
Boost Users : 
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 20210614 09:54:35
W dniu 14.06.2021 oÂ 11:33, Jens Thiele via Boostusers pisze:
> Adam Wulkiewicz via Boostusers <boostusers_at_[hidden]> writes:
>
>> W dniu 23.05.2021 oÂ 17:33, Adam Wulkiewicz pisze:
>>> W dniu 18.05.2021 oÂ 09:34, Jens Thiele via Boostusers pisze:
>>>> 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,
>
>>> 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 Rtree
and checking if this is the case.
The speed of the Rtree 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 Rtree created and what are the
parameters?
Some users confuse maximum number of parameters in node with
capacity/size of the Rtree. The result is the Rtree 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 RtreeÂ during a query.
Adam
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