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.

Answer 1

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.

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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')

Answer 2

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)

Answer 3

You can also rescale using sklearn. The advantages are that you can adjust normalize the standard deviation, in addition to mean-centering the data, and that you can do this on either axis, by features, or by records.

from sklearn.preprocessing import scale
X = scale( X, axis=0, with_mean=True, with_std=True, copy=True )

The keyword arguments axis, with_mean, with_std are self explanatory, and are shown in their default state. The argument copy performs the operation in-place if it is set to False. Documentation here.

Answer 4

You can use the "i" (as in idiv, imul..) version, and it doesn't look half bad:

image /= (image.max()/255.0)

For the other case you can write a function to normalize an n-dimensional array by colums:

def normalize_columns(arr):
    rows, cols = arr.shape
    for col in xrange(cols):
        arr[:,col] /= abs(arr[:,col]).max()

Answer 5

A simple solution is using the scalers offered by the sklearn.preprocessing library.

scaler = sk.MinMaxScaler(feature_range=(0, 250))
scaler = scaler.fit(X)
X_scaled = scaler.transform(X)
# Checking reconstruction
X_rec = scaler.inverse_transform(X_scaled)

The error X_rec-X will be zero. You can adjust the feature_range for your needs, or even use a standart scaler sk.StandardScaler()

Answer 6

You are trying to min-max scale the values of audio between -1 and +1 and image between 0 and 255.

Using sklearn.preprocessing.minmax_scale, should easily solve your problem.

e.g.:

audio_scaled = minmax_scale(audio, feature_range=(-1,1))

and

shape = image.shape
image_scaled = minmax_scale(image.ravel(), feature_range=(0,255)).reshape(shape)

note: Not to be confused with the operation that scales the norm (length) of a vector to a certain value (usually 1), which is also commonly referred to as normalization.

Answer 7

I tried following this, and got the error

TypeError: ufunc 'true_divide' output (typecode 'd') could not be coerced to provided output parameter (typecode 'l') according to the casting rule ''same_kind''

The numpy array I was trying to normalize was an integer array. It seems they deprecated type casting in versions > 1.10, and you have to use numpy.true_divide() to resolve that.

arr = np.array(img)
arr = np.true_divide(arr,[255.0],out=None)

img was an PIL.Image object.