dask.array.Array.map_overlap

Array.map_overlap(func, depth, boundary=None, trim=True, **kwargs)[source]

Map a function over blocks of the array with some overlap

We share neighboring zones between blocks of the array, then map a function, then trim away the neighboring strips.

Note that this function will attempt to automatically determine the output array type before computing it, please refer to the meta keyword argument in map_blocks if you expect that the function will not succeed when operating on 0-d arrays.

Parameters
func: function

The function to apply to each extended block

depth: int, tuple, or dict

The number of elements that each block should share with its neighbors If a tuple or dict then this can be different per axis

boundary: str, tuple, dict

How to handle the boundaries. Values include ‘reflect’, ‘periodic’, ‘nearest’, ‘none’, or any constant value like 0 or np.nan

trim: bool

Whether or not to trim depth elements from each block after calling the map function. Set this to False if your mapping function already does this for you

**kwargs:

Other keyword arguments valid in map_blocks.

Examples

>>> import dask.array as da
>>> x = np.array([1, 1, 2, 3, 3, 3, 2, 1, 1])
>>> x = da.from_array(x, chunks=5)
>>> def derivative(x):
...     return x - np.roll(x, 1)

>>> y = x.map_overlap(derivative, depth=1, boundary=0)
>>> y.compute()
array([ 1,  0,  1,  1,  0,  0, -1, -1,  0])

>>> import dask.array as da
>>> x = np.arange(16).reshape((4, 4))
>>> d = da.from_array(x, chunks=(2, 2))
>>> d.map_overlap(lambda x: x + x.size, depth=1).compute()
array([[16, 17, 18, 19],
       [20, 21, 22, 23],
       [24, 25, 26, 27],
       [28, 29, 30, 31]])

>>> func = lambda x: x + x.size
>>> depth = {0: 1, 1: 1}
>>> boundary = {0: 'reflect', 1: 'none'}
>>> d.map_overlap(func, depth, boundary).compute()  
array([[12,  13,  14,  15],
       [16,  17,  18,  19],
       [20,  21,  22,  23],
       [24,  25,  26,  27]])

>>> x = np.arange(16).reshape((4, 4))
>>> d = da.from_array(x, chunks=(2, 2))
>>> y = d.map_overlap(lambda x: x + x[2], depth=1, meta=np.array(()))
>>> y
dask.array<_trim, shape=(4, 4), dtype=float64, chunksize=(2, 2), chunktype=numpy.ndarray>
>>> y.compute()
array([[ 4,  6,  8, 10],
       [ 8, 10, 12, 14],
       [20, 22, 24, 26],
       [24, 26, 28, 30]])

>>> import cupy  
>>> x = cupy.arange(16).reshape((5, 4))  
>>> d = da.from_array(x, chunks=(2, 2))  
>>> y = d.map_overlap(lambda x: x + x[2], depth=1, meta=cupy.array(()))  
>>> y  
dask.array<_trim, shape=(4, 4), dtype=float64, chunksize=(2, 2), chunktype=cupy.ndarray>
>>> y.compute()  
array([[ 4,  6,  8, 10],
       [ 8, 10, 12, 14],
       [20, 22, 24, 26],
       [24, 26, 28, 30]])