[core] Do not index and place the off-screen symbols for overscaled tiles

For overscaled tiles the viewport might be showing only a small part of the
tile, so we filter out the off-screen symbols to improve the performance.

3 files changed, 47 insertions, 13 deletions

diff --git a/src/mbgl/text/cross_tile_symbol_index.cpp b/src/mbgl/text/cross_tile_symbol_index.cpp
index 5b9d6ae69c..f806c652e1 100644
--- a/src/mbgl/text/cross_tile_symbol_index.cpp
+++ b/src/mbgl/text/cross_tile_symbol_index.cpp
@@ -12,6 +12,7 @@ TileLayerIndex::TileLayerIndex(OverscaledTileID coord_,
                                std::string bucketLeaderId_)
     : coord(coord_), bucketInstanceId(bucketInstanceId_), bucketLeaderId(std::move(bucketLeaderId_)) {
     for (SymbolInstance& symbolInstance : symbolInstances) {
+        if (symbolInstance.crossTileID == SymbolInstance::invalidCrossTileID()) continue;
         indexedSymbolInstances[symbolInstance.key].emplace_back(symbolInstance.crossTileID,
                                                                 getScaledCoordinates(symbolInstance, coord));
     }
@@ -95,11 +96,29 @@ void CrossTileSymbolLayerIndex::handleWrapJump(float newLng) {
     lng = newLng;
 }
 
-bool CrossTileSymbolLayerIndex::addBucket(const OverscaledTileID& tileID, SymbolBucket& bucket) {
+namespace {
+
+bool isInVewport(const mat4& posMatrix, const Point<float>& point) {
+    vec4 p = {{point.x, point.y, 0, 1}};
+    matrix::transformMat4(p, p, posMatrix);
+
+    // buffer covers area of the next zoom level (current zoom - 1 covered area).
+    constexpr float buffer = 1.0f;
+    constexpr float edge = 1.0f + buffer;
+    float x = p[0] / p[3];
+    float y = p[1] / p[3];
+    return (x > -edge && y > -edge && x < edge && y < edge);
+}
+
+} // namespace
+
+bool CrossTileSymbolLayerIndex::addBucket(const OverscaledTileID& tileID,
+                                          const mat4& tileMatrix,
+                                          SymbolBucket& bucket) {
     auto& thisZoomIndexes = indexes[tileID.overscaledZ];
     auto previousIndex = thisZoomIndexes.find(tileID);
     if (previousIndex != thisZoomIndexes.end()) {
-        if (previousIndex->second.bucketInstanceId == bucket.bucketInstanceId) {
+        if (previousIndex->second.bucketInstanceId == bucket.bucketInstanceId && !bucket.hasUninitializedSymbols) {
             return false;
         } else {
             // We're replacing this bucket with an updated version
@@ -110,8 +129,23 @@ bool CrossTileSymbolLayerIndex::addBucket(const OverscaledTileID& tileID, Symbol
         }
     }
 
-    for (auto& symbolInstance: bucket.symbolInstances) {
-        symbolInstance.crossTileID = 0;
+    bucket.hasUninitializedSymbols = false;
+
+    if (tileID.overscaleFactor() > 1u) {
+        // For overscaled tiles the viewport might be showing only a small part of the tile,
+        // so we filter out the off-screen symbols to improve the performance.
+        for (auto& symbolInstance : bucket.symbolInstances) {
+            if (isInVewport(tileMatrix, symbolInstance.anchor.point)) {
+                symbolInstance.crossTileID = 0u;
+            } else {
+                symbolInstance.crossTileID = SymbolInstance::invalidCrossTileID();
+                bucket.hasUninitializedSymbols = true;
+            }
+        }
+    } else {
+        for (auto& symbolInstance : bucket.symbolInstances) {
+            symbolInstance.crossTileID = 0u;
+        }
     }
 
     auto& thisZoomUsedCrossTileIDs = usedCrossTileIDs[tileID.overscaledZ];
@@ -195,7 +229,7 @@ auto CrossTileSymbolIndex::addLayer(const RenderLayer& layer, float lng) -> AddL
     for (const auto& item : layer.getPlacementData()) {
         const RenderTile& renderTile = item.tile;
         Bucket& bucket = item.bucket;
-        auto pair = bucket.registerAtCrossTileIndex(layerIndex, renderTile.getOverscaledTileID());
+        auto pair = bucket.registerAtCrossTileIndex(layerIndex, renderTile);
         assert(pair.first != 0u);
         if (pair.second) result |= AddLayerResult::BucketsAdded;
         currentBucketIDs.insert(pair.first);
diff --git a/src/mbgl/text/cross_tile_symbol_index.hpp b/src/mbgl/text/cross_tile_symbol_index.hpp
index 3b7c367726..8dfdb8466f 100644
--- a/src/mbgl/text/cross_tile_symbol_index.hpp
+++ b/src/mbgl/text/cross_tile_symbol_index.hpp
@@ -2,8 +2,9 @@
 
 #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/geometry.hpp>
+#include <mbgl/util/mat4.hpp>
 #include <mbgl/util/optional.hpp>
 
 #include <map>
@@ -48,7 +49,7 @@ public:
 class CrossTileSymbolLayerIndex {
 public:
     CrossTileSymbolLayerIndex(uint32_t& maxCrossTileID);
-    bool addBucket(const OverscaledTileID&, SymbolBucket&);
+    bool addBucket(const OverscaledTileID&, const mat4& tileMatrix, SymbolBucket&);
     bool removeStaleBuckets(const std::unordered_set<uint32_t>& currentIDs);
     void handleWrapJump(float newLng);
 private:
diff --git a/src/mbgl/text/placement.cpp b/src/mbgl/text/placement.cpp
index 43d9713717..32edf679e8 100644
--- a/src/mbgl/text/placement.cpp
+++ b/src/mbgl/text/placement.cpp
@@ -157,10 +157,7 @@ void Placement::placeBucket(const SymbolBucket& bucket,
     const bool rotateIconWithMap = layout.get<style::IconRotationAlignment>() == style::AlignmentType::Map;
     const bool pitchIconWithMap = layout.get<style::IconPitchAlignment>() == style::AlignmentType::Map;
 
-    mat4 posMatrix;
-    const auto& projMatrix = state.getProjectionMatrix();
-    state.matrixFor(posMatrix, renderTile.id);
-    matrix::multiply(posMatrix, projMatrix, posMatrix);
+    const mat4& posMatrix = renderTile.matrix;
 
     mat4 textLabelPlaneMatrix =
         getLabelPlaneMatrix(posMatrix, pitchTextWithMap, rotateTextWithMap, state, pixelsToTileUnits);
@@ -212,8 +209,10 @@ void Placement::placeBucket(const SymbolBucket& bucket,
     std::vector<ProjectedCollisionBox> textBoxes;
     std::vector<ProjectedCollisionBox> iconBoxes;
 
-    auto placeSymbol = [&] (const SymbolInstance& symbolInstance) {
-        if (seenCrossTileIDs.count(symbolInstance.crossTileID) != 0u) return;
+    auto placeSymbol = [&](const SymbolInstance& symbolInstance) {
+        if (symbolInstance.crossTileID == SymbolInstance::invalidCrossTileID() ||
+            seenCrossTileIDs.count(symbolInstance.crossTileID) != 0u)
+            return;
 
         if (renderTile.holdForFade()) {
             // Mark all symbols from this tile as "not placed", but don't add to seenCrossTileIDs, because we don't