boost::geometry: nearest neighbors using a circle

Question

I am using the Rtree implementation of boost::geometry to store (lots of) 2D points. Now I need to do distance-based nearest neigbors queries.

However, the manual on

Answer 1

The manual documents some model::ring class that I thought at first might fit for a circle, but it is actually more a kind of a piece-wise line (a polygon). Is that assumption correct ?

I think that's correct.

I noticed that you can define a unary-predicate, but is is... unary (thus, not suitable for a condition on two points).

Would the 'second' (or reference) point not be fixed? Because then you can use a simple bind expression to supply the reference point.

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Additionally you can use the KNN algorithm with a very large n and add some kind of breaking condition on the predicate:

Breaking or pausing the query

for ( Rtree::const_query_iterator it = tree.qbegin(bgi::nearest(pt, 10000)) ;
      it != tree.qend() ; ++it )
{
    // do something with value
    if ( has_enough_nearest_values() )
        break;
}

This could work pretty well, assuming that the algorithm already traverses the points in order of ascending distance (you will want to check that assumption of course).