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I'm new to numpy and trying to understand the following example from here. I'm having trouble understanding the output of 

>>> palette[image]

When the indexed array a is multidimensional, a single array of indices refers to the first dimension of a. The following example shows this behavior by converting an image of labels into a color image using a palette.

>>> palette = array( [ [0,0,0],                # black
...                    [255,0,0],              # red
...                    [0,255,0],              # green
...                    [0,0,255],              # blue
...                    [255,255,255] ] )       # white
>>> image = array( [ [ 0, 1, 2, 0 ],           # each value corresponds to a color in the palette
...                  [ 0, 3, 4, 0 ]  ] )
>>> palette[image]                            # the (2,4,3) color image
array([[[  0,   0,   0],
        [255,   0,   0],
        [  0, 255,   0],
        [  0,   0,   0]],
       [[  0,   0,   0],
        [  0,   0, 255],
        [255, 255, 255],
        [  0,   0,   0]]])
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2 Answers 2

up vote 2 down vote accepted

You are creating a 3D array, where first 2D array (withing 3D array) is given by extracting rows from palette as given by indices of image[0] and the second array is given by extracting rows from palette as given by indices of image[1]. 

>>> palette = array( [ [0,0,0],                # black
...                    [255,0,0],              # red
...                    [0,255,0],              # green
...                    [0,0,255],              # blue
...                    [255,255,255] ] )       # white
>>> image = array( [ [ 0, 1, 2, 0 ],           # each value corresponds to a color in the palette
...                  [ 0, 3, 4, 0 ]  ] )
>>> palette[image]                            # the (2,4,3) color image
array([[[  0,   0,   0], # row at index 0 of palete
        [255,   0,   0], # index 1
        [  0, 255,   0], # index 2
        [  0,   0,   0]], # index 0
       [[  0,   0,   0], # index 0
        [  0,   0, 255], # index 3
        [255, 255, 255], # index 4
        [  0,   0,   0]]]) # index 0
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up vote 0 down vote

This might help you understand:

array([[[  0,   0,   0],   # palette[0]
        [255,   0,   0],   # palette[1]
        [  0, 255,   0],   # palette[2]
        [  0,   0,   0]],  # palette[0]

       [[  0,   0,   0],   # palette[0]
        [  0,   0, 255],   # palette[3]
        [255, 255, 255],   # palette[4]
        [  0,   0,   0]]]) # palette[0]
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