[core] Initiate new placement only when a new bucket appeares

src/mbgl/renderer/render_orchestrator.cpp

@@ -360,11 +360,14 @@ std::unique_ptr<RenderTree> RenderOrchestrator::createRenderTree(const UpdatePar
             crossTileSymbolIndex.reset();
         }
 
+        bool symbolBucketsAdded = false;
         bool symbolBucketsChanged = false;
         for (auto it = layersNeedPlacement.rbegin(); it != layersNeedPlacement.rend(); ++it) {
-            if (crossTileSymbolIndex.addLayer(*it, updateParameters.transformState.getLatLng().longitude())) symbolBucketsChanged = true;
+            auto result = crossTileSymbolIndex.addLayer(*it, updateParameters.transformState.getLatLng().longitude()); 
+            symbolBucketsAdded = symbolBucketsAdded || (result & CrossTileSymbolIndex::AddLayerResult::BucketsAdded);
+            symbolBucketsChanged = symbolBucketsChanged || (result != CrossTileSymbolIndex::AddLayerResult::NoChanges);
         }
-        renderTreeParameters->placementChanged = !placement->stillRecent(updateParameters.timePoint, updateParameters.transformState.getZoom()) || symbolBucketsChanged;
+        renderTreeParameters->placementChanged = !placement->stillRecent(updateParameters.timePoint, updateParameters.transformState.getZoom()) || symbolBucketsAdded;
 
         std::set<std::string> usedSymbolLayers;
         if (renderTreeParameters->placementChanged) {
@@ -389,7 +392,7 @@ std::unique_ptr<RenderTree> RenderOrchestrator::createRenderTree(const UpdatePar
         }
 
         for (auto it = layersNeedPlacement.rbegin(); it != layersNeedPlacement.rend(); ++it) {
-            placement->updateLayerBuckets(*it, updateParameters.transformState, renderTreeParameters->placementChanged);
+            placement->updateLayerBuckets(*it, updateParameters.transformState, renderTreeParameters->placementChanged || symbolBucketsChanged);
         }
 
         renderTreeParameters->symbolFadeChange = placement->symbolFadeChange(updateParameters.timePoint);

src/mbgl/text/cross_tile_symbol_index.cpp

@@ -163,10 +163,10 @@ bool CrossTileSymbolLayerIndex::removeStaleBuckets(const std::unordered_set<uint
 
 CrossTileSymbolIndex::CrossTileSymbolIndex() = default;
 
-bool CrossTileSymbolIndex::addLayer(const RenderLayer& layer, float lng) {
+auto CrossTileSymbolIndex::addLayer(const RenderLayer& layer, float lng) -> AddLayerResult {
     auto& layerIndex = layerIndexes[layer.getID()];
 
-    bool symbolBucketsChanged = false;
+    AddLayerResult result = AddLayerResult::NoChanges;
     std::unordered_set<uint32_t> currentBucketIDs;
 
     layerIndex.handleWrapJump(lng);
@@ -174,16 +174,15 @@ bool CrossTileSymbolIndex::addLayer(const RenderLayer& layer, float lng) {
     for (const auto& item : layer.getPlacementData()) {
         const RenderTile& renderTile = item.tile;
         Bucket& bucket = item.bucket;
-        auto result = bucket.registerAtCrossTileIndex(layerIndex, renderTile.getOverscaledTileID(), maxCrossTileID);
-        assert(result.first != 0u);
-        symbolBucketsChanged = symbolBucketsChanged || result.second;
-        currentBucketIDs.insert(result.first);
+        auto pair = bucket.registerAtCrossTileIndex(layerIndex, renderTile.getOverscaledTileID(), maxCrossTileID);
+        assert(pair.first != 0u);
+        if (pair.second) result |= AddLayerResult::BucketsAdded;
+        currentBucketIDs.insert(pair.first);
     }
 
-    if (layerIndex.removeStaleBuckets(currentBucketIDs)) {
-        symbolBucketsChanged = true;
-    }
-    return symbolBucketsChanged;
+    if (layerIndex.removeStaleBuckets(currentBucketIDs)) result |= AddLayerResult::BucketsRemoved;
+
+    return result;
 }
 
 void CrossTileSymbolIndex::pruneUnusedLayers(const std::set<std::string>& usedLayers) {

src/mbgl/text/cross_tile_symbol_index.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <mbgl/tile/tile_id.hpp>
+#include <mbgl/util/bitmask_operations.hpp>
 #include <mbgl/util/geometry.hpp>
 #include <mbgl/util/constants.hpp>
 #include <mbgl/util/optional.hpp>
@@ -58,7 +59,13 @@ class CrossTileSymbolIndex {
 public:
     CrossTileSymbolIndex();
 
-    bool addLayer(const RenderLayer& layer, float lng);
+    enum class AddLayerResult : uint8_t {
+        NoChanges = 0,
+        BucketsAdded = 1 << 0,
+        BucketsRemoved = 1 << 1
+    };
+
+    AddLayerResult addLayer(const RenderLayer& layer, float lng);
     void pruneUnusedLayers(const std::set<std::string>&);
 
     void reset();