Node's Pocket Monsters

    This package has been deprecated

    Author message:

    Package no longer supported. Contact Support at for more info.

    TypeScript icon, indicating that this package has built-in type declarations

    4.9.0 • Public • Published



    Build Status Build status Coverage npm download node version Slack

    By its nature, JavaScript lacks the performance to implement Computer Vision tasks efficiently. Therefore this package brings the performance of the native OpenCV library to your Node.js application. This project targets OpenCV 3 and provides an asynchronous as well as an synchronous API.

    The ultimate goal of this project is to provide a comprehensive collection of Node.js bindings to the API of OpenCV and the OpenCV-contrib modules. An overview of available bindings can be found in the API Documentation. Furthermore, contribution is highly appreciated. If you want to get involved you can have a look at the contribution guide.


    See examples for implementation.

    Face Detection

    face0 face1

    Face Recognition with the OpenCV face module

    Check out Node.js + OpenCV for Face Recognition.


    Face Landmarks with the OpenCV face module


    Face Recognition with face-recognition.js

    Check out Node.js + face-recognition.js : Simple and Robust Face Recognition using Deep Learning.


    Hand Gesture Recognition

    Check out Simple Hand Gesture Recognition using OpenCV and JavaScript.


    Object Recognition with Deep Neural Networks

    Check out Node.js meets OpenCV’s Deep Neural Networks — Fun with Tensorflow and Caffe.

    Tensorflow Inception

    husky car banana

    Single Shot Multibox Detector with COCO

    dishes-detection car-detection

    Machine Learning

    Check out Machine Learning with OpenCV and JavaScript: Recognizing Handwritten Letters using HOG and SVM.


    Object Tracking

    trackbgsubtract trackbycolor

    Feature Matching


    Image Histogram

    plotbgr plotgray

    How to install

    Important note: node-gyp won't handle whitespaces properly, thus make sure, that the path to your project directory does not contain any whitespaces. Installing opencv4nodejs under "C:\Program Files\some_dir" or similar will not work and will fail with: "fatal error C1083: Cannot open include file: 'opencv2/core.hpp'"!**


    • cmake (unless you are using a prebuilt OpenCV release)

    On Windows

    On windows you will need Windows Build Tools to compile OpenCV and opencv4nodejs. If you don't have Visual Studio or Windows Build Tools installed, you can easily install the VS2015 build tools:

    npm install --global windows-build-tools

    Auto build

    If you do not want to set up OpenCV on your own you can simply let this package auto install OpenCV 3.4 + OpenCV contrib 3.4 (might take some time):

    $ npm install --save opencv4nodejs

    Manual build

    Setting up OpenCV on your own will require you to set an environment variable: OPENCV4NODEJS_DISABLE_AUTOBUILD=1.

    You can either install any of the OpenCV 3+ releases (note, this will come without contrib) or build OpenCV with or without OpenCV contrib from source on your own. On Linux and MacOSX the library should be installed under usr/local (which is the default).

    On Windows

    If you choose to set up OpenCV on your own you have to set the following environment variables before installing opencv4nodejs:

    • OPENCV_INCLUDE_DIR pointing to the directory with the subfolders opencv and opencv2 containing the header files
    • OPENCV_LIB_DIR pointing to the lib directory containing the OpenCV .lib files

    Also you will need to add the OpenCV binaries to your system path:

    • add an environment variable OPENCV_BIN_DIR pointing to the binary directory containing the OpenCV .dll files
    • append ;%OPENCV_BIN_DIR%; to your system path variable

    Note: Restart your current console session after making changes to your environment.

    If you are running into issues also check the requirements for node-gyp specific to your OS:

    Usage with Docker

    opencv-express - example for opencv4nodejs with express.js and docker

    Or simply pull from justadudewhohacks/opencv-nodejs for opencv-3.2 + contrib-3.2 with opencv4nodejs globally installed:

    FROM justadudewhohacks/opencv-nodejs

    Note: The aforementioned Docker image already has opencv4nodejs installed globally. In order to prevent build errors during an npm install, your package.json should not include opencv4nodejs, and instead should include/require the global package either by requiring it by absolute path or setting the NODE_PATH environment variable to /usr/lib/node_modules in your Dockerfile and requiring the package as you normally would.

    Different OpenCV 3.x base images can be found here:

    Usage with Electron

    opencv-electron - example for opencv4nodejs with electron

    Add the following script to your package.json:

    "electron-rebuild": "electron-rebuild -w opencv4nodejs"

    Run the script:

    $ npm run electron-rebuild

    Require it in the application:

    const cv = require('opencv4nodejs');

    Usage with NW.js

    Any native modules, including opencv4nodejs, must be recompiled to be used with NW.js. Instructions on how to do this are available in the [Use Native Modules] ( section of the the NW.js documentation.

    Once recompiled, the module can be installed and required as usual:

    const cv = require('opencv4nodejs');

    Quick Start

    const cv = require('opencv4nodejs');

    Initializing Mat (image matrix), Vec, Point

    const rows = 100; // height
    const cols = 100; // width
    // empty Mat
    const emptyMat = new cv.Mat(rows, cols, cv.CV_8UC3);
    // fill the Mat with default value
    const whiteMat = new cv.Mat(rows, cols, cv.CV_8UC1, 255);
    const blueMat = new cv.Mat(rows, cols, cv.CV_8UC3, [255, 0, 0]);
    // from array (3x3 Matrix, 3 channels)
    const matData = [
      [[255, 0, 0], [255, 0, 0], [255, 0, 0]],
      [[0, 0, 0], [0, 0, 0], [0, 0, 0]],
      [[255, 0, 0], [255, 0, 0], [255, 0, 0]]
    const matFromArray = new cv.Mat(matData, cv.CV_8UC3);
    // from node buffer
    const charData = [255, 0, ...];
    const matFromArray = new cv.Mat(Buffer.from(charData), rows, cols, cv.CV_8UC3);
    // Point
    const pt2 = new cv.Point(100, 100);
    const pt3 = new cv.Point(100, 100, 0.5);
    // Vector
    const vec2 = new cv.Vec(100, 100);
    const vec3 = new cv.Vec(100, 100, 0.5);
    const vec4 = new cv.Vec(100, 100, 0.5, 0.5);

    Mat and Vec operations

    const mat0 = new cv.Mat(...);
    const mat1 = new cv.Mat(...);
    // arithmetic operations for Mats and Vecs
    const matMultipliedByScalar = mat0.mul(0.5);  // scalar multiplication
    const matDividedByScalar = mat0.div(2);       // scalar division
    const mat0PlusMat1 = mat0.add(mat1);          // addition
    const mat0MinusMat1 = mat0.sub(mat1);         // subtraction
    const mat0MulMat1 = mat0.hMul(mat1);          // elementwise multiplication
    const mat0DivMat1 = mat0.hDiv(mat1);          // elementwise division
    // logical operations Mat only
    const mat0AndMat1 = mat0.and(mat1);
    const mat0OrMat1 = mat0.or(mat1);
    const mat0bwAndMat1 = mat0.bitwiseAnd(mat1);
    const mat0bwOrMat1 = mat0.bitwiseOr(mat1);
    const mat0bwXorMat1 = mat0.bitwiseXor(mat1);
    const mat0bwNot = mat0.bitwiseNot();

    Accessing Mat data

    const matBGR = new cv.Mat(..., cv.CV_8UC3);
    const matGray = new cv.Mat(..., cv.CV_8UC1);
    // get pixel value as vector or number value
    const vec3 =, 100);
    const grayVal =, 100);
    // get raw pixel value as array
    const [b, g, r] = matBGR.atRaw(200, 100);
    // set single pixel values
    matBGR.set(50, 50, [255, 0, 0]);
    matBGR.set(50, 50, new Vec(255, 0, 0));
    matGray.set(50, 50, 255);
    // get a 25x25 sub region of the Mat at offset (50, 50)
    const width = 25;
    const height = 25;
    const region = matBGR.getRegion(new cv.Rect(50, 50, width, height));
    // get a node buffer with raw Mat data
    const matAsBuffer = matBGR.getData();
    // get entire Mat data as JS array
    const matAsArray = matBGR.getDataAsArray();


    // load image from file
    const mat = cv.imread('./path/img.jpg');
    cv.imreadAsync('./path/img.jpg', (err, mat) => {
    // save image
    cv.imwrite('./path/img.png', mat);
    cv.imwriteAsync('./path/img.jpg', mat,(err) => {
    // show image
    cv.imshow('a window name', mat);
    // load base64 encoded image
    const base64text='data:image/png;base64,R0lGO..';//Base64 encoded string
    const base64data =base64text.replace('data:image/jpeg;base64','')
                                .replace('data:image/png;base64','');//Strip image type prefix
    const buffer = Buffer.from(base64data,'base64');
    const image = cv.imdecode(buffer); //Image is now represented as Mat
    // convert Mat to base64 encoded jpg image
    const outBase64 =  cv.imencode('.jpg', croppedImage).toString('base64'); // Perform base64 encoding
    const htmlImg='<img src=data:image/jpeg;base64,'+outBase64 + '>'; //Create insert into HTML compatible <img> tag
    // open capture from webcam
    const devicePort = 0;
    const wCap = new cv.VideoCapture(devicePort);
    // open video capture
    const vCap = new cv.VideoCapture('./path/video.mp4');
    // read frames from capture
    const frame =;
    vCap.readAsync((err, frame) => {
    // loop through the capture
    const delay = 10;
    let done = false;
    while (!done) {
      let frame =;
      // loop back to start on end of stream reached
      if (frame.empty) {
        frame =;
      // ...
      const key = cv.waitKey(delay);
      done = key !== 255;

    Useful Mat methods

    const matBGR = new cv.Mat(..., cv.CV_8UC3);
    // convert types
    const matSignedInt = matBGR.convertTo(cv.CV_32SC3);
    const matDoublePrecision = matBGR.convertTo(cv.CV_64FC3);
    // convert color space
    const matGray = matBGR.bgrToGray();
    const matHSV = matBGR.cvtColor(cv.COLOR_BGR2HSV);
    const matLab = matBGR.cvtColor(cv.COLOR_BGR2Lab);
    // resize
    const matHalfSize = matBGR.rescale(0.5);
    const mat100x100 = matBGR.resize(100, 100);
    const matMaxDimIs100 = matBGR.resizeToMax(100);
    // extract channels and create Mat from channels
    const [matB, matG, matR] = matBGR.splitChannels();
    const matRGB = new cv.Mat([matR, matB, matG]);

    Drawing a Mat into HTML Canvas

    const img = ...
    // convert your image to rgba color space
    const matRGBA = img.channels === 1
      ? img.cvtColor(cv.COLOR_GRAY2RGBA)
      : img.cvtColor(cv.COLOR_BGR2RGBA);
    // create new ImageData from raw mat data
    const imgData = new ImageData(
      new Uint8ClampedArray(matRGBA.getData()),
    // set canvas dimensions
    const canvas = document.getElementById('myCanvas');
    canvas.height = img.rows;
    canvas.width = img.cols;
    // set image data
    const ctx = canvas.getContext('2d');
    ctx.putImageData(imgData, 0, 0);

    Method Interface

    OpenCV method interface from official docs or src:

    void GaussianBlur(InputArray src, OutputArray dst, Size ksize, double sigmaX, double sigmaY = 0, int borderType = BORDER_DEFAULT);

    translates to:

    const src = new cv.Mat(...);
    // invoke with required arguments
    const dst0 = src.gaussianBlur(new cv.Size(5, 5), 1.2);
    // with optional paramaters
    const dst2 = src.gaussianBlur(new cv.Size(5, 5), 1.2, 0.8, cv.BORDER_REFLECT);
    // or pass specific optional parameters
    const optionalArgs = {
      borderType: cv.BORDER_CONSTANT
    const dst2 = src.gaussianBlur(new cv.Size(5, 5), 1.2, optionalArgs);

    Async API

    The async API can be consumed by passing a callback as the last argument of the function call. By default, if an async method is called without passing a callback, the function call will yield a Promise.

    Async Face Detection

    const classifier = new cv.CascadeClassifier(cv.HAAR_FRONTALFACE_ALT2);
    // by nesting callbacks
    cv.imreadAsync('./faceimg.jpg', (err, img) => {
      if (err) { return console.error(err); }
      const grayImg = img.bgrToGray();
      classifier.detectMultiScaleAsync(grayImg, (err, res) => {
        if (err) { return console.error(err); }
        const { objects, numDetections } = res;
    // via Promise
      .then(img =>
          .then(grayImg => classifier.detectMultiScaleAsync(grayImg))
          .then((res) => {
            const { objects, numDetections } = res;
      .catch(err => console.error(err));
    // using async await
    try {
      const img = await cv.imreadAsync('./faceimg.jpg');
      const grayImg = await img.bgrToGrayAsync();
      const { objects, numDetections } = await classifier.detectMultiScaleAsync(grayImg);
    } catch (err) {

    With TypeScript

    import * as cv from 'opencv4nodejs'

    Check out the TypeScript examples.

    External Memory Tracking (v4.0.0)

    Since version 4.0.0 was released, external memory tracking has been enabled by default. Simply put, the memory allocated for Matrices (cv.Mat) will be manually reported to the node process. This solves the issue of inconsistent Garbage Collection, which could have resulted in spiking memory usage of the node process eventually leading to overflowing the RAM of your system, prior to version 4.0.0.

    Note, that in doubt this feature can be disabled by setting an environment variable OPENCV4NODEJS_DISABLE_EXTERNAL_MEM_TRACKING before requiring the module:


    Or directly in your code:

    const cv = require('opencv4nodejs')

    Available Modules

    API doc overview


    npm i @a-sync/opencv4nodejs

    DownloadsWeekly Downloads






    Unpacked Size

    10.9 MB

    Total Files


    Last publish


    • a-sync
    • peter.forgacs