mapwidgethelper: add means to obtain a small circle radius

Based on a QGeoCoordinate, calculateSmallCircleRadius() calls
some QML Map methods to obtain how big a circle is in meters if
a circle is drawn over the Map widget with radius SMALL_CIRCLE_RADIUS_PX
pixels.

This is called a "small circle" of a sphere:
https://en.wikipedia.org/wiki/Circle_of_a_sphere

This "small circle" radius becomes huge if the map is zoomed out, while
quite small if the map is really zoomed in.

The idea behind this circle is to be able to select multiple nearby dives,
when clicking on the map (TODO).

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>

2 files changed, 27 insertions, 0 deletions

diff --git a/mobile-widgets/qmlmapwidgethelper.cpp b/mobile-widgets/qmlmapwidgethelper.cpp
index 83a78582e..27505d192 100644
--- a/mobile-widgets/qmlmapwidgethelper.cpp
+++ b/mobile-widgets/qmlmapwidgethelper.cpp
@@ -10,6 +10,7 @@
 #include "qt-models/maplocationmodel.h"

 

 #define MIN_DISTANCE_BETWEEN_DIVE_SITES_M 50.0

+#define SMALL_CIRCLE_RADIUS_PX            26.0

 

 MapWidgetHelper::MapWidgetHelper(QObject *parent) : QObject(parent)

 {

@@ -61,6 +62,30 @@ void MapWidgetHelper::selectedLocationChanged(MapLocation *location)
 	qDebug() << location;

 }

 

+/*

+ * Based on a 2D Map widget circle with center "coord" and radius SMALL_CIRCLE_RADIUS_PX,

+ * obtain a "small circle" with radius m_smallCircleRadius in meters:

+ *     https://en.wikipedia.org/wiki/Circle_of_a_sphere

+ *

+ * The idea behind this circle is to be able to select multiple nearby dives, when clicking on

+ * the map. This code can be in QML, but it is in C++ instead for performance reasons.

+ *

+ * This can be made faster with an exponential regression [a * exp(b * x)], with a pretty

+ * decent R-squared, but it becomes bound to map provider zoom level mappings and the

+ * SMALL_CIRCLE_RADIUS_PX value, which makes the code hard to maintain.

+ */

+void MapWidgetHelper::calculateSmallCircleRadius(QGeoCoordinate coord)

+{

+	QPointF point;

+	QMetaObject::invokeMethod(m_map, "fromCoordinate", Q_RETURN_ARG(QPointF, point),

+	                          Q_ARG(QGeoCoordinate, coord), Q_ARG(bool, false));

+	QPointF point2(point.x() + SMALL_CIRCLE_RADIUS_PX, point.y());

+	QGeoCoordinate coord2;

+	QMetaObject::invokeMethod(m_map, "toCoordinate", Q_RETURN_ARG(QGeoCoordinate, coord2),

+	                          Q_ARG(QPointF, point2), Q_ARG(bool, false));

+	m_smallCircleRadius = coord2.distanceTo(coord);

+}

+

 void MapWidgetHelper::copyToClipboardCoordinates(QGeoCoordinate coord, bool formatTraditional)

 {

 	bool savep = prefs.coordinates_traditional;

diff --git a/mobile-widgets/qmlmapwidgethelper.h b/mobile-widgets/qmlmapwidgethelper.h
index 3a7f28eb0..d5cf669fc 100644
--- a/mobile-widgets/qmlmapwidgethelper.h
+++ b/mobile-widgets/qmlmapwidgethelper.h
@@ -21,10 +21,12 @@ public:
 	void centerOnDiveSite(struct dive_site *);

 	void reloadMapLocations();

 	Q_INVOKABLE void copyToClipboardCoordinates(QGeoCoordinate coord, bool formatTraditional);

+	Q_INVOKABLE void calculateSmallCircleRadius(QGeoCoordinate coord);

 

 private:

 	QObject *m_map;

 	MapLocationModel *m_mapLocationModel;

+	qreal m_smallCircleRadius;

 

 private slots:

 	void selectedLocationChanged(MapLocation *);