Using OpenCV.js {#tutorial_js_usage}
===============================
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.
@code{.js}
Hello OpenCV.js
Hello OpenCV.js
@endcode
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 \ tag.
@note For this tutorial, we host `opencv.js` at same folder as index.html.
Example for synchronous loading:
@code{.js}
@endcode
You may want to load `opencv.js` asynchronously by `async` attribute in \ tag. To be notified when `opencv.js` is ready, you can register a callback to `onload` attribute.
Example for asynchronous loading
@code{.js}
@endcode
### 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.
@code{.js}
imgElement.onload = function() {
let mat = cv.imread(imgElement);
}
@endcode
Many OpenCV functions can be used to process cv.Mat. You can refer to other tutorials, such as @ref tutorial_js_table_of_contents_imgproc, for details.
In this tutorial, we just show a cv.Mat on screen. To show a cv.Mat, you need a canvas element.
@code{.js}
@endcode
You can use cv.imshow to show cv.Mat on the canvas.
@code{.js}
cv.imshow(mat, "outputCanvas");
@endcode
Putting all of the steps together, the final index.html is shown below.
@code{.js}
Hello OpenCV.js
Hello OpenCV.js
OpenCV.js is loading...
@endcode
@note You have to call delete method of cv.Mat to free memory allocated in Emscripten's heap. Please refer to [Memory management of Emscripten](https://kripken.github.io/emscripten-site/docs/porting/connecting_cpp_and_javascript/embind.html#memory-management) for details.
Try it
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\endhtmlonly