[core] Simplify Screen to Geometry coordinate conversion

2 files changed, 8 insertions, 6 deletions

diff --git a/src/mbgl/annotation/point_annotation_impl.cpp b/src/mbgl/annotation/point_annotation_impl.cpp
index b0bce8c308..b54995bfdd 100644
--- a/src/mbgl/annotation/point_annotation_impl.cpp
+++ b/src/mbgl/annotation/point_annotation_impl.cpp
@@ -12,15 +12,15 @@ void PointAnnotationImpl::updateLayer(const TileID& tileID, AnnotationTileLayer&
     std::unordered_map<std::string, std::string> featureProperties;
     featureProperties.emplace("sprite", point.icon.empty() ? std::string("default_marker") : point.icon);
 
-    const mbgl::ScreenCoordinate pp = point.position.project();
-    const uint32_t z2 = 1 << tileID.z;
-    const uint32_t x = pp.x * z2;
-    const uint32_t y = pp.y * z2;
-    const GeometryCoordinate coordinate(GeometryTileFeature::defaultExtent * (pp.x * z2 - x), GeometryTileFeature::defaultExtent * (pp.y * z2 - y));
+    mbgl::ScreenCoordinate projected = point.position.project();
+    projected *= 1 << tileID.z;
+    projected.x -= int16_t(projected.x);
+    projected.y -= int16_t(projected.y);
+    projected *= GeometryTileFeature::defaultExtent;
 
     layer.features.emplace_back(
         std::make_shared<const AnnotationTileFeature>(FeatureType::Point,
-                                                      GeometryCollection {{ {{ coordinate }} }},
+                                                      GeometryCollection {{ {{ GeometryCoordinate { projected } }} }},
                                                       featureProperties));
 }
 
diff --git a/src/mbgl/tile/geometry_tile.hpp b/src/mbgl/tile/geometry_tile.hpp
index 3fcb910371..6236e4e889 100644
--- a/src/mbgl/tile/geometry_tile.hpp
+++ b/src/mbgl/tile/geometry_tile.hpp
@@ -25,6 +25,8 @@ enum class FeatureType : uint8_t {
 };
 
 // Normalized vector tile coordinates.
+// Each geometry coordinate represents a point in a bidimensional space,
+// varying from -V...0...+V, where V is the maximum extent applicable.
 using GeometryCoordinate  = vec2<int16_t>;
 using GeometryCoordinates = std::vector<GeometryCoordinate>;
 using GeometryCollection  = std::vector<GeometryCoordinates>;