[Numpy-discussion] Masked Array Usage Problems

Lane Brooks lane@brooks...
Sat Apr 10 02:49:03 CDT 2010

I am trying out masked arrays for the first time and having some 
problems. I have a 2-D image as dtype=numpy.int16

I create a mask of all False to not mask out any pixels.

I calculate the mean of the image original image and it comes out ~597.  
I calculate the mean of the masked array and it comes out differently 
around -179.  It produces the same negative mean value no matter what 
masks I try, e.g. (all True, all False, etc).  Furthermore there are no 
negative samples in the entire array.

Any ideas on what am I doing wrong?

Here is some sample code showing the behavior:

In [1]: img.dtype, img.shape
Out[1]: (dtype('int16'), (3200, 3456))

In [2]: mask = numpy.zeros(img.shape, dtype=bool)

In [3]: imgma = ma.masked_array(img, mask)

In [4]: img.mean()
Out[4]: 597.15437617549185

In [5]: imgma.mean()
Out[5]: -179.56858678747108

In [6]: imgma.min()
Out[6]: 25

In [7]: numpy.__version__
Out[7]: '1.3.0'

In [8]: numpy.ma.__version__
Out[8]: '1.0'

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