etutorialspoint
  • Home
  • PHP
  • MySQL
  • MongoDB
  • HTML
  • Javascript
  • Node.js
  • Express.js
  • Python
  • Jquery
  • R
  • Kotlin
  • DS
  • Blogs
  • Theory of Computation

Python OpenCV Histogram of Grayscale Image

In this post, you will learn how to compute the histogram of an image using Python OpenCV or cv2.calchist function.

A histogram is a graphical representation showing how frequently various color values occur in an image. It is 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 the X-axis and the corresponding number of pixels in the image on the 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 an image source of type uint8 or float32.
channels: It represents the index of the channel. It is given in a square bracket. For the grayscale image, its value is [0].
mask: It represents a mask image. To find the 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 the 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()
OpenCV Histogram OpenCV Histogram

Example of grayscale image histogram with mask

The mask consists of a black image with the same dimensions as the loaded image and some white regions corresponding to the image where we want to calculate the histogram. The following code creates a 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 the image with mask. In this example, it will be very clear for you to draw a histogram with and without a 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()
OpenCV Mask Histogram



Related Articles

Bitwise and OpenCV
Capture a video in Python OpenCV and save
Python OpenCV Histogram of Color Image
Arithmetic Operations on Images using Python OpenCV
Contour Detection using Python OpenCV
Python overlay two images
Python OpenCV Gaussian Blur Filtering
Python OpenCV Overview and Examples
Draw different shapes on image using Python OpenCV
How to convert MySQL query result to JSON in Python
How to display data from MongoDB in HTML table using Python
CRUD operations in Python using MongoDB connector
Write Python Pandas Dataframe to CSV
Quick Introduction to Python Pandas
Python Pandas DataFrame
Python3 Tkinter Messagebox
Python get visitor information by IP address
Python Webbrowser




Most Popular Development Resources
Retrieve Data From Database Without Page refresh Using AJAX, PHP and Javascript
-----------------
PHP Create Word Document from HTML
-----------------
How to get data from XML file in PHP
-----------------
Hypertext Transfer Protocol Overview
-----------------
PHP code to send email using SMTP
-----------------
Characteristics of a Good Computer Program
-----------------
How to encrypt password in PHP
-----------------
Create Dynamic Pie Chart using Google API, PHP and MySQL
-----------------
PHP MySQL PDO Database Connection and CRUD Operations
-----------------
Splitting MySQL Results Into Two Columns Using PHP
-----------------
Dynamically Add/Delete HTML Table Rows Using Javascript
-----------------
How to get current directory, filename and code line number in PHP
-----------------
How to add multiple custom markers on google map
-----------------
Get current visitor\'s location using HTML5 Geolocation API and PHP
-----------------
Fibonacci Series Program in PHP
-----------------
Simple star rating system using PHP, jQuery and Ajax
-----------------
How to Sort Table Data in PHP and MySQL
-----------------
Simple pagination in PHP with MySQL
-----------------
How to generate QR Code in PHP
-----------------
Submit a form data using PHP, AJAX and Javascript
-----------------
PHP MYSQL Advanced Search Feature
-----------------
jQuery loop over JSON result after AJAX Success
-----------------
Recover forgot password using PHP7 and MySQLi
-----------------
PHP Server Side Form Validation
-----------------
jQuery File upload progress bar with file size validation
-----------------
PHP user registration and login/ logout with secure password encryption
-----------------
To check whether a year is a leap year or not in php
-----------------
Php file based authentication
-----------------
Simple File Upload Script in PHP
-----------------
Simple PHP File Cache
-----------------
PHP User Authentication by IP Address
-----------------
Calculate the distance between two locations using PHP
-----------------
PHP Secure User Registration with Login/logout
-----------------
Polling system using PHP, Ajax and MySql
-----------------
How to print specific part of a web page in javascript
-----------------
Detect Mobile Devices in PHP
-----------------
Simple Show Hide Menu Navigation
-----------------
Simple way to send SMTP mail using Node.js
-----------------
SQL Injection Prevention Techniques
-----------------
Get Visitor\'s location and TimeZone
-----------------
Preventing Cross Site Request Forgeries(CSRF) in PHP
-----------------
Google Street View API Example
-----------------
PHP Sending HTML form data to an Email
-----------------
Driving route directions from source to destination using HTML5 and Javascript
-----------------
CSS Simple Menu Navigation Bar
-----------------
Date Timestamp Formats in PHP
-----------------
Convert MySQL to JSON using PHP
-----------------
PHP Programming Error Types
-----------------
Set and Get Cookies in PHP
-----------------
How to add google map on your website and display address on click marker
-----------------
How to select/deselect all checkboxes using Javascript
-----------------
PHP Getting Document of Remote Address
-----------------
How to display PDF file in web page from Database in PHP
-----------------
File Upload Validation in PHP
-----------------
PHP FTP Connection and File Handling
-----------------


Most Popular Blogs
Most in demand programming languages
Best mvc PHP frameworks in 2019
MariaDB vs MySQL
Most in demand NoSQL databases for 2019
Best AI Startups In India
Kotlin : Android App Development Choice
Kotlin vs Java which one is better
Top Android App Development Languages in 2019
Web Robots
Data Science Recruitment of Freshers - 2019


Interview Questions Answers
Basic PHP Interview
Advanced PHP Interview
MySQL Interview
Javascript Interview
HTML Interview
CSS Interview
Programming C Interview
Programming C++ Interview
Java Interview
Computer Networking Interview
NodeJS Interview
ExpressJS Interview
R Interview


Popular Tutorials
PHP Tutorial (Basic & Advance)
MySQL Tutorial & Exercise
MongoDB Tutorial
Python Tutorial & Exercise
Kotlin Tutorial & Exercise
R Programming Tutorial
HTML Tutorial
jQuery Tutorial
NodeJS Tutorial
ExpressJS Tutorial
Theory of Computation Tutorial
Data Structure Tutorial
Javascript Tutorial




General Knowledge

listen
listen
listen
listen
listen
listen
listen
listen
listen


Learn Popular Language

listen
listen
listen
listen
listen

Blogs

  • Jan 3

    Stateful vs Stateless

    A Stateful application recalls explicit subtleties of a client like profile, inclinations, and client activities...

  • Dec 29

    Best programming language to learn in 2021

    In this article, we have mentioned the analyzed results of the best programming language for 2021...

  • Dec 20

    How is Python best for mobile app development?

    Python has a set of useful Libraries and Packages that minimize the use of code...

  • July 18

    Learn all about Emoji

    In this article, we have mentioned all about emojis. It's invention, world emoji day, emojicode programming language and much more...

  • Jan 10

    Data Science Recruitment of Freshers

    In this article, we have mentioned about the recruitment of data science. Data Science is a buzz for every technician...

Follow us

  • etutorialspoint facebook
  • etutorialspoint twitter
  • etutorialspoint linkedin
etutorialspoint youtube
About Us      Contact Us


  • eTutorialsPoint©Copyright 2016-2022. All Rights Reserved.