neighbours_ring

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_ring.pro 

FORMAT

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

QUALIFIERS

Nside

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

Ipix0

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

Listpix

output: list of neighbouring pixel (RING 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_ring calls ring2nest, neighbours_nest and nest2ring

RELATED ROUTINES

This section lists the routines related to neighbours_ring

idl

version 6.4 or more is necessary to run neighbours_ring .

neighbours_nest

returns topological immediate neighbouring pixels of a given central pixel, using NESTED 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_ring , 4, 1, list, nneigh
print,nneigh,list

will return:8 16 6 5 0 3 2 8 7 listing the RING-indexed 8 neighbors of pixel #1 for Nside=4