[core] increase padding in CollisionIndex for MapMode::Tile (#15880)

* [core] fix cutoff cross-tile labels in MapMode::Tile

Increase the CollisionIndex's padding so that more of a tile's data gets
considered when checking for collisions. This should fix clipped
variable placement text labels.

3 files changed, 27 insertions, 16 deletions

diff --git a/src/mbgl/text/collision_index.cpp b/src/mbgl/text/collision_index.cpp
index c83b117b9a..4ffe66b98f 100644
--- a/src/mbgl/text/collision_index.cpp
+++ b/src/mbgl/text/collision_index.cpp
@@ -22,20 +22,30 @@ namespace mbgl {
 // the viewport for collision detection so that the bulk of the changes
 // occur offscreen. Making this constant greater increases label
 // stability, but it's expensive.
-static const float viewportPadding = 100;
-
-CollisionIndex::CollisionIndex(const TransformState& transformState_)
-    : transformState(transformState_)
-    , collisionGrid(transformState.getSize().width + 2 * viewportPadding, transformState.getSize().height + 2 * viewportPadding, 25)
-    , ignoredGrid(transformState.getSize().width + 2 * viewportPadding, transformState.getSize().height + 2 * viewportPadding, 25)
-    , screenRightBoundary(transformState.getSize().width + viewportPadding)
-    , screenBottomBoundary(transformState.getSize().height + viewportPadding)
-    , gridRightBoundary(transformState.getSize().width + 2 * viewportPadding)
-    , gridBottomBoundary(transformState.getSize().height + 2 * viewportPadding)
-    , pitchFactor(std::cos(transformState.getPitch()) * transformState.getCameraToCenterDistance())
-{}
-
-float CollisionIndex::approximateTileDistance(const TileDistance& tileDistance, const float lastSegmentAngle, const float pixelsToTileUnits, const float cameraToAnchorDistance, const bool pitchWithMap) {
+static const float viewportPaddingDefault = 100;
+// Viewport padding must be much larger for static tiles to avoid clipped labels.
+static const float viewportPaddingForStaticTiles = 1024;
+
+CollisionIndex::CollisionIndex(const TransformState& transformState_, MapMode& mapMode)
+    : transformState(transformState_),
+      viewportPadding(mapMode == MapMode::Tile ? viewportPaddingForStaticTiles : viewportPaddingDefault),
+      collisionGrid(transformState.getSize().width + 2 * viewportPadding,
+                    transformState.getSize().height + 2 * viewportPadding,
+                    25),
+      ignoredGrid(transformState.getSize().width + 2 * viewportPadding,
+                  transformState.getSize().height + 2 * viewportPadding,
+                  25),
+      screenRightBoundary(transformState.getSize().width + viewportPadding),
+      screenBottomBoundary(transformState.getSize().height + viewportPadding),
+      gridRightBoundary(transformState.getSize().width + 2 * viewportPadding),
+      gridBottomBoundary(transformState.getSize().height + 2 * viewportPadding),
+      pitchFactor(std::cos(transformState.getPitch()) * transformState.getCameraToCenterDistance()) {}
+
+float CollisionIndex::approximateTileDistance(const TileDistance& tileDistance,
+                                              const float lastSegmentAngle,
+                                              const float pixelsToTileUnits,
+                                              const float cameraToAnchorDistance,
+                                              const bool pitchWithMap) {
     // This is a quick and dirty solution for chosing which collision circles to use (since collision circles are
     // laid out in tile units). Ideally, I think we should generate collision circles on the fly in viewport coordinates
     // at the time we do collision detection.
diff --git a/src/mbgl/text/collision_index.hpp b/src/mbgl/text/collision_index.hpp
index 4e8a2fdb62..b9e618bd24 100644
--- a/src/mbgl/text/collision_index.hpp
+++ b/src/mbgl/text/collision_index.hpp
@@ -20,7 +20,7 @@ class CollisionIndex {
 public:
     using CollisionGrid = GridIndex<IndexedSubfeature>;
 
-    explicit CollisionIndex(const TransformState&);
+    explicit CollisionIndex(const TransformState&, MapMode&);
 
     std::pair<bool,bool> placeFeature(const CollisionFeature& feature,
                                       Point<float> shift,
@@ -72,6 +72,7 @@ private:
 
     const TransformState transformState;
 
+    const float viewportPadding;
     CollisionGrid collisionGrid;
     CollisionGrid ignoredGrid;
     
diff --git a/src/mbgl/text/placement.cpp b/src/mbgl/text/placement.cpp
index ecc8f93032..7964ad24c7 100644
--- a/src/mbgl/text/placement.cpp
+++ b/src/mbgl/text/placement.cpp
@@ -95,7 +95,7 @@ Placement::Placement(const TransformState& state_,
                      style::TransitionOptions transitionOptions_,
                      const bool crossSourceCollisions,
                      optional<Immutable<Placement>> prevPlacement_)
-    : collisionIndex(state_),
+    : collisionIndex(state_, mapMode_),
       mapMode(mapMode_),
       transitionOptions(std::move(transitionOptions_)),
       placementZoom(state_.getZoom()),