This notebook shows how to use the functionality in the HealpixTree class. This is a useful function to find groups of Healpixels at high resolution which are connected and nearby. In particular this notebook illustrates two useful methods:

How to find the descendent pixels of a sequence of pixels at the next higher resolution using pixelsAtNextLevel
How to find the descendent pixels at a sequence of pixels at resolution res above the resolution using pixelsAtResolutionLevel



In [1]:

    
from mpl_toolkits.basemap import Basemap



In [2]:

    
import opsimsummary as oss



In [3]:

    
oss.__VERSION__









    Out[3]:





'0.8.3'



In [4]:

    
from opsimsummary import HealpixTree, pixelsForAng, HealpixTiles



In [5]:

    
import numpy as np



In [6]:

    
%matplotlib inline
import matplotlib.pyplot as plt



In [7]:

    
import healpy as hp

Instantiate the object



In [8]:

    
htree = HealpixTree(nside=1, nest=True)

Find all the healpixels at the resolution one level higher



In [9]:

    
# By default the nside argument to this function is the nside at which htree was instantiated
print(htree.nside)
ipix =  np.array([0, 1])
htree.pixelsAtNextLevel(ipix)









    



1






    Out[9]:





(2, array([0, 1, 2, 3, 4, 5, 6, 7]))



In [10]:

    
# We can also be specific, and do this for a particular NSIDE
htree.pixelsAtNextLevel(ipix, nside=128)









    Out[10]:





(256, array([0, 1, 2, 3, 4, 5, 6, 7]))

Find all the pixels at NSIDE=256, which are owned by pixelid =1 at NSIDE=1



In [11]:

    
# How many subdivisions required to go to NSIDE =256 ?
desideredNSIDE = 256
res = int(np.log2(desideredNSIDE))



In [12]:

    
# nsidenew should be the NSIDE at the resolution we want
nsidenew, pixels = htree.pixelsAtResolutionLevel(1, res, 1)



In [13]:

    
assert nsidenew == desideredNSIDE

Visusalize this



In [14]:

    
n256 = hp.nside2npix(256)
n1 = hp.nside2npix(1)
arr1 = np.ones(n1) * hp.UNSEEN
arr1[1] = 1
arr256 = np.ones(n256) * hp.UNSEEN
arr256[pixels] = -2
hp.mollview(arr1, nest=True)
hp.mollview(arr256, nest=True)



In [ ]:

Contents

Instantiate the object

Find all the healpixels at the resolution one level higher

Find all the pixels at NSIDE=256, which are owned by pixelid =1 at NSIDE=1

Visusalize this