[Numpy-discussion] subsampling array without loops

[Zachary Pincus]

> This almost works.  Is there a way to do some masking on tiles, for
> instance taking the maximum height of each 2x2 square that is an
> odd number?   I've tried playing around with masking and where, but
> they don't return an array of the original size and shape of "tiles"
> below.

Could you provide an example (maybe not code, just descriptive) of  
what you want to do?

I'm not sure what specifically you mean by "height" and "odd" above...

Zach


> Catherine
>
>> Perhaps you could take the relevant slices of the arrays with
>> appropriate striding:
>>
>> a = numpy.arange(128*512).reshape((128, 512))
>> top_left = a[::2, ::2]
>> top_right = a[::2, 1::2]
>> bottom_left = a[1::2, ::2]
>> bottom_right = a[1::2, 1::2]
>>
>> tiles = numpy.array([top_left, top_right, bottom_left, bottom_right])
>> maxes = tiles.max(axis=0)
>>
>> Similarly if you want overlapping tiles, you could leave out the
>> final :2 in the slice specifications above.
>>
>> Zach
>>
>>
>>
>>> I'm looking for a way to acccomplish the following task without lots
>>> of loops involved, which are really slowing down my code.
>>>
>>> I have a 128x512 array which I want to break down into 2x2 squares.
>>> Then, for each 2x2 square I want to do some simple calculations
>>> such as finding the maximum value, which I then store in a 64x256
>>> array.
>>>
>>> Here is the actual code involved.  It's only slightly more complex
>>> than what I described above, since it also involves doing a bit of
>>> masking on the 2x2 sub-arrays.
>>>
>>> Any hints are much appreciated!  An inordinate amount of time is
>>> being spent in this function and another one like it.
>>>
>>> Catherine
>>>
>>>    def calc_sdcm_at_rlra(self,iblock):
>>>
>>>        npixl = self.nlineht/self.nlinerl
>>>        npixs = self.nsmpht/self.nsmprl
>>>
>>>        sdcm_out = numpy.array([Constants.CLOUDMASK_NR] \
>>>                               *self.nlinerl*self.nsmprl,'int8') \
>>>                        .reshape(self.nlinerl,self.nsmprl)
>>>
>>>        for ilinerl in range(0,self.nlinerl):
>>>            for ismprl in range(0,self.nsmprl):
>>>
>>>                height = self.data[iblock].height
>>> [ilinerl*2:ilinerl*2+2, \
>>>                                                 ismprl*2:ismprl*2+2]
>>>                sdcm   = self.data[iblock].sdcm[ilinerl*2:ilinerl*2
>>> +2, \
>>>                                                ismprl*2:ismprl*2+2]
>>>                source = self.data[iblock].heightsrc
>>> [ilinerl*2:ilinerl*2+2, \
>>>
>>> ismprl*2:ismprl*2+2]
>>>
>>>                mask1 = (source == Constants.HEIGHT_STEREO)
>>>                mask2 = ( (source == Constants.HEIGHT_SURFACE) | \
>>>                          (source == Constants.HEIGHT_DEFAULT) )
>>>
>>>                if (mask1.any()):
>>>                    loc = height[mask1].argmax()
>>>                    sdcm_out[ilinerl,ismprl] = sdcm[mask1].ravel()
>>> [loc]
>>>                elif (mask2.any()):
>>>                    loc = height[mask2].argmax()
>>>                    sdcm_out[ilinerl,ismprl] = sdcm[mask2].ravel()
>>> [loc]
>>>
>>>        return sdcm_out
>>
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