CGAL users discussion list

[Sebastien Loriot <>]

You should not iterate on a range and remove elements at the same time.
Put elements to be removed in a std::vector and remove them after the
main loop over all elements. Removing an element may invalidate its
iterator for any container.

Something like that:
 for ( auto it = cloud.begin(); it != cloud.end(); ++it )
 {
   if ( c_size <= size_limit ) to_remove.push_back(*it);
 }
 for(auto i : to_remove)
    cloud.remove(i);

Best,

Sebastien.


On 11/18/21 4:18 PM, Yaoyu Hu ( via cgal-discuss 
Mailing List) wrote:
> Hi,
>
> I am trying to clean the points of a Point_set_3 by removing small 
> clusters of points.
>
> The way I did it is first computing the clusters 
> by CGAL::cluster_point_set() and saving the cluster indices to a 
> property map with the name "cluster". Then I count the number of points 
> in each cluster. Finally, the points belonging to a cluster that has a 
> small cluster size are removed by issuing the remove() function ( 
> followed by collect_garbage() after all remove() calls).
>
> The problem is when I iterate through the Point_set_3 point cloud and 
> send the de-referenced iterator value to the property map, the property 
> map throws an exception triggered by a CGAL::assertion_fail(). The 
> following is the code I use to do the small cluster removal. And I also 
> create a gist if that is more convenient.
>
> https://gist.github.com/huyaoyu/ea9919a1ff6e4c4b49d55020998d1e46 
> <https://gist.github.com/huyaoyu/ea9919a1ff6e4c4b49d55020998d1e46>
>
> === Code begins. ===
>
> template < typename Iterable_t >
> static std::map<int, int>
> compute_cluster_size(const Iterable_t& cluster_map) {
>      std::map<int, int> cluster_size;
>
>      for ( const auto& idx : cluster_map ) {
>          if ( cluster_size.find(idx) == cluster_size.end() ) {
>              // New idx.
>              cluster_size[idx] = 1;
>          } else {
>              // Existing cluster.
>              cluster_size[idx]++;
>          }
>      }
>
>      return cluster_size;
> }
>
> // sr::PointSet_t is CGAL::Point_set_3.
> void sr::compute_and_remove_small_clusters(
>      sr::PointSet_t& cloud,
>      double avg_spacing,
>      int size_limit,
>      std::ostream& os ) {
>      sr::PointSet_t::Property_map<int> cluster_map;
>
>      // Reset or create a new property map.
>      if ( cloud.has_property_map<int>("cluster") ) {
>          boost::tie( cluster_map, boost::tuples::ignore ) =
>              cloud.property_map<int>("cluster");
>
>          cloud.remove_property_map(cluster_map);
>      }
>
>      bool success = false;
>      boost::tie( cluster_map, success ) =
>          cloud.add_property_map<int>("cluster", -1);
>
>      if ( !success ) {
>          os << "Cannot add cluster property map. \n";
>          return;
>      }
>
>      // Clustering.
>      CGAL::Real_timer rt; rt.start();
>      std::size_t n_clusters =
>          CGAL::cluster_point_set(
>              cloud,
>              cluster_map,
>              cloud.parameters().neighbor_radius(avg_spacing)
>          );
>      rt.stop();
>      os << "Find " << n_clusters << " clusters in " << rt.time() << " 
> seoncds. \n";
>
>      // Compute the cluster size.
>      std::map<int, int> cluster_size = compute_cluster_size( cluster_map );
>
>      // Remove small clusters.
>      for ( auto it = cloud.begin(); it != cloud.end(); ++it ) {
>          // Get the cluster index.
>          const auto c_idx = cluster_map[*it];
>
>          // Get the cluster size.
>          const auto c_size = cluster_size[c_idx];
>
>          if ( c_size <= size_limit ) cloud.remove(it);
>      }
>
>      cloud.collect_garbage();
> }
>
> === Code ends. ===
>
> In the above code, the line *const auto c_idx = 
> cluster_map[*it];* triggers the assertion failure in 
> *CGAL::Property_array<int>::operator[]()*.
>
> I am assuming that *cluster_map* has all the points in the point cloud 
> as the key and as long as the iterator *it* comes from the point cloud, 
> **it* should be a valid key for *cluster_map*.
>
> The assertion failure does not always get triggered. Most of my point 
> clouds are OK. Only some point clouds trigger this error. I have 
> uploaded one sample point cloud that causes this error to my Google 
> Drive for your reference.
> https://drive.google.com/file/d/1rfQP0mo7pj84YgAX1CZtorr6IMpsslyo/view?usp=sharing 
> <https://drive.google.com/file/d/1rfQP0mo7pj84YgAX1CZtorr6IMpsslyo/view?usp=sharing>
>
> Please help me. Any suggestions are appreciated. Thank you!
>
> Yaoyu
>
>
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