CODEWITHSUNDEEP
pythonnumpyindexingslice
cageyKJ
cageyKJ

Reputation: 46

What does this slicing mean [:, :, 0]?

I am trying to understand this code

I can't get my head around what this line is doing. The flow variable is an array of flow vectors with one for each pixel in the image (so a 2d array).

fx, fy = flow[:, :, 0], flow[:, :, 1]

Any help would be appreciated

Upvotes: 1

Views: 1250

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477883

Let us first simplify the expression. Your code:

fx, fy = flow[:, :, 0], flow[:, :, 1]

is equivalent to:

fx = flow[:, :, 0]
fy = flow[:, :, 1]

So now it boils down on what flow[:, :, 0]. It means that flow is a numpy array with at least three dimensions (let us define N as the number of dimensions). Then flow[:,:,0] is an N-1-dimensional array, where we pick 0 always as the third dimension.

In the context of image processing, an image is usually a 3d-array (given it has color) with dimensions w × h × 3 (three color channels). So here it means that flow[:,:,0] will generate a w × h view where for each pixel, we select the red channel (given the red channel is the first channel).

So if flow is a 5 × 4 × 3 matrix, like:

>>> flow
array([[[ 0,  1,  2],
        [ 3,  4,  5],
        [ 6,  7,  8],
        [ 9, 10, 11]],

       [[12, 13, 14],
        [15, 16, 17],
        [18, 19, 20],
        [21, 22, 23]],

       [[24, 25, 26],
        [27, 28, 29],
        [30, 31, 32],
        [33, 34, 35]],

       [[36, 37, 38],
        [39, 40, 41],
        [42, 43, 44],
        [45, 46, 47]],

       [[48, 49, 50],
        [51, 52, 53],
        [54, 55, 56],
        [57, 58, 59]]])

Then we will obtain for each 3-tuple the first element, making it:

>>> flow[:,:,0]
array([[ 0,  3,  6,  9],
       [12, 15, 18, 21],
       [24, 27, 30, 33],
       [36, 39, 42, 45],
       [48, 51, 54, 57]])

and by querying flow[:,:,1], we obtain:

>>> flow[:,:,1]
array([[ 1,  4,  7, 10],
       [13, 16, 19, 22],
       [25, 28, 31, 34],
       [37, 40, 43, 46],
       [49, 52, 55, 58]])

mind that these are views: if you alter flow, it will have impact on fx and fy as well, even if you did these assignments before.

Upvotes: 6

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