Python OpenCV Histogram of Grayscale Image

A histogram is a graphical representation showing how frequently various color values occur in an image. It a graph or plot which represents the intensity distribution of an image. It is a plot with pixel values (ranging from 0 to 255, not always) in X-axis and corresponding number of pixels in the image on Y-axis.

Python OpenCV provides cv2.calcHist() function to calculate the histogram of one or more arrays. We can use this function to calculate the histogram of both single channel images and multi-channel images. In this article, we have used only single channel image.

Syntax of calcHist()

cv2.calcHist(images, channels, mask, histSize, ranges)

images - It is image source of type uint8 or float32.
channels - It represents the index of the channel. It is given in square bracket. For grayscale image, it's value is [0].
mask - It represents a mask image. To find histogram of the full image, it is given as "None".
histSize - It represents the number of bins provided as a list.
ranges - It represents the range of intensity values.

Required Modules

• OpenCV (cv2)
• Numpy
• Matplotlib

Example of grayscale image histogram without Mask

Here is the code for calculating the histogram of a full grayscale image. In this case, hist is a (256,1) array. Each value of the array corresponds to the number of pixels with the corresponding tone value.

In this example, the matplotlib library is used to plot the histograms. It provides hist() function for plotting.

import cv2
import numpy as np
from matplotlib import pyplot as plt
  
# image path 
path = r'cat.jpg'

# using imread()   
img = cv2.imread(path, cv2.IMREAD_GRAYSCALE)
cv2.imshow('Cat',img)

dst = cv2.calcHist(img, [0], None, [256], [0,256])

plt.hist(img.ravel(),256,[0,256])
plt.title('Histogram for gray scale image')
plt.show()

Example of grayscale image histogram with mask

The mask consists of a black image with the same dimensions as the loaded image and with some white regions corresponding to the image where we want to calculate the histogram. The following code creates mask -

mask = np.zeros(img.shape[:2], np.uint8)
mask[100:300, 100:400] = 255

Here is the complete code to draw a histogram of image with mask. In this example, it will be very clear for you to draw a histogram with and without mask.

import cv2
import numpy as np
from matplotlib import pyplot as plt
  
img = cv2.imread('nature.jpg',0)

# create a mask
mask = np.zeros(img.shape[:2], np.uint8)
mask[100:300, 100:400] = 255
masked_img = cv2.bitwise_and(img,img,mask = mask)

# Calculate histogram with mask and without mask
hist_full = cv2.calcHist([img],[0],None,[256],[0,256])
hist_mask = cv2.calcHist([img],[0],mask,[256],[0,256])

plt.subplot(221), plt.imshow(img, 'gray')
plt.subplot(222), plt.imshow(mask,'gray')
plt.subplot(223), plt.imshow(masked_img, 'gray')
plt.subplot(224), plt.plot(hist_full), plt.plot(hist_mask)
plt.xlim([0,256])

plt.show()

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