fixes #476 & #853: pixel/meter/latlng conversion routines in core & iOS

3 files changed, 96 insertions, 0 deletions

diff --git a/include/mbgl/util/constants.hpp b/include/mbgl/util/constants.hpp
index 78c92643d2..069e6e41ae 100644
--- a/include/mbgl/util/constants.hpp
+++ b/include/mbgl/util/constants.hpp
@@ -8,6 +8,13 @@ namespace mbgl {
 namespace util {
 
 extern const float tileSize;
+
+extern const double DEG2RAD;
+extern const double RAD2DEG;
+extern const double M2PI;
+extern const double EARTH_RADIUS_M;
+extern const double LATITUDE_MAX;
+
 }
 
 namespace debug {
diff --git a/include/mbgl/util/geo.hpp b/include/mbgl/util/geo.hpp
new file mode 100644
index 0000000000..1d9986bd91
--- /dev/null
+++ b/include/mbgl/util/geo.hpp
@@ -0,0 +1,24 @@
+#ifndef MBGL_UTIL_GEO
+#define MBGL_UTIL_GEO
+
+namespace mbgl {
+
+struct LatLng {
+    double latitude = 0;
+    double longitude = 0;
+
+    inline LatLng(double lat = 0, double lon = 0)
+        : latitude(lat), longitude(lon) {}
+};
+
+struct ProjectedMeters {
+    double northing = 0;
+    double easting = 0;
+
+    inline ProjectedMeters(double n = 0, double e = 0)
+        : northing(n), easting(e) {}
+};
+
+}
+
+#endif
diff --git a/include/mbgl/util/projection.hpp b/include/mbgl/util/projection.hpp
new file mode 100644
index 0000000000..f301d93005
--- /dev/null
+++ b/include/mbgl/util/projection.hpp
@@ -0,0 +1,65 @@
+#ifndef MBGL_UTIL_PROJECTION
+#define MBGL_UTIL_PROJECTION
+
+#include <mbgl/util/constants.hpp>
+#include <mbgl/util/geo.hpp>
+
+#include <cmath>
+
+namespace mbgl {
+
+class Projection {
+
+public:
+    static inline void getWorldBoundsMeters(ProjectedMeters &sw, ProjectedMeters &ne) {
+        const double d = util::EARTH_RADIUS_M * M_PI;
+
+        sw.easting  = -d;
+        sw.northing = -d;
+
+        ne.easting  =  d;
+        ne.northing =  d;
+    }
+
+    static inline void getWorldBoundsLatLng(LatLng &sw, LatLng &ne) {
+        ProjectedMeters projectedMetersSW = ProjectedMeters();
+        ProjectedMeters projectedMetersNE = ProjectedMeters();
+
+        getWorldBoundsMeters(projectedMetersSW, projectedMetersNE);
+
+        sw = latLngForProjectedMeters(projectedMetersSW);
+        ne = latLngForProjectedMeters(projectedMetersNE);
+    }
+
+    static inline double getMetersPerPixelAtLatitude(const double lat, const double zoom) {
+        const double mapPixelWidthAtZoom = std::pow(2.0, zoom) * util::tileSize;
+        const double constrainedLatitude = std::fmin(std::fmax(lat, -util::LATITUDE_MAX), util::LATITUDE_MAX);
+
+        return std::cos(constrainedLatitude * util::DEG2RAD) * util::M2PI * util::EARTH_RADIUS_M / mapPixelWidthAtZoom;
+    }
+
+    static inline const ProjectedMeters projectedMetersForLatLng(const LatLng latLng) {
+        const double constrainedLatitude = std::fmin(std::fmax(latLng.latitude, -util::LATITUDE_MAX), util::LATITUDE_MAX);
+
+        const double m = 1 - 1e-15;
+        const double f = std::fmin(std::fmax(std::sin(util::DEG2RAD * constrainedLatitude), -m), m);
+
+        const double easting  = util::EARTH_RADIUS_M * latLng.longitude * util::DEG2RAD;
+        const double northing = 0.5 * util::EARTH_RADIUS_M * std::log((1 + f) / (1 - f));
+
+        return ProjectedMeters(northing, easting);
+    }
+
+    static inline const LatLng latLngForProjectedMeters(const ProjectedMeters projectedMeters) {
+        double latitude = (2 * std::atan(std::exp(projectedMeters.northing / util::EARTH_RADIUS_M)) - (M_PI / 2)) * util::RAD2DEG;
+        double longitude = projectedMeters.easting * util::RAD2DEG / util::EARTH_RADIUS_M;
+
+        latitude = std::fmin(std::fmax(latitude, -util::LATITUDE_MAX), util::LATITUDE_MAX);
+        
+        return LatLng(latitude, longitude);
+    }
+};
+
+}
+
+#endif