[core] avoid symbol flickering when longitude is wrapped

3 files changed, 34 insertions, 3 deletions

diff --git a/src/mbgl/renderer/renderer_impl.cpp b/src/mbgl/renderer/renderer_impl.cpp
index 4380ef24a1..ca9e809977 100644
--- a/src/mbgl/renderer/renderer_impl.cpp
+++ b/src/mbgl/renderer/renderer_impl.cpp
@@ -380,7 +380,8 @@ void Renderer::Impl::render(const UpdateParameters& updateParameters) {
     }
     for (auto it = order.rbegin(); it != order.rend(); ++it) {
         if (it->layer.is<RenderSymbolLayer>()) {
-            if (crossTileSymbolIndex.addLayer(*it->layer.as<RenderSymbolLayer>())) symbolBucketsChanged = true;
+            const float lng = parameters.state.getLatLng().longitude();
+            if (crossTileSymbolIndex.addLayer(*it->layer.as<RenderSymbolLayer>(), lng)) symbolBucketsChanged = true;
         }
     }
 
diff --git a/src/mbgl/text/cross_tile_symbol_index.cpp b/src/mbgl/text/cross_tile_symbol_index.cpp
index b0c3511ce3..8df26ca117 100644
--- a/src/mbgl/text/cross_tile_symbol_index.cpp
+++ b/src/mbgl/text/cross_tile_symbol_index.cpp
@@ -61,6 +61,32 @@ void TileLayerIndex::findMatches(std::vector<SymbolInstance>& symbolInstances, c
 CrossTileSymbolLayerIndex::CrossTileSymbolLayerIndex() {
 }
 
+/*
+ * Sometimes when a user pans across the antimeridian the longitude value gets wrapped.
+ * To prevent labels from flashing out and in we adjust the tileID values in the indexes
+ * so that they match the new wrapped version of the map.
+ */
+void CrossTileSymbolLayerIndex::handleWrapJump(float newLng) {
+
+    const int wrapDelta = ::round((newLng - lng) / 360);
+    if (wrapDelta != 0) {
+        std::map<uint8_t, std::map<OverscaledTileID,TileLayerIndex>> newIndexes;
+        for (auto& zoomIndex : indexes) {
+            std::map<OverscaledTileID,TileLayerIndex> newZoomIndex;
+            for (auto& index : zoomIndex.second) {
+                // change the tileID's wrap and move its index
+                index.second.coord = index.second.coord.unwrapTo(index.second.coord.wrap + wrapDelta);
+                newZoomIndex.emplace(index.second.coord, std::move(index.second));
+            }
+            newIndexes.emplace(zoomIndex.first, std::move(newZoomIndex));
+        }
+
+        indexes = std::move(newIndexes);
+    }
+
+    lng = newLng;
+}
+
 bool CrossTileSymbolLayerIndex::addBucket(const OverscaledTileID& tileID, SymbolBucket& bucket, uint32_t& maxCrossTileID) {
     const auto& thisZoomIndexes = indexes[tileID.overscaledZ];
     auto previousIndex = thisZoomIndexes.find(tileID);
@@ -138,13 +164,15 @@ bool CrossTileSymbolLayerIndex::removeStaleBuckets(const std::unordered_set<uint
 
 CrossTileSymbolIndex::CrossTileSymbolIndex() {}
 
-bool CrossTileSymbolIndex::addLayer(RenderSymbolLayer& symbolLayer) {
+bool CrossTileSymbolIndex::addLayer(RenderSymbolLayer& symbolLayer, float lng) {
 
     auto& layerIndex = layerIndexes[symbolLayer.getID()];
 
     bool symbolBucketsChanged = false;
     std::unordered_set<uint32_t> currentBucketIDs;
 
+    layerIndex.handleWrapJump(lng);
+
     for (RenderTile& renderTile : symbolLayer.renderTiles) {
         if (!renderTile.tile.isRenderable()) {
             continue;
diff --git a/src/mbgl/text/cross_tile_symbol_index.hpp b/src/mbgl/text/cross_tile_symbol_index.hpp
index 541c2e3661..051573e1d2 100644
--- a/src/mbgl/text/cross_tile_symbol_index.hpp
+++ b/src/mbgl/text/cross_tile_symbol_index.hpp
@@ -45,18 +45,20 @@ public:
     CrossTileSymbolLayerIndex();
     bool addBucket(const OverscaledTileID&, SymbolBucket&, uint32_t& maxCrossTileID);
     bool removeStaleBuckets(const std::unordered_set<uint32_t>& currentIDs);
+    void handleWrapJump(float newLng);
 private:
     void removeBucketCrossTileIDs(uint8_t zoom, const TileLayerIndex& removedBucket);
 
     std::map<uint8_t, std::map<OverscaledTileID,TileLayerIndex>> indexes;
     std::map<uint8_t, std::set<uint32_t>> usedCrossTileIDs;
+    float lng = 0;
 };
 
 class CrossTileSymbolIndex {
 public:
     CrossTileSymbolIndex();
 
-    bool addLayer(RenderSymbolLayer&);
+    bool addLayer(RenderSymbolLayer&, float lng);
     void pruneUnusedLayers(const std::set<std::string>&);
 
     void reset();