Fix the fitViewportToGeoShape calculation

The current implementation is calculating the bounding box of
a geocircle based on moving along a circle of latitude by the
distance of a circle's radius. Unfortunately the distances
on a small circle of a sphere are greater than great circle
distances. Reimplement the calculation using tangential
points between a geocircle and meridians.

Do not center the viewport in the center of the geoshape,
use the center of the bounding box instead.

Simplify zoom level adjustment calculations, use the reference
world plane to get rid of longitude wrapping and rounding
erros.

Finally update a viewport unit test, adjust the minimum map
size to 256x256, so fitToViewport calls have chance to succeed.
Fix out of order test execution.

Task-number: QTBUG-54337
Change-Id: I61726a4eb7183470c493ceb03d101f3a75305121
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

1 files changed, 73 insertions, 32 deletions

diff --git a/src/imports/location/qdeclarativegeomap.cpp b/src/imports/location/qdeclarativegeomap.cpp
index 7647a70b..082eaf98 100644
--- a/src/imports/location/qdeclarativegeomap.cpp
+++ b/src/imports/location/qdeclarativegeomap.cpp
@@ -168,6 +168,8 @@ QT_BEGIN_NAMESPACE
     application should open the link in a browser or display its contents to the user.
 */
 
+static const qreal EARTH_MEAN_RADIUS = 6371007.2;
+
 QDeclarativeGeoMap::QDeclarativeGeoMap(QQuickItem *parent)
         : QQuickItem(parent),
         m_plugin(0),
@@ -771,32 +773,63 @@ QColor QDeclarativeGeoMap::color() const
 
 void QDeclarativeGeoMap::fitViewportToGeoShape()
 {
-    if (!m_map) return;
+    int margins  = 10;
+    if (!m_map || width() <= margins || height() <= margins)
+        return;
 
-    double bboxWidth;
-    double bboxHeight;
-    QGeoCoordinate centerCoordinate;
+    QGeoCoordinate topLeft;
+    QGeoCoordinate bottomRight;
 
     switch (m_region.type()) {
     case QGeoShape::RectangleType:
     {
         QGeoRectangle rect = m_region;
-        QDoubleVector2D topLeftPoint = m_map->coordinateToItemPosition(rect.topLeft(), false);
-        QDoubleVector2D botRightPoint = m_map->coordinateToItemPosition(rect.bottomRight(), false);
-        bboxWidth = qAbs(topLeftPoint.x() - botRightPoint.x());
-        bboxHeight = qAbs(topLeftPoint.y() - botRightPoint.y());
-        centerCoordinate = rect.center();
+        topLeft = rect.topLeft();
+        bottomRight = rect.bottomRight();
         break;
     }
     case QGeoShape::CircleType:
     {
+        const double pi = M_PI;
         QGeoCircle circle = m_region;
-        centerCoordinate = circle.center();
-        QGeoCoordinate edge = centerCoordinate.atDistanceAndAzimuth(circle.radius(), 90);
-        QDoubleVector2D centerPoint = m_map->coordinateToItemPosition(centerCoordinate, false);
-        QDoubleVector2D edgePoint = m_map->coordinateToItemPosition(edge, false);
-        bboxWidth = qAbs(centerPoint.x() - edgePoint.x()) * 2;
-        bboxHeight = bboxWidth;
+        QGeoCoordinate centerCoordinate = circle.center();
+
+        // calculate geo bounding box of the circle
+        // circle tangential points with meridians and the north pole create
+        // spherical triangle, we use spherical law of sines
+        // sin(lon_delta_in_rad)/sin(r_in_rad) =
+        // sin(alpha_in_rad)/sin(pi/2 - lat_in_rad), where:
+        // * lon_delta_in_rad - delta of longitudes of circle center
+        //   and tangential points
+        // * r_in_rad - angular radius of the circle
+        // * lat_in_rad - latitude of circle center
+        // * alpha_in_rad - angle between meridian and radius to the circle =>
+        //   this is tangential point => sin(alpha) = 1
+        // * lat_delta_in_rad - delta of latitudes of circle center and
+        //   latitude of points where great circle (going through circle
+        //   center) crosses circle and the pole
+
+        double r_in_rad = circle.radius() / EARTH_MEAN_RADIUS; // angular r
+        double lat_delta_in_deg = r_in_rad * 180 / pi;
+        double lon_delta_in_deg = std::asin(std::sin(r_in_rad) /
+               std::cos(centerCoordinate.latitude() * pi / 180)) * 180 / pi;
+
+        topLeft.setLatitude(centerCoordinate.latitude() + lat_delta_in_deg);
+        topLeft.setLongitude(centerCoordinate.longitude() - lon_delta_in_deg);
+        bottomRight.setLatitude(centerCoordinate.latitude()
+                                - lat_delta_in_deg);
+        bottomRight.setLongitude(centerCoordinate.longitude()
+                                 + lon_delta_in_deg);
+
+        // adjust if circle reaches poles => cross all meridians and
+        // fit into Mercator projection bounds
+        if (topLeft.latitude() > 90 || bottomRight.latitude() < -90) {
+            topLeft.setLatitude(qMin(topLeft.latitude(), 85.05113));
+            topLeft.setLongitude(-180.0);
+            bottomRight.setLatitude(qMax(bottomRight.latitude(),
+                                                 -85.05113));
+            bottomRight.setLongitude(180.0);
+        }
         break;
     }
     case QGeoShape::UnknownType:
@@ -805,28 +838,36 @@ void QDeclarativeGeoMap::fitViewportToGeoShape()
         return;
     }
 
-    // position camera to the center of bounding box
-    setProperty("center", QVariant::fromValue(centerCoordinate));
+    // adjust zoom, use reference world to keep things simple
+    // otherwise we would need to do the error prone longitudes
+    // wrapping
+    QDoubleVector2D topLeftPoint =
+            m_map->referenceCoordinateToItemPosition(topLeft);
+    QDoubleVector2D bottomRightPoint =
+            m_map->referenceCoordinateToItemPosition(bottomRight);
 
-    //If the shape is empty we just change centerposition, not zoom
-    if (bboxHeight == 0 && bboxWidth == 0)
-        return;
+    double bboxWidth = bottomRightPoint.x() - topLeftPoint.x();
+    double bboxHeight = bottomRightPoint.y() - topLeftPoint.y();
 
-    // adjust zoom
-    double bboxWidthRatio = bboxWidth / (bboxWidth + bboxHeight);
-    double mapWidthRatio = width() / (width() + height());
-    double zoomRatio;
+    // find center of the bounding box
+    QGeoCoordinate centerCoordinate =
+            m_map->referenceItemPositionToCoordinate(
+                (topLeftPoint + bottomRightPoint)/2);
 
-    if (bboxWidthRatio > mapWidthRatio)
-        zoomRatio = bboxWidth / width();
-    else
-        zoomRatio = bboxHeight / height();
-
-    qreal newZoom = std::log10(zoomRatio) / std::log10(0.5);
+    // position camera to the center of bounding box
+    setCenter(centerCoordinate);
 
-    newZoom = std::floor(qMax(minimumZoomLevel(), (m_map->mapController()->zoom() + newZoom)));
-    setProperty("zoomLevel", QVariant::fromValue(newZoom));
+    // if the shape is empty we just change center position, not zoom
+    if (bboxHeight == 0 && bboxWidth == 0)
+        return;
 
+    double zoomRatio = qMax(bboxWidth / (width() - margins),
+                            bboxHeight / (height() - margins));
+    // fixme: use log2 with c++11
+    zoomRatio = std::log(zoomRatio) / std::log(2.0);
+    double newZoom = qMax(minimumZoomLevel(), m_map->mapController()->zoom()
+                          - zoomRatio);
+    setZoomLevel(newZoom);
     m_validRegion = true;
 }