[core] make symbols fade out faster while zooming out

Zooming outcan make symbols overlap quickly. The area previously covered
by the viewport is covered by a lot of colliding labels while the
surrounding area has no labels. This difference produces an unwanted
effect.

This reduces that effect by:
- reducing the fade duration while zooming out
- doing placement more frequently while zooming out

1 files changed, 5 insertions, 2 deletions

diff --git a/src/mbgl/text/placement.hpp b/src/mbgl/text/placement.hpp
index 722a4a0926..2349fd3785 100644
--- a/src/mbgl/text/placement.hpp
+++ b/src/mbgl/text/placement.hpp
@@ -102,14 +102,14 @@ class Placement {
 public:
     Placement(const TransformState&, MapMode, style::TransitionOptions, const bool crossSourceCollisions, std::unique_ptr<Placement> prevPlacementOrNull = nullptr);
     void placeLayer(const RenderLayer&, const mat4&, bool showCollisionBoxes);
-    void commit(TimePoint);
+    void commit(TimePoint, const double zoom);
     void updateLayerBuckets(const RenderLayer&, const TransformState&,  bool updateOpacities);
     float symbolFadeChange(TimePoint now) const;
     bool hasTransitions(TimePoint now) const;
 
     const CollisionIndex& getCollisionIndex() const;
 
-    bool stillRecent(TimePoint now) const;
+    bool stillRecent(TimePoint now, const float zoom) const;
     void setRecent(TimePoint now);
     void setStale();
     
@@ -125,6 +125,7 @@ private:
     void updateBucketOpacities(SymbolBucket&, const TransformState&, std::set<uint32_t>&);
     void markUsedJustification(SymbolBucket&, style::TextVariableAnchorType, const SymbolInstance&, style::TextWritingModeType orientation);
     void markUsedOrientation(SymbolBucket&, style::TextWritingModeType, const SymbolInstance&);
+    float zoomAdjustment(const float zoom) const;
 
     CollisionIndex collisionIndex;
 
@@ -133,6 +134,8 @@ private:
 
     TimePoint fadeStartTime;
     TimePoint commitTime;
+    float placementZoom;
+    float prevZoomAdjustment = 0;
 
     std::unordered_map<uint32_t, JointPlacement> placements;
     std::unordered_map<uint32_t, JointOpacityState> opacities;