OpenCV: Using OpenCV.js

| OpenCV 3.4.2 Open Source Computer Vision

Using OpenCV.js

## Steps

In this tutorial, you will learn how to include and start to use `opencv.js` inside a web page.

### Create a web page

First, let's create a simple web page that is able to upload an image.

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Hello OpenCV.js</title>
</head>
<body>
<h2>Hello OpenCV.js</h2>
<div>
<div class="inputoutput">
<img id="imageSrc" alt="No Image" />
<div class="caption">imageSrc <input type="file" id="fileInput" name="file" /></div>
</div>
</div>
<script type="text/javascript">
let imgElement = document.getElementById("imageSrc")
let inputElement = document.getElementById("fileInput");
inputElement.addEventListener("change", (e) => {
imgElement.src = URL.createObjectURL(e.target.files[0]);
}, false);
</script>
</body>
</html>

To run this web page, copy the content above and save to a local index.html file. To run it, open it using your web browser.

Note
  It is a better practice to use a local web server to host the index.html.

### Include OpenCV.js

Set the URL of `opencv.js` to `src` attribute of <script> tag.

Note
  For this tutorial, we host `opencv.js` at same folder as index.html.

Example for synchronous loading:

<script src="opencv.js" type="text/javascript"></script>

You may want to load `opencv.js` asynchronously by `async` attribute in <script> tag. To be notified when `opencv.js` is ready, you can register a callback to `onload` attribute.

Example for asynchronous loading

<script async src="opencv.js" onload="onOpenCvReady();" type="text/javascript"></script>

### Use OpenCV.js

Once `opencv.js` is ready, you can access OpenCV objects and functions through `cv` object.

For example, you can create a cv.Mat from an image by cv.imread.

Note
  Because image loading is asynchronous, you need to put cv.Mat creation inside the `onload` callback.

imgElement.onload = function() {
let mat = cv.imread(imgElement);
}

Many OpenCV functions can be used to process cv.Mat. You can refer to other tutorials, such as Image Processing, for details.

In this tutorial, we just show a cv.Mat on screen. To show a cv.Mat, you need a canvas element.

<canvas id="outputCanvas"></canvas>

You can use cv.imshow to show cv.Mat on the canvas.

cv.imshow(mat, "outputCanvas");

Putting all of the steps together, the final index.html is shown below.

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Hello OpenCV.js</title>
</head>
<body>
<h2>Hello OpenCV.js</h2>
<p id="status">OpenCV.js is loading...</p>
<div>
<div class="inputoutput">
<img id="imageSrc" alt="No Image" />
<div class="caption">imageSrc <input type="file" id="fileInput" name="file" /></div>
</div>
<div class="inputoutput">
<canvas id="canvasOutput" ></canvas>
<div class="caption">canvasOutput</div>
</div>
</div>
<script type="text/javascript">
let imgElement = document.getElementById('imageSrc');
let inputElement = document.getElementById('fileInput');
inputElement.addEventListener('change', (e) => {
imgElement.src = URL.createObjectURL(e.target.files[0]);
}, false);
imgElement.onload = function() {
let mat = cv.imread(imgElement);
cv.imshow('canvasOutput', mat);
mat.delete();
};
function onOpenCvReady() {
document.getElementById('status').innerHTML = 'OpenCV.js is ready.';
}
</script>
<script async src="opencv.js" onload="onOpenCvReady();" type="text/javascript"></script>
</body>
</html>

Note
  You have to call delete method of cv.Mat to free memory allocated in Emscripten's heap. Please refer to Memeory management of Emscripten for details.

## Try it