[AstroPy] Image combine

[Wolfgang Kerzendorf]

Hello Jose,

What you're trying to do is already half implemented in numpy and called masked arrays:

Let's say you want to stack images and you have put them together in a 3d cube (first axis being the images):

myimages = numpy.ma.MaskedArray(rand(5,100,100), mask=zeros((5,100,100).astype(bool))

let's do a simple sigma clipping algorithm:

image_mean = mean(myimages)
image_std = std(myimages)

#now let's adjust the mask to mask the pixels that are more than 1 sigma out

new_mask = abs(myimages - image_mean) > 1.

myimages.mask = new_mask

#if you do a mean now it will ignore the values where the mask = True

mean(myimages, axis=0)

This will be all near C-speeds (I guess a factor of 100 is easily in there).
----

As other's have pointed out using i,j in a loop is a very bad idea for numpy arrays. Numpy arrays are much more easy to handle than in most other languages, there is a bit of getting used to required though. I recommend reviewing the different operations on http://www.scipy.org/Tentative_NumPy_Tutorial. 

Hope that helps,

    Wolfgang 
On 2012-03-07, at 4:41 AM, Sergio Pascual wrote:

> Hello
> 
> I have written a image combine implementation for the pipeline of
> EMIR[1], a near infrared instrument for the 10m GTC Telescope. It does
> scaling,
> weighting and masking. It's written as a C/C++ extension. It's part of
> a larger package, numina, used for other GTC instruments, but I may
> split combine if there is interest in it. The package, called numina,
> its under heavy development, but the combine part is fairly stable.
> 
> You can see the code here https://guaix.fis.ucm.es/hg/numina/
> 
> The combine part is the module numina.array.combine, fairly all the
> C/C++ code is in src/
> 
> Regards, Sergio
> 
> 
> [1] http://www.gtc.iac.es/pages/instrumentacion/emir.php
> [2] http://www.gtc.iac.es/
> 
> 
> 
> El día 6 de marzo de 2012 12:00, Jose Miguel Ibáñez <ppmime at gmail.com> escribió:
>> Hello everyone,
>> 
>> does anyone know of an implementation of the iraf.imcombine task in
>> python+numpy ? (of course, not using pyraf.imcombine call)
>> 
>> 
>> Thanks !
>> Jose
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> 
> 
> 
> -- 
> Sergio Pascual     http://guaix.fis.ucm.es/~spr    +34 91 394 5018
> gpg fingerprint: 5203 B42D 86A0 5649 410A F4AC A35F D465 F263 BCCC
> Departamento de Astrofísica -- Universidad Complutense de Madrid (Spain)
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