Flexibly normalize arrays across any combination of dimensions
Author brendanjohnharris
6 Stars
Updated Last
12 Months Ago
Started In
March 2022


Build Status Coverage

This package allows you to easily normalize an array over any dimensions. It also provides a bunch of normalization methods, such as the z-score, sigmoid, robust, mixed, and NaN-safe normalizations.


Each normalization method is a subtype of AbstractNormalization. Instances of a normalization, including any parameters (such as the mean of a dataset) are stored in a variable of the AbstractNormalization type. For example, to normalize a 2D array using the ZScore normalization method (or any other <: AbstractNormalization) over all dimensions:

X = rand(100, 100)
N = ZScore(X) # A normalization fit to X, NOT the normalized array
N = ZScore()(X) # An alternative to the line above
Y = N(X) # The normalized array
Z = N(rand(100, 100)) # Apply a normalization with parameters fit to X on a new array

There is also an alternative, preferred, syntax:

using Statistics
N = fit(ZScore, X)
Y = normalize(X, N)
normalize!(X, N) # In place, writing over X

A normalization can also be reversed:

_X = denormalize(X, N) # Apply the inverse normalization
denormalize!(X, N) # Or do the inverse in place

Both syntaxes allow you to specify the dimensions to normalize over. For example, to normalize each 2D slice (i.e. iterating over the 3rd dimension) of a 3D array:

X = rand(100, 100, 10)
N = fit(ZScore, X; dims=[1, 2])
normalize!(X, N) # Each [1, 2] slice is normalized independently
all(std(X; dims=[1, 2]) .โ‰ˆ 1) # true

Normalization methods

Any of these normalizations will work in place of ZScore in the examples above:

Normalization Formula Description
ZScore $(x - \mu)/\sigma$ Subtract the mean and scale by the standard deviation (aka standardisation)
Sigmoid $(1 + \exp(-\frac{x-\mu}{\sigma}))^{-1}$ Map to the interval $(0, 1)$ by applying a sigmoid transformation
MinMax $(x-\inf{x})/(\sup{x}-\inf{x})$ Scale to the unit interval
Center $x - \mu$ Subtract the mean
UnitEnergy $x/\sum x^2$ Scale to have unit energy

Robust normalizations

This package also defines robust versions of any normalization methods that have $\mu$ (the mean) and $\sigma$ (the standard deviation) parameters. Robust normalizations, including RobustZScore and RobustSigmoid, use the median and iqr/1.35 rather than the mean and std for a normalization that is less sensitive to outliers. There are also Mixed methods, such as MixedZScore and MixedSigmoid, that default to the Robust versions but use the regular parameters (mean and std) if the iqr is 0.

NaN-safe normalizations

If the input array contains any NaN values, the normalizations given above will fit with NaN parameters and return NaN arrays. To circumvent this, any normalization can be made 'NaN-safe', meaning it ignores NaN values in the input array. Using the ZScore example:

N = nansafe(ZScore)
fit!(N, X)
Y = N(X)

New normalizations

Finally, there is also a macro to define your own normalization (honestly you could just make the struct directly). For example, the ZScore is defined as:

@_Normalization ZScore (mean, std)  (x, ๐œ‡, ๐œŽ) -> x .= (x .- ๐œ‡)./๐œŽ  #=
                                 =# (y, ๐œ‡, ๐œŽ) -> y .= y.*๐œŽ .+ ๐œ‡

Here, the first argument is a name for the normalization, the second is a tuple of parameter functions, the third is a vectorised, in-place function of an array x and any parameters, and the fourth is a function for the inverse transformation.

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