healpix_geo.nested.vertices#

healpix_geo.nested.vertices(ipix, depth, ellipsoid, step=1, num_threads=0)#

Get the longitudes and latitudes of the vertices of some HEALPix cells at a given depth.

This method returns the 4 vertices of each cell in ipix.

Parameters:

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

  • depth (int, or numpy.ndarray) – The depth of the HEALPix cells. If given as an array, should have the same shape than ipix

  • ellipsoid (ellipsoid-like, default: "sphere") – Reference ellipsoid to evaluate healpix on. If the reference ellipsoid is spherical, this will return the same result as cdshealpix.nested.vertices().

  • step (int, default: 1) – The number of vertices returned per HEALPix side. By default it is set to 1 meaning that it will only return the vertices of the cell. 2 means that it will return the vertices of the cell plus one more vertex per edge (the center of the edge). More generally, the number of vertices returned is 4 * step.

  • num_threads (int, optional) – 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:

longitude, latitude (numpy.ndarray) – The sky coordinates of the 4 vertices of the HEALPix cells. lon and lat are of shape \(N\) x \(4\) numpy arrays where N is the number of HEALPix cell given in ipix.

Raises:

ValueError – When the HEALPix cell indexes given have values out of \([0, 4^{29 - depth}[\).

Examples

>>> from healpix_geo.nested import vertices
>>> import numpy as np
>>> ipix = np.array([42, 6, 10])
>>> depth = 12
>>> lon, lat = vertices(ipix, depth, ellipsoid="sphere")
>>> np.stack([lon, lat], axis=-1)
array([[[4.49230957e+01, 6.52784088e-02],
        [4.49340820e+01, 7.46039007e-02],
        [4.49230957e+01, 8.39293945e-02],
        [4.49121094e+01, 7.46039007e-02]],

       [[4.50109863e+01, 2.79764560e-02],
        [4.50219727e+01, 3.73019424e-02],
        [4.50109863e+01, 4.66274299e-02],
        [4.50000000e+01, 3.73019424e-02]],

       [[4.49670410e+01, 2.79764560e-02],
        [4.49780273e+01, 3.73019424e-02],
        [4.49670410e+01, 4.66274299e-02],
        [4.49560547e+01, 3.73019424e-02]]])