healpix_geo.auto.kth_neighbourhood

healpix_geo.auto.kth_neighbourhood(ipix, grid, *, ring, num_threads=0)

Get the kth ring neighbouring cells of some HEALPix cells.

This method returns a \(N\) x \((2 k + 1)^2\) np.uint64 numpy array containing the neighbours of each cell of the \(N\) sized ipix array. This method is wrapped around the kth_neighbourhood method from the cdshealpix Rust crate.

Parameters:

• ipix (numpy.ndarray) – The HEALPix cell indexes given as a np.uint64 numpy array.

• grid (Grid) – The definition of the HEALPix grid.

• ring (int) – The number of rings. ring=0 returns just the input cell ids, ring=1 returns the 8 (or 7) immediate neighbours, ring=2 returns the 8 (or 7) immediate neighbours plus their immediate neighbours (a total of 24 cells), and so on.

• num_threads (int, default: 0) – Specifies the number of threads to use for the computation. Default to 0 means it will choose the number of threads based on the RAYON_NUM_THREADS environment variable (if set), or the number of logical CPUs (otherwise)

Returns:

neighbours (numpy.ndarray) – A \(N\) x \((2 k + 1)^2\) np.int64 numpy array containing the kth ring neighbours of each cell. The \(5^{th}\) element corresponds to the index of HEALPix cell from which the neighbours are evaluated. All its 8 neighbours occup the remaining elements of the line.

Examples

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>>> import healpix_geo.auto as hg
>>> import numpy as np
>>> ipix = np.array([42, 6, 10])
>>> grid = hg.Grid(level=12, indexing_scheme="nested", ellipsoid="sphere")
>>> grid
Grid(level=12, indexing_scheme='nested', ellipsoid='sphere')
>>> ring = 3
>>> neighbours = hg.kth_neighbourhood(ipix, grid, ring=ring)
>>> neighbours
array([[       42,  72701309,  72701311,  72701397,       128,       129,
               43,        41,        40,  72701302,  72701308,  72701310,
         72701396,  72701398,  72701399,  72701303,       130,       131,
              134,       132,        46,        44,        38,        35,
               34,  72701297,  72701299,  72701305,  72701307,  72701393,
         72701395,  72701401,  72701404,  72701405,  72701301,  72701300,
              136,       137,       140,       141,       135,       133,
               47,        45,        39,        37,        36,        33,
               32],
       [        6,         1,         3,         9,        12,        13,
                7,         5,         4,  95070890,  95070906,  95070895,
         95070894,  95070891,         0,         2,         8,        10,
               11,        14,        15,        26,        24,        18,
               16, 150994941, 150994943,  72701269,  72701271,  72701277,
         72701279,  72701301,  95070907,  95070905,  95070904,  95070893,
         95070892,  95070889,  95070888,        32,        33,        36,
               37,        48,        49,        27,        25,        19,
               17],
       [       10,  72701277,  72701279,  72701301,        32,        33,
               11,         9,         8,  72701270,  72701276,  72701278,
         72701300,  72701302,  72701303,  72701271,        34,        35,
               38,        36,        14,        12,         6,         3,
                2,  72701265,  72701267,  72701273,  72701275,  72701297,
         72701299,  72701305,  72701308,  72701309,  72701269,  72701268,
               40,        41,        44,        45,        39,        37,
               15,        13,         7,         5,         4,         1,
                0]])

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