resize a 2D numpy array excluding NaN

Question

I'm trying to resize a 2D numpy array of a given factor, obtaining a smaller array in output.

The array is read from an image file and some of the values should be NaN (Not a Number, np.nan from numpy): it is the result of remote sensing measurements from satellite and simply some pixels weren't measured.

The suitable package I found for this is scypy.misc.imresize, but each pixel in the output array containing a NaN is set to NaN, even if there are some valid data in the original pixels interpolated together.

My solution is appended here, what I've done is essentially :

• create a new array based on the original array shape and the desired reduction factor
• create an index array to address all the pixels of the original array to be averaged for each pixel in the new
• cycle through the new array pixels and average all the not-NaN pixel to obtain the new array pixel value; it there are only NaN, the output will be NaN.

I'm planning to add keyword to choice between different output (average, median, standard deviation of the input pixels and so on).

It is working as expected, but on a ~1Mpx image it takes around 3 seconds. Due to my lack of experience in python I'm searching for improvements.

Do anyone have suggestion how to do it better and more efficiently?

Do anyone know a library that already implements all that stuff?

Thanks.

Here you have an example output for random pixel input generated with the code here below:

import numpy as np
import pylab as plt
from scipy import misc

def resize_2d_nonan(array,factor):
    """
    Resize a 2D array by different factor on two axis sipping NaN values.
    If a new pixel contains only NaN, it will be set to NaN


    Parameters
    ----------

    array : 2D np array

    factor : int or tuple. If int x and y factor wil be the same

    Returns
    -------
    array : 2D np array scaled by factor

    Created on Mon Jan 27 15:21:25 2014

    @author: damo_ma
    """
    xsize, ysize = array.shape

    if isinstance(factor,int):
        factor_x = factor
        factor_y = factor
    elif isinstance(factor,tuple):
        factor_x , factor_y = factor[0], factor[1]
    else:
        raise NameError('Factor must be a tuple (x,y) or an integer')

    if not (xsize %factor_x == 0 or ysize % factor_y == 0) :
        raise NameError('Factors must be intger multiple of array shape')

    new_xsize, new_ysize = xsize/factor_x, ysize/factor_y

    new_array = np.empty([new_xsize, new_ysize])
    new_array[:] = np.nan # this saves us an assignment in the loop below

    # submatrix indexes : is the average box on the original matrix
    subrow, subcol  = np.indices((factor_x, factor_y))

     # new matrix indexs
    row, col  = np.indices((new_xsize, new_ysize))

    # some output for testing
    #for i, j, ind in zip(row.reshape(-1), col.reshape(-1),range(row.size)) :
    #    print '----------------------------------------------'
    #    print 'i: %i, j: %i, ind: %i ' % (i, j, ind)    
    #    print 'subrow+i*new_ysize, subcol+j*new_xsize :'    
    #    print i,'*',new_xsize,'=',i*factor_x
    #    print j,'*',new_ysize,'=',j*factor_y
    #    print subrow+i*factor_x,subcol+j*factor_y
    #    print '---'
    #    print 'array[subrow+i*factor_x,subcol+j*factor_y] : '    
    #    print array[subrow+i*factor_x,subcol+j*factor_y]

    for i, j, ind in zip(row.reshape(-1), col.reshape(-1),range(row.size)) :
        # define the small sub_matrix as view of input matrix subset
        sub_matrix = array[subrow+i*factor_x,subcol+j*factor_y]
        # modified from any(a) and all(a) to a.any() and a.all()
        # see https://stackoverflow.com/a/10063039/1435167
        if not (np.isnan(sub_matrix)).all(): # if we haven't all NaN
            if (np.isnan(sub_matrix)).any(): # if we haven no NaN at all
                msub_matrix = np.ma.masked_array(sub_matrix,np.isnan(sub_matrix))
                (new_array.reshape(-1))[ind] = np.mean(msub_matrix)
            else: # if we haven some NaN
                (new_array.reshape(-1))[ind] = np.mean(sub_matrix)
        # the case assign NaN if we have all NaN is missing due 
        # to the standard values of new_array

    return new_array


row , cols = 6, 4

a = 10*np.random.random_sample((row , cols))
a[0:3,0:2] = np.nan
a[0,2] = np.nan

factor_x = 2
factor_y = 2
a_misc = misc.imresize(a, .5, interp='nearest', mode='F')
a_2d_nonan = resize_2d_nonan(a,(factor_x,factor_y))

print a
print
print a_misc
print
print a_2d_nonan

plt.subplot(131)
plt.imshow(a,interpolation='nearest')
plt.title('original')
plt.xticks(arange(a.shape[1]))
plt.yticks(arange(a.shape[0]))
plt.subplot(132)
plt.imshow(a_misc,interpolation='nearest')
plt.title('scipy.misc')
plt.xticks(arange(a_misc.shape[1]))
plt.yticks(arange(a_misc.shape[0]))
plt.subplot(133)
plt.imshow(a_2d_nonan,interpolation='nearest')
plt.title('my.func')
plt.xticks(arange(a_2d_nonan.shape[1]))
plt.yticks(arange(a_2d_nonan.shape[0]))

EDIT

I add some modification to address ChrisProsser comment.

If I substitute the NaN with some other value, let say the average of the not-NaN pixels, it will affect all the subsequent calculation: the difference between the resampled original array and the resampled array with NaN substituted shows that 2 pixels changed their values.

My goal is simply skip all the NaN pixels.

# substitute NaN with the average value 

ind_nonan , ind_nan = np.where(np.isnan(a) == False), np.where(np.isnan(a) == True)
a_substitute = np.copy(a)

a_substitute[ind_nan] = np.mean(a_substitute[ind_nonan]) # substitute the NaN with average on the not-Nan

a_substitute_misc = misc.imresize(a_substitute, .5, interp='nearest', mode='F')
a_substitute_2d_nonan = resize_2d_nonan(a_substitute,(factor_x,factor_y))

print a_2d_nonan-a_substitute_2d_nonan

[[        nan -0.02296697]
 [ 0.23143208  0.        ]
 [ 0.          0.        ]]

** 2nd EDIT**

To address the Hooked's answer I put some additional code. It is an iteresting idea, sadly it interpolates new values over pixels that should be "empty" (NaN) and for my small example generate more NaN than good values.

X , Y  = np.indices((row , cols))
X_new , Y_new  = np.indices((row/factor_x , cols/factor_y))

from scipy.interpolate import CloughTocher2DInterpolator as intp
C = intp((X[ind_nonan],Y[ind_nonan]),a[ind_nonan])

a_interp = C(X_new , Y_new)

print a
print
print a_interp

[[        nan,         nan],
 [        nan,         nan],
 [        nan,  6.32826577]])

Answer 1

You are operating on small windows of the array. Instead of looping through the array to make the windows, the array can be efficiently restructured by manipulating its strides. The numpy library provides the as_strided() function to help with that. An example is provided in the SciPy CookBook Stride tricks for the Game of Life.

The following will use a generalized sliding window function which I will include it at the end.

Determine the shape of the new array:

rows, cols = a.shape
new_shape = rows / 2, cols / 2

Restructure the array into the windows you need, and create an indexing array identifying NaNs:

# 2x2 windows of the original array
windows = sliding_window(a, (2,2))
# make a windowed boolean array for indexing
notNan = sliding_window(np.logical_not(np.isnan(a)), (2,2))

The new array can be made using a list comprehension or a generator expression.

# using a list comprehension
# make a list of the means of the windows, disregarding the Nan's
means = [window[index].mean() for window, index in zip(windows, notNan)]
new_array = np.array(means).reshape(new_shape)

# generator expression
# produces the means of the windows, disregarding the Nan's
means = (window[index].mean() for window, index in zip(windows, notNan))
new_array = np.fromiter(means, dtype = np.float32).reshape(new_shape)

The generator expression should conserve memory. Using itertools.izip() instead of ```zip`` should also help if memory is a problem. I just used the list comprehension for your solution.

Your function:

def resize_2d_nonan(array,factor):
    """
    Resize a 2D array by different factor on two axis skipping NaN values.
    If a new pixel contains only NaN, it will be set to NaN

    Parameters
    ----------
    array : 2D np array

    factor : int or tuple. If int x and y factor wil be the same

    Returns
    -------
    array : 2D np array scaled by factor

    Created on Mon Jan 27 15:21:25 2014

    @author: damo_ma
    """
    xsize, ysize = array.shape

    if isinstance(factor,int):
        factor_x = factor
        factor_y = factor
        window_size = factor, factor
    elif isinstance(factor,tuple):
        factor_x , factor_y = factor
        window_size = factor
    else:
        raise NameError('Factor must be a tuple (x,y) or an integer')

    if (xsize % factor_x or ysize % factor_y) :
        raise NameError('Factors must be integer multiple of array shape')

    new_shape = xsize / factor_x, ysize / factor_y

    # non-overlapping windows of the original array
    windows = sliding_window(a, window_size)
    # windowed boolean array for indexing
    notNan = sliding_window(np.logical_not(np.isnan(a)), window_size)

    #list of the means of the windows, disregarding the Nan's
    means = [window[index].mean() for window, index in zip(windows, notNan)]
    # new array
    new_array = np.array(means).reshape(new_shape)

    return new_array

I haven't done any time comparisons with your original function, but it should be faster.

Many solutions I've seen here on SO vectorize the operations to increase speed/efficiency - I don't quite have a handle on that and don't know if it can be applied to your problem. Searching SO for window, array, moving average, vectorize, and numpy should produce similar questions and answers for reference.

sliding_window() see attribution below:

import numpy as np
from numpy.lib.stride_tricks import as_strided as ast
from itertools import product

def norm_shape(shape):
    '''
    Normalize numpy array shapes so they're always expressed as a tuple, 
    even for one-dimensional shapes.
     
    Parameters
        shape - an int, or a tuple of ints
     
    Returns
        a shape tuple
    '''
    try:
        i = int(shape)
        return (i,)
    except TypeError:
        # shape was not a number
        pass
 
    try:
        t = tuple(shape)
        return t
    except TypeError:
        # shape was not iterable
        pass
     
    raise TypeError('shape must be an int, or a tuple of ints')
 

def sliding_window(a,ws,ss = None,flatten = True):
    '''
    Return a sliding window over a in any number of dimensions
     
    Parameters:
        a  - an n-dimensional numpy array
        ws - an int (a is 1D) or tuple (a is 2D or greater) representing the size 
             of each dimension of the window
        ss - an int (a is 1D) or tuple (a is 2D or greater) representing the 
             amount to slide the window in each dimension. If not specified, it
             defaults to ws.
        flatten - if True, all slices are flattened, otherwise, there is an 
                  extra dimension for each dimension of the input.
     
    Returns
        an array containing each n-dimensional window from a
    '''
     
    if None is ss:
        # ss was not provided. the windows will not overlap in any direction.
        ss = ws
    ws = norm_shape(ws)
    ss = norm_shape(ss)
     
    # convert ws, ss, and a.shape to numpy arrays so that we can do math in every 
    # dimension at once.
    ws = np.array(ws)
    ss = np.array(ss)
    shape = np.array(a.shape)
     
     
    # ensure that ws, ss, and a.shape all have the same number of dimensions
    ls = [len(shape),len(ws),len(ss)]
    if 1 != len(set(ls)):
        raise ValueError(\
        'a.shape, ws and ss must all have the same length. They were %s' % str(ls))
     
    # ensure that ws is smaller than a in every dimension
    if np.any(ws > shape):
        raise ValueError(\
        'ws cannot be larger than a in any dimension.\
 a.shape was %s and ws was %s' % (str(a.shape),str(ws)))
     
    # how many slices will there be in each dimension?
    newshape = norm_shape(((shape - ws) // ss) + 1)
    # the shape of the strided array will be the number of slices in each dimension
    # plus the shape of the window (tuple addition)
    newshape += norm_shape(ws)
    # the strides tuple will be the array's strides multiplied by step size, plus
    # the array's strides (tuple addition)
    newstrides = norm_shape(np.array(a.strides) * ss) + a.strides
    strided = ast(a,shape = newshape,strides = newstrides)
    if not flatten:
        return strided
     
    # Collapse strided so that it has one more dimension than the window.  I.e.,
    # the new array is a flat list of slices.
    meat = len(ws) if ws.shape else 0
    firstdim = (np.product(newshape[:-meat]),) if ws.shape else ()
    dim = firstdim + (newshape[-meat:])
    # remove any dimensions with size 1
    dim = filter(lambda i : i != 1,dim)
    return strided.reshape(dim)

---

sliding_window() attribution
I originally found this on a blog page that is now a broken link:

Efficient Overlapping Windows with Numpy - http://www.johnvinyard.com/blog/?p=268

With a little searching it looks like it now resides in the Zounds github repository. Thanks John Vinyard.

---

Note this post is pretty old and there are a lot of SO Q&A's regarding sliding windows, rolling windows, and for images- patch extraction. There are a lot of one-offs using numpy's as_strided but this function still seems the only one to handle n-d windowing. scikits sklearn.feature_extraction.image library seems to be often cited for extracting or viewing image patches.