[core] Fix tileCover / TileCoordinate fromScreenCoordinate(#15442)

Remove unecessary tile coordinate unproject -> project  step.
Unproject+project isn't always a noop and this was the issue leading to wrong cover calculation in high pitch values.

Fixes: https://github.com/mapbox/mapbox-gl-native/issues/15442

2 files changed, 13 insertions, 3 deletions

diff --git a/src/mbgl/map/transform_state.cpp b/src/mbgl/map/transform_state.cpp
index 77309a2a55..951c5a6202 100644
--- a/src/mbgl/map/transform_state.cpp
+++ b/src/mbgl/map/transform_state.cpp
@@ -3,6 +3,7 @@
 #include <mbgl/util/constants.hpp>
 #include <mbgl/util/interpolate.hpp>
 #include <mbgl/util/projection.hpp>
+#include <mbgl/util/tile_coordinate.hpp>
 #include <mbgl/math/log2.hpp>
 #include <mbgl/math/clamp.hpp>
 
@@ -289,9 +290,9 @@ ScreenCoordinate TransformState::latLngToScreenCoordinate(const LatLng& latLng)
     return { p[0] / p[3], size.height - p[1] / p[3] };
 }
 
-LatLng TransformState::screenCoordinateToLatLng(const ScreenCoordinate& point, LatLng::WrapMode wrapMode) const {
+TileCoordinate TransformState::screenCoordinateToTileCoordinate(const ScreenCoordinate& point, uint8_t atZoom) const {
     if (size.isEmpty()) {
-        return {};
+        return { {}, 0 };
     }
 
     float targetZ = 0;
@@ -325,7 +326,13 @@ LatLng TransformState::screenCoordinateToLatLng(const ScreenCoordinate& point, L
     double z1 = coord1[2] / w1;
     double t = z0 == z1 ? 0 : (targetZ - z0) / (z1 - z0);
 
-    return Projection::unproject(util::interpolate(p0, p1, t), scale / util::tileSize, wrapMode);
+    Point<double> p = util::interpolate(p0, p1, t) / scale * static_cast<double>(1 << atZoom);
+    return { { p.x, p.y }, static_cast<double>(atZoom) };
+}
+
+LatLng TransformState::screenCoordinateToLatLng(const ScreenCoordinate& point, LatLng::WrapMode wrapMode) const {
+    auto coord = screenCoordinateToTileCoordinate(point, 0);
+    return Projection::unproject(coord.p, 1 / util::tileSize, wrapMode);
 }
 
 mat4 TransformState::coordinatePointMatrix() const {
diff --git a/src/mbgl/map/transform_state.hpp b/src/mbgl/map/transform_state.hpp
index cca42db20f..e81ebed27e 100644
--- a/src/mbgl/map/transform_state.hpp
+++ b/src/mbgl/map/transform_state.hpp
@@ -17,6 +17,7 @@
 namespace mbgl {
 
 class UnwrappedTileID;
+class TileCoordinate;
 
 class TransformState {
     friend class Transform;
@@ -81,6 +82,8 @@ public:
     // Conversion
     ScreenCoordinate latLngToScreenCoordinate(const LatLng&) const;
     LatLng screenCoordinateToLatLng(const ScreenCoordinate&, LatLng::WrapMode = LatLng::Unwrapped) const;
+    // Implements mapbox-gl-js pointCoordinate() : MercatorCoordinate.
+    TileCoordinate screenCoordinateToTileCoordinate(const ScreenCoordinate&, uint8_t atZoom) const;
 
     double zoomScale(double zoom) const;
     double scaleZoom(double scale) const;