType type, const QMessageLogContext& context, const QString& msg)
{
	if (type == QtWarningMsg &&
	    // suppress following warnings
	    (msg.contains("QGeoTileRequestManager: Failed to fetch tile"))) {
		return;
	}

	(*QT_DEFAULT_MESSAGE_HANDLER)(type, context, msg);
}

And then

qInstallMessageHandler(customMessageHandler);

eduardo-araujo

@imbachb Hi, I'm facing the same problem, I want to increase the value of maximumZoomLevel, but I can't, the maximum is 19.
Did you manage to solve this problem?