[core] re-assign ids when lng jumps to avoid flicker

include/mbgl/tile/tile_id.hpp

@@ -85,9 +85,10 @@ public:
     std::array<UnwrappedTileID, 4> children() const;
     OverscaledTileID overscaleTo(uint8_t z) const;
     float pixelsToTileUnits(float pixelValue, float zoom) const;
+    UnwrappedTileID unwrapTo(int16_t wrap);
 
-    const int16_t wrap;
-    const CanonicalTileID canonical;
+    int16_t wrap;
+    CanonicalTileID canonical;
 };
 
 ::std::ostream& operator<<(::std::ostream& os, const UnwrappedTileID& rhs);
@@ -220,6 +221,10 @@ inline bool UnwrappedTileID::operator<(const UnwrappedTileID& rhs) const {
     return std::tie(wrap, canonical) < std::tie(rhs.wrap, rhs.canonical);
 }
 
+inline UnwrappedTileID UnwrappedTileID::unwrapTo(int16_t newWrap) {
+    return { newWrap, canonical };
+}
+
 inline bool UnwrappedTileID::isChildOf(const UnwrappedTileID& parent) const {
     return wrap == parent.wrap && canonical.isChildOf(parent.canonical);
 }

src/mbgl/renderer/render_tile.hpp

@@ -22,7 +22,7 @@ public:
     RenderTile& operator=(const RenderTile&) = delete;
     RenderTile& operator=(RenderTile&&) = default;
 
-    const UnwrappedTileID id;
+    UnwrappedTileID id;
     Tile& tile;
     ClipID clip;
     mat4 matrix;

src/mbgl/renderer/tile_pyramid.cpp

@@ -90,6 +90,8 @@ void TilePyramid::update(const std::vector<Immutable<style::Layer::Impl>>& layer
         return;
     }
 
+    handleWrapJump(parameters.transformState.getLatLng().longitude());
+
     // Determine the overzooming/underzooming amounts and required tiles.
     int32_t overscaledZoom = util::coveringZoomLevel(parameters.transformState.getZoom(), type, tileSize);
     int32_t tileZoom = overscaledZoom;
@@ -238,6 +240,44 @@ void TilePyramid::update(const std::vector<Immutable<style::Layer::Impl>>& layer
     }
 }
 
+void TilePyramid::handleWrapJump(float lng) {
+    // On top of the regular z/x/y values, TileIDs have a `wrap` value that specify
+    // which cppy of the world the tile belongs to. For example, at `lng: 10` you
+    // might render z/x/y/0 while at `lng: 370` you would render z/x/y/1.
+    //
+    // When lng values get wrapped (going from `lng: 370` to `long: 10`) you expect
+    // to see the same thing on the screen (370 degrees and 10 degrees is the same
+    // place in the world) but all the TileIDs will have different wrap values.
+    //
+    // In order to make this transition seamless, we calculate the rounded difference of
+    // "worlds" between the last frame and the current frame. If the map panned by
+    // a world, then we can assign all the tiles new TileIDs with updated wrap values.
+    // For example, assign z/x/y/1 a new id: z/x/y/0. It is the same tile, just rendered
+    // in a different position.
+    //
+    // This enables us to reuse the tiles at more ideal locations and prevent flickering.
+
+    const float lngDifference = lng - prevLng;
+    const float worldDifference = lngDifference / 360;
+    const int wrapDelta = ::round(worldDifference);
+    prevLng = lng;
+
+    if (wrapDelta) {
+        std::map<OverscaledTileID, std::unique_ptr<Tile>> newTiles;
+        for (auto& tile : tiles) {
+            auto newID = tile.second->id.unwrapTo(tile.second->id.wrap + wrapDelta);
+            tile.second->id = newID;
+            newTiles.emplace(newID, std::move(tile.second));
+        }
+        tiles = std::move(newTiles);
+
+        for (auto& renderTile : renderTiles) {
+            renderTile.id = renderTile.id.unwrapTo(renderTile.id.wrap + wrapDelta);
+        }
+    }
+}
+
+
 std::unordered_map<std::string, std::vector<Feature>> TilePyramid::queryRenderedFeatures(const ScreenLineString& geometry,
                                            const TransformState& transformState,
                                            const std::vector<const RenderLayer*>& layers,

src/mbgl/renderer/tile_pyramid.hpp

@@ -49,6 +49,8 @@ public:
     std::vector<std::reference_wrapper<RenderTile>> getRenderTiles();
     Tile* getTile(const OverscaledTileID&);
 
+    void handleWrapJump(float lng);
+
     std::unordered_map<std::string, std::vector<Feature>>
     queryRenderedFeatures(const ScreenLineString& geometry,
                           const TransformState& transformState,
@@ -72,6 +74,8 @@ public:
     std::vector<RenderTile> renderTiles;
 
     TileObserver* observer = nullptr;
+
+    float prevLng = 0;
 };
 
 } // namespace mbgl

src/mbgl/tile/tile.hpp

@@ -113,7 +113,7 @@ public:
     
     void dumpDebugLogs() const;
 
-    const OverscaledTileID id;
+    OverscaledTileID id;
     optional<Timestamp> modified;
     optional<Timestamp> expires;