How to normalize a NumPy array to within a certain range?


Question

After doing some processing on an audio or image array, it needs to be normalized within a range before it can be written back to a file. This can be done like so:

# Normalize audio channels to between -1.0 and +1.0
audio[:,0] = audio[:,0]/abs(audio[:,0]).max()
audio[:,1] = audio[:,1]/abs(audio[:,1]).max()

# Normalize image to between 0 and 255
image = image/(image.max()/255.0)

Is there a less verbose, convenience function way to do this? matplotlib.colors.Normalize() doesn't seem to be related.

1
107
8/20/2013 4:48:07 AM

Accepted Answer

audio /= np.max(np.abs(audio),axis=0)
image *= (255.0/image.max())

Using /= and *= allows you to eliminate an intermediate temporary array, thus saving some memory. Multiplication is less expensive than division, so

image *= 255.0/image.max()    # Uses 1 division and image.size multiplications

is marginally faster than

image /= image.max()/255.0    # Uses 1+image.size divisions

Since we are using basic numpy methods here, I think this is about as efficient a solution in numpy as can be.


In-place operations do not change the dtype of the container array. Since the desired normalized values are floats, the audio and image arrays need to have floating-point point dtype before the in-place operations are performed. If they are not already of floating-point dtype, you'll need to convert them using astype. For example,

image = image.astype('float64')
118
8/11/2019 3:21:05 PM

If the array contains both positive and negative data, I'd go with:

import numpy as np

a = np.random.rand(3,2)

# Normalised [0,1]
b = (a - np.min(a))/np.ptp(a)

# Normalised [0,255] as integer
c = 255*(a - np.min(a))/np.ptp(a).astype(int)

# Normalised [-1,1]
d = 2.*(a - np.min(a))/np.ptp(a)-1

also, worth mentioning even if it's not OP's question, standardization:

e = (a - np.mean(a)) / np.std(a)

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