numpy - Pixel neighbors in 2d array (image) using Python -

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i have numpy array this:

x = np.array([[1,2,3],[4,5,6],[7,8,9]])

i need create function let's call "neighbors" following input parameter:

  • x: numpy 2d array
  • (i,j): index of element in 2d array
  • d: neighborhood radius

as output want neighbors of cell i,j given distance d. if run 

neighbors(im, i, j, d=1) = 1 , j = 1 (element value = 5)

i should indices of following values: [1,2,3,4,6,7,8,9]. hope make clear. there library scipy deal this?

i've done working it's rough solution.

def pixel_neighbours(self, p):      rows, cols = self.im.shape      i, j = p[0], p[1]      rmin = - 1 if - 1 >= 0 else 0     rmax = + 1 if + 1 < rows else      cmin = j - 1 if j - 1 >= 0 else 0     cmax = j + 1 if j + 1 < cols else j      neighbours = []      x in xrange(rmin, rmax + 1):         y in xrange(cmin, cmax + 1):             neighbours.append([x, y])     neighbours.remove([p[0], p[1]])      return neighbours

how can improve this?

edit: ah crap, answer writing im[i-d:i+d+1, j-d:j+d+1].flatten() written in incomprehensible way :)

the old sliding window trick may here:

import numpy np numpy.lib.stride_tricks import as_strided  def sliding_window(arr, window_size):     """ construct sliding window view of array"""     arr = np.asarray(arr)     window_size = int(window_size)     if arr.ndim != 2:         raise valueerror("need 2-d input")     if not (window_size > 0):         raise valueerror("need positive window size")     shape = (arr.shape[0] - window_size + 1,              arr.shape[1] - window_size + 1,              window_size, window_size)     if shape[0] <= 0:         shape = (1, shape[1], arr.shape[0], shape[3])     if shape[1] <= 0:         shape = (shape[0], 1, shape[2], arr.shape[1])     strides = (arr.shape[1]*arr.itemsize, arr.itemsize,                arr.shape[1]*arr.itemsize, arr.itemsize)     return as_strided(arr, shape=shape, strides=strides)  def cell_neighbors(arr, i, j, d):     """return d-th neighbors of cell (i, j)"""     w = sliding_window(arr, 2*d+1)      ix = np.clip(i - d, 0, w.shape[0]-1)     jx = np.clip(j - d, 0, w.shape[1]-1)      i0 = max(0, - d - ix)     j0 = max(0, j - d - jx)     i1 = w.shape[2] - max(0, d - + ix)     j1 = w.shape[3] - max(0, d - j + jx)      return w[ix, jx][i0:i1,j0:j1].ravel()  x = np.arange(8*8).reshape(8, 8) print x  d in [1, 2]:     p in [(0,0), (0,1), (6,6), (8,8)]:         print "-- d=%d, %r" % (d, p)         print cell_neighbors(x, p[0], p[1], d=d)

didn't timings here, it's possible version has reasonable performance.

for more info, search net phrases "rolling window numpy" or "sliding window numpy".


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