Better label placement for polygons

Using the y-coordinate of the polygon's bounding box, this
simple algorithm intersects the polygon with the horizontal
center line of its bounding box. The x-coordinate of the label
point is the center of the longest segment of this intersection
that is inside the polygon.

src/ol/geom/polygon.js

@@ -1,6 +1,7 @@
 goog.provide('ol.geom.Polygon');
 
 goog.require('goog.asserts');
+goog.require('ol.extent');
 goog.require('ol.geom.Geometry');
 goog.require('ol.geom.GeometryType');
 goog.require('ol.geom.LinearRing');
@@ -35,6 +36,12 @@ ol.geom.Polygon = function(coordinates, opt_shared) {
     vertices = new ol.geom.SharedVertices({dimension: dimension});
   }
 
+  /**
+   * @private
+   * @type {ol.Coordinate}
+   */
+  this.labelPoint_ = null;
+
   /**
    * @type {ol.geom.SharedVertices}
    */
@@ -126,3 +133,48 @@ ol.geom.Polygon.prototype.containsCoordinate = function(coordinate) {
   }
   return containsCoordinate;
 };
+
+
+/**
+ * Calculates a label point that is guaranteed to be inside the polygon.
+ * @return {ol.Coordinate} The label point.
+ */
+ol.geom.Polygon.prototype.getLabelPoint = function() {
+  if (goog.isNull(this.labelPoint_)) {
+    var center = ol.extent.getCenter(this.getBounds()),
+        resultY = center[1],
+        vertices = this.rings[0].getCoordinates(),
+        intersections = [],
+        maxLength = 0,
+        i, vertex1, vertex2, x, segmentLength, resultX;
+
+    // Calculate intersections with the horizontal bounding box center line
+    for (i = vertices.length - 1; i >= 1; --i) {
+      vertex1 = vertices[i];
+      vertex2 = vertices[i - 1];
+      if ((vertex1[1] >= resultY && vertex2[1] <= resultY) ||
+          (vertex1[1] <= resultY && vertex2[1] >= resultY)) {
+        x = (resultY - vertex1[1]) / (vertex2[1] - vertex1[1]) *
+            (vertex2[0] - vertex1[0]) + vertex1[0];
+        intersections.push(x);
+      }
+    }
+
+    // Find the longest segment of the horizontal bounding box center line that
+    // has its center point inside the polygon
+    intersections.sort();
+    for (i = intersections.length - 1; i >= 1; --i) {
+      segmentLength = Math.abs(intersections[i] - intersections[i - 1]);
+      if (segmentLength > maxLength) {
+        x = (intersections[i] + intersections[i - 1]) / 2;
+        if (this.containsCoordinate([x, resultY])) {
+          maxLength = segmentLength;
+          resultX = x;
+        }
+      }
+    }
+    this.labelPoint_ = [resultX, resultY];
+  }
+
+  return this.labelPoint_;
+};

src/ol/renderer/canvas/canvasvectorrenderer.js

@@ -8,7 +8,6 @@ goog.require('goog.events');
 goog.require('goog.events.EventType');
 goog.require('goog.vec.Mat4');
 goog.require('ol.Feature');
-goog.require('ol.extent');
 goog.require('ol.geom.AbstractCollection');
 goog.require('ol.geom.Geometry');
 goog.require('ol.geom.GeometryType');
@@ -446,8 +445,7 @@ ol.renderer.canvas.VectorRenderer.getLabelVectors = function(geometry) {
     return [[geometry.get(0), geometry.get(1), 0]];
   }
   if (type == ol.geom.GeometryType.POLYGON) {
-    // TODO: better label placement
-    var coordinates = ol.extent.getCenter(geometry.getBounds());
+    var coordinates = geometry.getLabelPoint();
     return [[coordinates[0], coordinates[1], 0]];
   }
   throw new Error('Label rendering not implemented for geometry type: ' +