How To Plot A Histogram Of A Grayscale Image In 2 Ways In Python Using OpenCV - 2024

In today’s very short blog we will see that how we can plot a histogram of a grayscale image.

The first way is using NumPy and the second way is using matplotlib. This is going to be a very interesting blog, so without any further due, Let’s do it…

Table of Contents

Step 1 – Import the libraries required for the histogram of a grayscale image.

import cv2
import matplotlib.pyplot as plt
import numpy as np

Step 2 – Let’s read the image.

imgpath = "4.2.07.tiff"
img = cv2.imread(imgpath,0)

Here while reading the image, we passed the second argument as 0 to read the image as a grayscale image.

Step 3 – Let’s plot the image and the histogram of a grayscale image.

Syntax: numpy.histogram(a, bins=10, range=None, normed=None, weights=None, density=None)

Parameters:

data	Input data. The histogram is computed over the flattened array.
bins	int or sequence or str defines the number of equal-width bins in a range, default is 10
range	The lower and upper range of the bins. If not provided, range is simply `(a.min(), a.max())`. Values outside the range are ignored.
normed	optional parameter same as density attribute, gives incorrect result for unequal bin width
weights	optional parameter defines array of weights having same dimensions as data
density	optional parameter if False result contains number of samples in each bin, if True result contain probability density function at bin

plt.subplot(1,2,1)
plt.imshow(img,cmap='gray')
plt.title('image')
plt.xticks([])
plt.yticks([])

plt.subplot(1,2,2)
hist,bin = np.histogram(img.ravel(),256,[0,255])
plt.xlim([0,255])
plt.plot(hist)
plt.title('histogram')

plt.show()

img.ravel is basically used to flatten the 2-D matrix(grayscale image) to a 1-D array.
Now we will be having our image as something like [125,113,8,45,63…] with no. of elements as n*m where n is the height of the grayscale image and m is the width of the grayscale image.
Read more about np.histogram here.

histogram of a grayscale image — Histogram using NumPy

Let’s see the whole code of 1st way using NumPy…

import cv2
import matplotlib.pyplot as plt
import numpy as np

imgpath = "test.tiff"
img = cv2.imread(imgpath,0)

plt.subplot(1,2,1)
plt.imshow(img,cmap='gray')
plt.title('image')
plt.xticks([])
plt.yticks([])

plt.subplot(1,2,2)
hist,bin = np.histogram(img.ravel(),256,[0,255])
plt.xlim([0,255])
plt.plot(hist)
plt.title('histogram')
plt.show()

Now let’s see the whole code of 2nd way using matplotlib.

import cv2
import matplotlib.pyplot as plt

imgpath = "test.tiff"
img = cv2.imread(imgpath,0)

plt.subplot(1,2,1)
plt.imshow(img,cmap='gray')
plt.title('image')
plt.xticks([])
plt.yticks([])

plt.subplot(1,2,2)
plt.hist(img.ravel(),256,[0,255])
plt.title('histogram')

plt.show()