CODEWITHSUNDEEP
pythonimage-processingnumpyhistogram
pbu
pbu

Reputation: 3060

Histogram equalization of grayscale images with NumPy

How to do histogram equalization for multiple grayscaled images stored in a NumPy array easily?

I have the 96x96 pixel NumPy data in this 4D format:

(1800, 1, 96,96)

Upvotes: 11

Views: 39596

Answers (4)

NoDataDumpNoContribution
NoDataDumpNoContribution

Reputation: 10859

Moose's comment which points to this blog entry does the job quite nicely.

For completeness, I give an example here using nicer variable names and a looped execution on 1000 96x96 images which are in a 4D array as in the question. It is fast (1-2 seconds on my computer) and only needs NumPy.

import numpy as np

def image_histogram_equalization(image, number_bins=256):
    # from http://www.janeriksolem.net/histogram-equalization-with-python-and.html

    # get image histogram
    image_histogram, bins = np.histogram(image.flatten(), number_bins, density=True)
    cdf = image_histogram.cumsum() # cumulative distribution function
    cdf = (number_bins-1) * cdf / cdf[-1] # normalize

    # use linear interpolation of cdf to find new pixel values
    image_equalized = np.interp(image.flatten(), bins[:-1], cdf)

    return image_equalized.reshape(image.shape), cdf

if __name__ == '__main__':

    # generate some test data with shape 1000, 1, 96, 96
    data = np.random.rand(1000, 1, 96, 96)

    # loop over them
    data_equalized = np.zeros(data.shape)
    for i in range(data.shape[0]):
        image = data[i, 0, :, :]
        data_equalized[i, 0, :, :] = image_histogram_equalization(image)[0]

Upvotes: 19

Isaac Berrios
Isaac Berrios

Reputation: 165

Here's an alternate implementation for a single channel image that is fast. See skimage.exposure.histogram for reference. Using timeit, 'image_histogram_equalization' in Trilarion's answer has a mean execution time was 0.3696 seconds, while this function has a mean execution time of 0.0534 seconds. However this implementation also relies on skimage.

import numpy as np
from skimage import exposure

def hist_eq(image):
    hist, bins = exposure.histogram(image, nbins=256, normalize=False)
    # append any remaining 0 values to the histogram
    hist = np.hstack((hist, np.zeros((255 - bins[-1])))) 
    cdf = 255*(hist/hist.sum()).cumsum()
    equalized = cdf[image].astype(np.uint8)

    return equalized

Upvotes: 1

Diego Ferri
Diego Ferri

Reputation: 2787

As of today janeriksolem's url is broken.

I found however this gist that links the same page and claims to perform histogram equalization without computing the histogram.

The code is:

img_eq = np.sort(img.ravel()).searchsorted(img)

Upvotes: 2

Dodo
Dodo

Reputation: 147

Very fast and easy way is to use the cumulative distribution function provided by the skimage module. Basically what you do mathematically to proof it.

from skimage import exposure
import numpy as np
def histogram_equalize(img):
    img = rgb2gray(img)
    img_cdf, bin_centers = exposure.cumulative_distribution(img)
    return np.interp(img, bin_centers, img_cdf)

Upvotes: 4

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