Re: geometry: knn search of point to many linestrings?

From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 2021-06-14 09:54:35

W dniu 14.06.2021 o 11:33, Jens Thiele via Boost-users pisze:
> Adam Wulkiewicz via Boost-users <boost-users_at_[hidden]> writes:
>
>> W dniu 23.05.2021 o 17:33, Adam Wulkiewicz pisze:
>>> W dniu 18.05.2021 o 09:34, Jens Thiele via Boost-users 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 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