improve line label density

Place the first label as close to the beginning of the line as possible.

js: 316ed0bd9bff8cd430ad81d0b6397f396fbce124

src/mbgl/geometry/resample.cpp

@@ -17,17 +17,18 @@ const std::array<std::vector<float>, 4> minScaleArrays = {{
 
 Anchors resample(const std::vector<Coordinate> &vertices, float spacing,
                  const float /*minScale*/, float maxScale, const float tilePixelRatio,
-                 const int start) {
+                 int offset) {
 
     maxScale = std::round(std::fmax(std::fmin(8.0f, maxScale / 2.0f), 1.0f));
     spacing *= tilePixelRatio / maxScale;
+    offset *= tilePixelRatio / maxScale;
     const size_t index = util::clamp<size_t>(std::floor(std::log(maxScale) / std::log(2)), 0, minScaleArrays.size() - 1);
     const std::vector<float> &minScales = minScaleArrays[index];
     const size_t len = minScales.size();
 
-    float distance = 0.0f;
+    float distance = spacing - offset;;
     float markedDistance = 0.0f;
-    int added = start;
+    int added = 0;
 
     Anchors points;
 

src/mbgl/geometry/resample.hpp

@@ -7,7 +7,7 @@
 namespace mbgl {
 
 Anchors resample(const std::vector<Coordinate> &vertices, float spacing,
-                 float minScale, float maxScale, float tilePixelRatio, int start = 0);
+                 float minScale, float maxScale, float tilePixelRatio, int offset);
 }
 
 #endif

src/mbgl/renderer/symbol_bucket.cpp

@@ -238,9 +238,26 @@ void SymbolBucket::addFeature(const std::vector<Coordinate> &line, const Shaping
     Anchors anchors;
 
     if (properties.placement == PlacementType::Line) {
+        float textResampleOffset = 0;
+        float iconResampleOffset = 0;
+
+        if (shaping.size()) {
+            float minX = std::numeric_limits<float>::infinity();
+            float maxX = -std::numeric_limits<float>::infinity();
+            for (const auto &glyph : shaping) {
+                minX = std::min(minX, glyph.x);
+                maxX = std::max(maxX, glyph.x);
+            }
+            const float labelLength = maxX - minX;
+            textResampleOffset = (labelLength / 2.0 + glyphSize) * fontScale;
+        }
+        if (image) {
+            iconResampleOffset = image.w;
+        }
+
         // Line labels
         anchors = resample(line, properties.min_distance, minScale, collision.maxPlacementScale,
-                           collision.tilePixelRatio);
+                           collision.tilePixelRatio, std::max(iconResampleOffset, textResampleOffset));
 
         // Sort anchors by segment so that we can start placement with the
         // anchors that can be shown at the lowest zoom levels.