Re: [Boost-users] geometry: knn search of point to many linestrings?

W dniu 14.06.2021 o 11:33, Jens Thiele via Boost-users pisze:

Adam Wulkiewicz via Boost-users writes:

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W dniu 23.05.2021 o 17:33, Adam Wulkiewicz pisze:

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W dniu 18.05.2021 o 09:34, Jens Thiele via Boost-users pisze:

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I have lots of linestrings and want to find the k nearest linestrings to some point.

Looking at the example /libs/geometry/doc/index/src/examples/rtree/polygons_shared_ptr.cpp I first thought this should be close to the solution and I just could replace the polygons with linestrings. But now I think the nearest query in that example only finds the nearest polygon bounding boxes and not the nearest polygons. Am I correct?

If yes, how would one extend that example to find the nearest polygons? Hi Jens, Hi,

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Yes, your understanding is correct. Bounding boxes of polygons together with pointers to polygons are stored in the rtree. This is also what is returned by the query. So you need to calculate the distances to actual linestrings by yourself.

I propose you to use query iterators instead of query function. Then you can iterate over nearest boxes (passing the number of values stored in the rtree into the nearest predicate). In the loop calculate distances to linestrings and break when you have enough of them. You should probably break when the number of linestrings you have is equal to your K and the distance to the furthest linestring is lesser than the distance to the current box returned by the rtree query (because then you know that you will not get any closer linestring). To track the furthest linestring you can use std::priority_queue.

Adam Correction: "the current box returned by the rtree query"

I of course had in mind: "the current box returned by the query iterator" I followed that route and the results look correct but performance is really bad. Nearly all time (0.5s with a really small test) is consumed by qbegin: rtree_t::const_query_iterator it = t->qbegin(idx::nearest(pt, tree_size));

Any ideas how to improve performance?

Are you compiling with optimizations enabled? Without knowing your code I can't esstimate if this is long time or not, I can only guess. In general qbegin() loop gathering some number of nearest elements should take similar time to query() call doing the same. So you can try getting some number of knn boxes from the R-tree and checking if this is the case. The speed of the R-tree is determined by the tree internal structure which is created during balancing or packing (packing is the fastest), minimum and maximum number of elements in nodes and the data itself (sparsity vs overlap). How is your R-tree created and what are the parameters? Some users confuse maximum number of parameters in node with capacity/size of the R-tree. The result is the R-tree with very big root node which is effectively a vector storing all of the elements. So the query has to check all of the elements of this node and by extension the whole R-tree  during a query. Adam