Labeling centroid calculation update (provided by Serge Dikiy)

Author: dakcarto

src/core/pal/pointset.cpp

@@ -962,18 +962,28 @@ namespace pal
     // http://local.wasp.uwa.edu.au/~pbourke/geometry/polyarea/
 
     int i, j;
-    double cx = 0, cy = 0, A = 0, tmp;
+    double cx = 0, cy = 0, A = 0, tmp, sumx = 0, sumy = 0;
     for ( i = 0; i < nbPoints; i++ )
     {
       j = i + 1; if ( j == nbPoints ) j = 0;
-      tmp = ( x[i] * y[j] - x[j] * y[i] );
-      cx += ( x[i] + x[j] ) * tmp;
-      cy += ( y[i] + y[j] ) * tmp;
+      tmp = (( x[i] - x[0] ) * ( y[j] - y[0] ) - ( x[j] - x[0] ) * ( y[i] - y[0] ) );
+      cx += ( x[i] + x[j] - 2 * x[0] ) * tmp;
+      cy += ( y[i] + y[j] - 2 * y[0] ) * tmp;
       A += tmp;
+      sumx += x[i];
+      sumy += y[i];
     }
 
-    px = cx / ( 3 * A );
-    py = cy / ( 3 * A );
+    if ( A == 0 )
+    {
+      px = sumx / nbPoints;
+      py = sumy / nbPoints;
+    }
+    else
+    {
+      px = cx / ( 3 * A ) + x[0];
+      py = cy / ( 3 * A ) + y[0];
+    }
   }
 
 } // end namespace