flanders 0.3.0

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Flanders: Fast 2D nearest neighbor search with an angle

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Installation

$ pip install flanders

Example

In this example we have 6 points (numbered 0 to 5) and two observer points with a certain view vector and view angle (90 degrees). The first observer point finds point 2. The second observer point does not find any neighbor within the view angle and returns -1.

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Example code:

import flanders


# as a first step we build the search tree
# we can later reuse the search tree many times

points = [
    (60.4, 51.3),
    (173.9, 143.8),
    (132.9, 124.9),
    (19.5, 108.9),
    (196.5, 9.9),
    (143.3, 53.3),
]

tree = flanders.build_search_tree(points)


# now we will search the indices of nearest neighbor points
# for two observer points

observer_coordinates = [(119.2, 59.7), (155.2, 30.2)]
view_vectors = [(0.0, 1.0), (-1.0, -1.0)]
view_angles_deg = [90.0, 90.0]

indices = flanders.nearest_indices_from_coordinates(
    tree, observer_coordinates, view_vectors, view_angles_deg
)

assert indices == [2, -1]


# instead of using observer coordinates, also the original
# points themselves can be observers and we can select them
# by their index

observer_indices = [0, 1, 2, 3, 4, 5]
view_vectors = [(1.0, 1.0) for _ in observer_indices]
view_angles_deg = [90.0 for _ in observer_indices]

indices = flanders.nearest_indices_from_indices(
    tree, observer_indices, view_vectors, view_angles_deg
)

assert indices == [5, -1, 1, 2, -1, 1]

Efficiency considerations

The above example is very small and simple but this library starts to shine once you have very many points and/or very many observers where a noddy implementation would take too long to compute.

Example timing for 1 M points and 10 k observers (on i7-10710U):

• constructing the search tree: 3.0 s
• nearest neighbor search: 9.6 s

If you compute nearest neighbors for many observers it is a good idea to send in an entire batch of observers instead of computing one by one. If you send in an entire batch, the code will shared-memory parallelize the loop over the observers.

References used during development

• https://en.wikipedia.org/wiki/Nearest_neighbor_search
• https://en.wikipedia.org/wiki/K-d_tree
• http://www.slideshare.net/awebneck/the-post-office-problem
• http://www.cs.nyu.edu/~roweis/papers/Ahmed_msc_thesis.pdf
• http://dl.acm.org/citation.cfm?doid=361002.361007