numpy multiply array elements with another array

Question

I am trying to code a multispectral image.

Every pixel's value is coded in 33 channels.

I have two numpy arrays image and spectral_range

For example one image has 4 x 4 pixels:

image = np.array([[[1,2,4,3],[2,2,2,1],[1,2,3,2],[5,4,3,2]])

And for each pixel should be associated the 33 values of the spectral range covered by the image:

spectral_range = np.array([0,0,0,0,0,0,0,1,23,99,166,86,54,12,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0])

How can I simply create a np.array of shape (width, height, 33), where each pixel's 33 values are the 33 values of the array spectrum multiplied by the individual values of the array image?

The expected result looks like:

result = np.array([[[0,0,0,0,0,0,0,1,23,99,166,86,54,12,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0],[0,0,0,0,0,0,0,2,46,198,...etc.]]])

Thanks for helping

Alex Riley · Accepted Answer · 2015-04-17 20:10:46Z

5

You just need to add an extra axis to image and then multiply this array with spectral_range. The extra axis makes the two arrays broadcastable with each other:

>>> result = image[:, :, np.newaxis] * spectral_range
>>> result.shape
(4, 4, 33)
>>> result
array([[[  0,   0,   0,   0,   0,   0,   0,   1,  23,  99, 166,  86,  54,
          12,   1,   1,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
           0,   0,   0,   0,   0,   0,   0],
        [  0,   0,   0,   0,   0,   0,   0,   2,  46, 198, 332, 172, 108,
          24,   2,   2,   0,   0,   0,   0,   0,   0,   0,   0,   0,   0,
           0,   0,   0,   0,   0,   0,   0],
        ...

edited Apr 17, 2015 at 20:10

answered Apr 17, 2015 at 20:03

Alex Riley

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