[core] Coalesce buckets updates from several tiles

src/mbgl/renderer/render_orchestrator.cpp

@@ -367,7 +367,11 @@ std::unique_ptr<RenderTree> RenderOrchestrator::createRenderTree(const UpdatePar
             symbolBucketsAdded = symbolBucketsAdded || (result & CrossTileSymbolIndex::AddLayerResult::BucketsAdded);
             symbolBucketsChanged = symbolBucketsChanged || (result != CrossTileSymbolIndex::AddLayerResult::NoChanges);
         }
-        renderTreeParameters->placementChanged = !placement->stillRecent(updateParameters.timePoint, updateParameters.transformState.getZoom()) || symbolBucketsAdded;
+        // We want new symbols to show up faster, however simple setting `placementChanged` to `true` would
+        // initiate placement too often as new buckets ususally come from several rendered tiles in a row within
+        // a short period of time. Instead, we squeeze placement update period to coalesce buckets updates from several tiles.
+        if (symbolBucketsAdded) placement->setMaximumUpdatePeriod(Milliseconds(30));
+        renderTreeParameters->placementChanged = !placement->stillRecent(updateParameters.timePoint, updateParameters.transformState.getZoom());
 
         std::set<std::string> usedSymbolLayers;
         if (renderTreeParameters->placementChanged) {

src/mbgl/text/placement.cpp

@@ -964,6 +964,17 @@ float Placement::zoomAdjustment(const float zoom) const {
     return std::max(0.0, (placementZoom - zoom) / 1.5);
 }
 
+Duration Placement::getUpdatePeriod(const float zoom) const {
+    // Even if transitionOptions.duration is set to a value < 300ms, we still wait for this default transition duration
+    // before attempting another placement operation.
+    const auto fadeDuration = std::max(util::DEFAULT_TRANSITION_DURATION, transitionOptions.duration.value_or(util::DEFAULT_TRANSITION_DURATION));
+    const auto adjustedDuration = std::chrono::duration_cast<Duration>(fadeDuration * (1.0 - zoomAdjustment(zoom)));
+    if (maximumUpdatePeriod) {
+        return std::min(*maximumUpdatePeriod, adjustedDuration);
+    }
+    return adjustedDuration;
+}
+
 bool Placement::hasTransitions(TimePoint now) const {
     if (mapMode == MapMode::Continuous && transitionOptions.enablePlacementTransitions) {
         return stale || std::chrono::duration<float>(now - fadeStartTime) < transitionOptions.duration.value_or(util::DEFAULT_TRANSITION_DURATION);
@@ -973,12 +984,13 @@ bool Placement::hasTransitions(TimePoint now) const {
 }
 
 bool Placement::stillRecent(TimePoint now, const float zoom) const {
-    // Even if transitionOptions.duration is set to a value < 300ms, we still wait for this default transition duration
-    // before attempting another placement operation.
-    const auto fadeDuration = std::max(util::DEFAULT_TRANSITION_DURATION, transitionOptions.duration.value_or(util::DEFAULT_TRANSITION_DURATION));
     return mapMode == MapMode::Continuous &&
         transitionOptions.enablePlacementTransitions &&
-        commitTime + fadeDuration * (1.0 - zoomAdjustment(zoom)) > now;
+        commitTime + getUpdatePeriod(zoom) > now;
+}
+
+void Placement::setMaximumUpdatePeriod(Duration duration) {
+    maximumUpdatePeriod = duration;
 }
 
 void Placement::setStale() {

src/mbgl/text/placement.hpp

@@ -110,7 +110,7 @@ public:
     const CollisionIndex& getCollisionIndex() const;
 
     bool stillRecent(TimePoint now, const float zoom) const;
-    void setRecent(TimePoint now);
+    void setMaximumUpdatePeriod(Duration);
     void setStale();
     
     const RetainedQueryData& getQueryData(uint32_t bucketInstanceId) const;
@@ -126,6 +126,7 @@ private:
     void markUsedJustification(SymbolBucket&, style::TextVariableAnchorType, const SymbolInstance&, style::TextWritingModeType orientation);
     void markUsedOrientation(SymbolBucket&, style::TextWritingModeType, const SymbolInstance&);
     float zoomAdjustment(const float zoom) const;
+    Duration getUpdatePeriod(const float zoom) const;
 
     CollisionIndex collisionIndex;
 
@@ -147,6 +148,7 @@ private:
     std::unordered_map<uint32_t, RetainedQueryData> retainedQueryData;
     CollisionGroups collisionGroups;
     std::unique_ptr<Placement> prevPlacement;
+    optional<Duration> maximumUpdatePeriod;
 
     // Used for debug purposes.
     std::unordered_map<const CollisionFeature*, std::vector<ProjectedCollisionBox>> collisionCircles;