neighbours_nest

This IDL facility returns the number and indices of the topological immediate neighbours of a central pixel. The pixels are ordered in a clockwise sense (when watching the sphere from the outside) about the central pixel with the southernmost pixel in first element. For the four pixels in the southern corners of the equatorial faces which have two equally southern neighbours the routine returns the southwestern pixel first and proceeds clockwise.

Location in HEALPix directory tree: src/idl/toolkit/neighbours_nest.pro 

FORMAT

IDL> neighbours_nest (Nside, Ipix0, Listpix[,Nneigh])

QUALIFIERS

Nside

HEALPix resolution parameter (scalar integer), should be a valid Nside (power of 2)

Ipix0

NESTED-scheme index of central pixel in [0,12*Nside²-1]

Listpix

output: list of neighbouring pixel (NESTED scheme index) of size Nneigh

Nneigh

optional output: number of neighbours of pixel #Ipix0. Usually 8, sometimes 7 (for 8 particular pixels) or 6 (if Nside=1)

DESCRIPTION

neighbours_nest calls pix2xy_nest to find location of central pixel within the pixelation base-face, and then xy2pix_nest to find neighbouring pixels within the same face, or one of the bit manipulation routines if the neighbouring pixel is on a different base-face.

RELATED ROUTINES

This section lists the routines related to neighbours_nest

idl

version 6.4 or more is necessary to run neighbours_nest .

neighbours_ring

returns topological immediate neighbouring pixels of a given central pixel, using RING indexing.

query_disc, query_polygon,

query_strip, query_triangle

render the list of pixels enclosed respectively in a given disc, polygon, latitude strip and triangle

nest2ring, ring2nest

conversion between NESTED and RING indices

EXAMPLE:

neighbours_nest , 4, 1, list, nneigh
print,nneigh,list

will return:8 90 0 2 3 6 4 94 91, listing the NESTED-indexed 8 neighbors of pixel #1 for Nside=4