Python OpenCV: Draw Grayscale Image Histogram Using cv2.calcHist()

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In this tutorial, we will use an example to show python opencv beginners on how to draw grayscale image histogram using cv2.calcHist().

Python OpenCV - Draw Grayscale Image Histogram Using cv2.calcHist() for Beginners

1.Open an grayscale image using opencv

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

gray_img = cv2.imread('images/GoldenGateSunset.png', cv2.IMREAD_GRAYSCALE)
cv2.imshow('GoldenGate',gray_img)

2.Calculate histogram value using cv2.calcHist()

hist = cv2.calcHist([gray_img],[0],None,[256],[0,256])

We should notice:

cv2.calcHist() is defined as:

cv2.calcHist(images, channels, mask, histSize, ranges[, hist[, accumulate]])

Parameters are:

  • images: source image
  • channels: the index of channel for which we calculate histogram. For example, if input is grayscale image, its value is [0]. For color image, you can pass [0],[1] or [2] to calculate histogram of blue,green or red channel, respectively.
  • mask: mask image.
  • histSize: this represents our BIN count.For full scale, we pass [256].
  • ranges: Normally, it is [0,256].

3.Display histogram plot using matplotlib

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

4.Release resources

while True:
    k = cv2.waitKey(0) & 0xFF     
    if k == 27: break             # ESC key to exit 
cv2.destroyAllWindows()

Run this code, you may get this histogram:

Python OpenCV: Draw Grayscale Image Histogram Using cv2.calcHist()