Masked arrays
Robert Kern
robert.kern at gmail.com
Wed May 10 20:45:29 CEST 2006
Tommy Grav wrote:
> I am trying to get the flux of a star in an image. I have been using numpy
> and pyfits and have the code.
You will probably want to ask on numpy-discussion, instead.
http://www.scipy.org/Mailing_Lists
> def distance(im,xc,yc):
> (rows,cols) = im.shape
> dist = zeros([rows,cols]).astype(Float64)
> for row in range(rows):
> for col in range(cols):
> dist[row,col] = sqrt(((row + 0.5) - yc)**2 + ((col + 0.5) -
> xc)**2)
> return dist
You will probably want to use numpy.fromfunction() instead.
> def apphot(im,x,y,r):
> dist = distance(im,x,y)
> appmask = where(dist <= r,1,0)
You don't have to use where() here. (dist <= r) returns an array of bools, which
are already 1 and 0 (for most purposes).
> fluxim = where(appmask,im,0)
> appflux = sum(sum(fluxim))
> skymask = where(dist > r, 1,0)
> skyim = where(skymask,im,0)
> sky = mean(skyim)
> print skyim
> print sky
>
> return 1
>
> Output:
>> array (20,20) , type = f, has 400 elements
>> [ 45.89742126, 45.92555847, 45.8874054 , 45.88538208, 45.88244934,
> 45.9353241 ,
> 36.75245361, 29.85816345, 27.53547668, 22.93712311, 22.93178101,
> 22.93699799, 22.91038208, 27.4988739 , 29.84021606, 36.71789551,
> 45.86646729, 45.86741638, 45.85328979, 45.823349 ,]
>
> where im is a ndarray, x and y are the position of the star and r is
> the radius of the aperture. I calculate the flux inside the aperture,
> but when I want to calculate the mean of the pixels outside the
> aperture I run into problems as the pixels values inside the aperture
> is 0 and is still considered in the mean calculation. Is there a way to
> do this without using masked arrays?
Sure! But ...
> How would I use a masked array
> to do it?
... masked arrays already do the appropriate tasks for you.
In [1]: from numpy import ma
In [2]: ma.average?
Type: function
Base Class: <type 'function'>
String Form: <function average at 0x697ef0>
Namespace: Interactive
File:
/Library/Frameworks/Python.framework/Versions/2.4/lib/python2.4/site-packages/numpy-0.9.7.2476-py2.4-macosx-10.4-ppc.egg/numpy/core/ma.py
Definition: ma.average(a, axis=0, weights=None, returned=0)
Docstring:
average(a, axis=0, weights=None)
Computes average along indicated axis.
If axis is None, average over the entire array
Inputs can be integer or floating types; result is of type float.
If weights are given, result is sum(a*weights)/(sum(weights)*1.0)
weights must have a's shape or be the 1-d with length the size
of a in the given axis.
If returned, return a tuple: the result and the sum of the weights
or count of values. Results will have the same shape.
masked values in the weights will be set to 0.0
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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