Add additional transformState::latLngToScreenCoordinate getter (#16311)

This change adds a latLngToScreenCoordinate overload, to retrieve
the projected vec4 in additional to the ScreenCoordinate object,
that is useful to detect whether the projected latLng is in front
or behind the camera.

3 files changed, 17 insertions, 2 deletions

diff --git a/src/mbgl/map/transform_state.cpp b/src/mbgl/map/transform_state.cpp
index c5bb38653a..da21487d67 100644
--- a/src/mbgl/map/transform_state.cpp
+++ b/src/mbgl/map/transform_state.cpp
@@ -81,7 +81,7 @@ void TransformState::getProjMatrix(mat4& projMatrix, uint16_t nearZ, bool aligne
     }
 
     const double cameraToCenterDistance = getCameraToCenterDistance();
-    auto offset = getCenterOffset();
+    const ScreenCoordinate offset = getCenterOffset();
 
     // Find the Z distance from the viewport center point
     // [width/2 + offset.x, height/2 + offset.y] to the top edge; to point
@@ -498,11 +498,15 @@ double TransformState::scaleZoom(double s) const {
 }
 
 ScreenCoordinate TransformState::latLngToScreenCoordinate(const LatLng& latLng) const {
+    vec4 p;
+    return latLngToScreenCoordinate(latLng, p);
+}
+
+ScreenCoordinate TransformState::latLngToScreenCoordinate(const LatLng& latLng, vec4& p) const {
     if (size.isEmpty()) {
         return {};
     }
 
-    vec4 p;
     Point<double> pt = Projection::project(latLng, scale) / util::tileSize;
     vec4 c = {{pt.x, pt.y, 0, 1}};
     matrix::transformMat4(p, c, getCoordMatrix());
diff --git a/src/mbgl/map/transform_state.hpp b/src/mbgl/map/transform_state.hpp
index 1561ffc904..183400d417 100644
--- a/src/mbgl/map/transform_state.hpp
+++ b/src/mbgl/map/transform_state.hpp
@@ -196,6 +196,7 @@ public:
 
     // Conversion
     ScreenCoordinate latLngToScreenCoordinate(const LatLng&) const;
+    ScreenCoordinate latLngToScreenCoordinate(const LatLng&, vec4&) const;
     LatLng screenCoordinateToLatLng(const ScreenCoordinate&, LatLng::WrapMode = LatLng::Unwrapped) const;
     // Implements mapbox-gl-js pointCoordinate() : MercatorCoordinate.
     TileCoordinate screenCoordinateToTileCoordinate(const ScreenCoordinate&, uint8_t atZoom) const;
diff --git a/test/map/transform.test.cpp b/test/map/transform.test.cpp
index 506e61aca7..41bf31a032 100644
--- a/test/map/transform.test.cpp
+++ b/test/map/transform.test.cpp
@@ -130,6 +130,16 @@ TEST(Transform, PerspectiveProjection) {
     point = transform.getState().latLngToScreenCoordinate({37.692872969426375, -76.75823239205641});
     ASSERT_NEAR(point.x, 1000.0, 1e-5);
     ASSERT_NEAR(point.y, 0.0, 1e-4);
+
+    mbgl::vec4 p;
+    point = transform.getState().latLngToScreenCoordinate({37.692872969426375, -76.75823239205641}, p);
+    ASSERT_NEAR(point.x, 1000.0, 1e-5);
+    ASSERT_NEAR(point.y, 0.0, 1e-4);
+    ASSERT_GT(p[3], 0.0);
+
+    transform.jumpTo(CameraOptions().withCenter(LatLng{38.0, -77.0}).withZoom(18.0).withPitch(51.56620156));
+    point = transform.getState().latLngToScreenCoordinate({7.692872969426375, -76.75823239205641}, p);
+    ASSERT_LT(p[3], 0.0);
 }
 
 TEST(Transform, UnwrappedLatLng) {