OpenCV Python Program to analyze an image using Histogram

Last Updated : 11 Aug, 2025

In this article, we will learn how to analyze an image using histograms with OpenCV and Matplotlib in Python. A histogram represents the distribution of pixel intensity values in an image, helping us understand brightness, contrast and overall image composition.

Before getting started, make sure you have the following installed:

Importing image data

For demonstration, we will use a 24-bit RGB PNG image (8 bits for each of R, G, B channels). Matplotlib supports image arrays in float32 and uint8 formats.

Python

import matplotlib.pyplot as plt
import cv2

img = plt.imread('flower.png')  
plt.imshow(img)
plt.title("Original Image")
plt.show()

Output

Explanation: plt.imread() loads the image as an array, plt.imshow() displays it, plt.title() adds a title, and plt.show() renders the output.

Creating Histogram using Numpy & Matplotlib

To analyze intensity distribution, we can flatten (ravel()) the image into a 1D array and use plt.hist() to compute the histogram.

For grayscale images, this shows the spread of pixel values (0–255).
For RGB images, we can plot separate histograms for the Red, Green and Blue channels.

Python

import cv2, matplotlib.pyplot as plt
img = cv2.imread("flower.png")

# Grayscale + Histogram
g = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
plt.subplot(121), plt.imshow(g, cmap='gray'), plt.axis("off"), plt.title("Grayscale")
plt.subplot(122), plt.hist(g.ravel(),256,[0,256],color='k'), plt.title("Gray Histogram")
plt.show()

# RGB Histograms
for i,c in enumerate(('r','g','b')):
    plt.plot(cv2.calcHist([img],[i],None,[256],[0,256]), color=c)
plt.title("RGB Histograms"), plt.xlabel("Intensity"), plt.ylabel("Frequency")
plt.show()

Output

Explanation:

cv2.imread() loads the image, cv2.cvtColor() makes it grayscale, plt.imshow() displays it, and plt.hist() shows its intensity histogram.
A loop over ('r','g','b') uses cv2.calcHist() and plt.plot() to display RGB channel histograms.

Histogram Calculation using OpenCV

OpenCV provides an in-built function cv2.calcHist() for histogram calculation. It is more efficient and widely used in image processing tasks.

Python

img = cv2.imread('flower.jpg', 0)
histg = cv2.calcHist([img], [0], None, [256], [0, 256])

# Plot histogram
plt.plot(histg)
plt.title("Histogram using OpenCV calcHist()")
plt.xlabel("Pixel Intensity")
plt.ylabel("Frequency")
plt.show()

Output

Explanation: cv2.imread() loads the grayscale image, cv2.calcHist() computes its histogram and plt.plot() displays it with title and labels.

Histogram for Color Images

For color images, histograms can be calculated separately for Blue, Green and Red channels.

Python

img = cv2.imread('ex.jpg')

# colors for channels
colors = ('b', 'g', 'r')

for i, col in enumerate(colors):
    hist = cv2.calcHist([img], [i], None, [256], [0, 256])
    plt.plot(hist, color=col)
    plt.xlim([0, 256])

plt.title("RGB Color Histogram")
plt.xlabel("Pixel Intensity")
plt.ylabel("Frequency")
plt.show()

Output

Explanation: cv2.imread() loads the image, a loop over ('b','g','r') uses cv2.calcHist() to compute each channel’s histogram and plt.plot() displays them with axis labels and title.

Plotting Histograms

Apart from analyzing images with NumPy and OpenCV, we can also plot histograms using two common methods:

1. Using OpenCV (cv2.calcHist())

cv2.calcHist() calculate histograms efficiently. It gives full control over the number of bins, intensity range and even allows applying masks for region-based histogram analysis.

Python

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

histr = cv2.calcHist([img],[0],None,[256],[0,256])
plt.plot(histr)
plt.show()

Input:

Output:

Explanation: cv2.imread() loads the image in grayscale, cv2.calcHist() computes its intensity histogram and plt.plot() displays it.

2. Using plt.hist()

plt.hist(), an image is flattened into a 1D array and its pixel intensity distribution is quickly visualized, making it ideal for simple exploration tasks.

Python

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

# alternative way to find histogram of an image
plt.hist(img.ravel(),256,[0,256])
plt.show()