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How to use OpenCV with Node.js?

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  • Difficulty Level : Basic
  • Last Updated : 12 May, 2021

OpenCV is a free-to-use cross-platform programming library that is mainly utilized for computer vision.  It is written in C/C++. Computer vision is widely used in applications like face detection, autonomous vehicles, etc. To use this with Node.js we will use opencv4nodejs library. Opencv4nodejs allows us to use the native OpenCV library with our Node.js application. This is a great way of implementing computer vision on your existing website and a great way to make projects. 

Advantages of using OpenCV with Node.js

  1. As OpenCV is written in C/C++ it is very efficient and hence does not slow down the site.
  2. Implementing on web applications that already have a Node.js backend is super easy hence any project can have features like facial recognition and computer vision.
  3. Easily use prebuilt classifiers which makes it easy to implement
  4. It is an open-source library that means it is continuously worked on and updated to improve performance.

Installing Required Modules:

  • Installing Express Module.
    npm install express
  • Installing Windows Build Tools Installing VS Code required built tools.
    npm install --global windows-build-tools
  • Installing module.
    npm install
  • Installing opencv4nodejs module.
    npm install --save opencv4nodejs

First, we’ll create a web express server that hosts the whole application, then we will use to update the image and then use OpenCV to read the image.

Example This HTML file will be used to display the image.


<!DOCTYPE html>
  <img id="image">
  <! --Connecting html page to -->
  <script src=
    // Connecting to localhost 
       const socket = io.connect('http://localhost:3000');
    // listening to the image event in index.js
    socket.on('image', (data)=>{
        console.log('data', data);
      // Now we are getting the image and 
      // displaying it via img tag
      const imageEle = document.getElementById('image');
      // Also we are decoding the base64 encoding 
      // set in javascript file.
      imageEle.src = data:image/jpeg;base64,${image};

This is the Node.js entry point file which will use express,, and OpenCV to display the image.



// importing OpenCv library
const cv = require('opencv4nodejs');
const path = require('path')
const express = require('express');
const app = express();
const server = require('http').Server(app);
const io = require('')(server);
// We will now create a video capture object.
const wCap = new cv.VideoCapture(0);
//Setting the height and width of object
wCap.set(cv.CAP_PROP_FRAME_WIDTH, 300);
wCap.set(cv.CAP_PROP_FRAME_HEIGHT, 300);
// Creating get request simple route
app.get('/', (req, res)=>{
    res.sendFile(path.join(__dirname, 'index.html'));
// Using setInterval to read the image every one second.
    // Reading image from video capture device
    const frame =;
    // Encoding the image with base64.
    const image = cv.imencode('.jpg', frame).toString('base64');
    io.emit('image', image);
}, 1000)

Run index.js file using below command:

node index.js

Output: Open any browser on our machine and type localhost:3000.

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