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    5.0.1 • Public • Published

    Demo of Layout Container


    Automatic layout of video elements (publisher and subscriber) minimising white-space for the OpenTok on WebRTC API. This is intended for use with the OpenTok on WebRTC JS API but can be used for any case where you're trying to fit multiple videos into a box. It could possibly be used for Photos as well.

    It automatically detects when an element is added or when the container is resized and it adjusts the layout of its' children accordingly. It can also arrange items with different aspect ratios and maintain their aspect ratios without cropping.

    The difference between this layout algorithm and other similar photo layout algorithms, such as the Flickr Justified Layout algorithm, is that this has a constrained height as well as width. The assumption is that when you're viewing video you want all videos to be visible at the same time. You don't want to scroll down to continue to view the list. So this algorithm fits everything inside a box and tries to minimize the amount of whitespace.


    Demo of layout container in action


    The OpenTok for WebRTC JS API is required.


    Call initLayoutContainer and pass it the element you want it to layout. It works best if you set the position of the element to be absolute or relative. You will then be returned an object with a layout method that you can call to layout the elements inside.

    const initLayoutContainer = require('opentok-layout-js');
    const options = {
        maxRatio: 3/2,             // The narrowest ratio that will be used (default 2x3)
        minRatio: 9/16,            // The widest ratio that will be used (default 16x9)
        fixedRatio: false,         // If this is true then the aspect ratio of the video is maintained and minRatio and maxRatio are ignored (default false)
        scaleLastRow: true,        // If there are less elements on the last row then we can scale them up to take up more space
        alignItems: 'center',      // Can be 'start', 'center' or 'end'. Determines where to place items when on a row or column that is not full
        bigClass: "OT_big",        // The class to add to elements that should be sized bigger
        bigPercentage: 0.8,        // The maximum percentage of space the big ones should take up
        minBigPercentage: 0,       // If this is set then it will scale down the big space if there is left over whitespace down to this minimum size
        bigFixedRatio: false,      // fixedRatio for the big ones
        bigScaleLastRow: true,     // scale last row for the big elements
        bigAlignItems: 'center',   // How to align the big items
        smallAlignItems: 'center', // How to align the small row or column of items if there is a big one
        maxWidth: Infinity,        // The maximum width of the elements
        maxHeight: Infinity,       // The maximum height of the elements
        smallMaxWidth: Infinity,   // The maximum width of the small elements
        smallMaxHeight: Infinity,  // The maximum height of the small elements
        bigMaxWidth: Infinity,     // The maximum width of the big elements
        bigMaxHeight: Infinity,    // The maximum height of the big elements
        bigMaxRatio: 3/2,          // The narrowest ratio to use for the big elements (default 2x3)
        bigMinRatio: 9/16,         // The widest ratio to use for the big elements (default 16x9)
        bigFirst: true,            // Whether to place the big one in the top left (true) or bottom right (false).
                                   // You can also pass 'column' or 'row' to change whether big is first when you are in a row (bottom) or a column (right) layout
        animate: true,             // Whether you want to animate the transitions using jQuery (not recommended, use CSS transitions instead)
        window: window,            // Lets you pass in your own window object which should be the same window that the element is in
        ignoreClass: 'OT_ignore',  // Elements with this class will be ignored and not positioned. This lets you do things like picture-in-picture
        onLayout: null,            // A function that gets called every time an element is moved or resized, (element, { left, top, width, height }) => {} 
    const layout = initLayoutContainer(document.getElementById("layout"), options);

    The other way to use this library is to just use the getLayout() method to get the layout data and position the elements yourself. getLayout() takes an array of objects with a width and height property along with a big property indicating whether it should be treated as a bigger element. eg.

    const layout = initLayoutContainer(options);
    const boxes = layout.getLayout([
            width: 640,     // The native width of this element (eg. subscriber.videoWidth())
            height: 480,    // The native height of this element (eg. subscriber.videoHeight())
            big: false      // Whether to treat this element as a bigger element

    It will return an object { boxes: Box[], areas: { small: Box, big: Box } }. This object includes an array of boxes which will be in the same order as the array you passed in. It will also contain an areas property which tells you about the dimensions of the small and big areas. Each Box in the array and in the areas will look like:



    Cropping videos to fit best.

    In this example we crop the videos to take up the most space in the window.


    Cropping videos to fit best

    Respecting different aspect ratios

    In this example we are respecting the native aspect ratio of multiple different videos.

    initLayoutContainer(document.getElementById("layout"), {
      fixedRatio: true

    Respecting different aspect ratios

    Some big videos

    This example has 2 big videos focused and the other 4 small.

    Some big videos

    Some big respecting aspect ratios

    This example has 2 big and we're respecting the native aspect ratios of the different videos.

    Some big respecting aspect ratios

    Using in an OpenTok Application

    In an OpenTok application you would do something like:

        <title>Layout Container Example</title>
        <script src=""></script>
        <script src="js/opentok-layout.min.js"></script>
        <style type="text/css" media="screen">
            #layoutContainer {
                width: 320px;
                height: 240px;
                background-color: #DDD;
        <div id="layoutContainer">
            <div id="publisherContainer"></div>
    <script type="text/javascript" charset="utf-8">
        var layoutContainer = document.getElementById("layoutContainer");
        // Initialize the layout container and get a reference to the layout method
        var layout = initLayoutContainer(layoutContainer).layout;
        // Below is a normal hello world OpenTok application for v2 of the API
        // The layout container will redraw when the layout mtehod is called and
        // adjust the layout accordingly
        var sessionId = "mySessionId";
        var token = "myToken";
        var apiKey = "myAPIKey";
        var session = OT.initSession(sessionId);
        session.on("streamCreated", function(event){
            session.subscribe(, "layoutContainer", {
                insertMode: "append"
        }).connect(apiKey, token, function (err) {
            if (!err) {

    Now any time you call the layout method the layout container will automatically be positioned and sized so that it minimises the white-space within the box given.

    Resizing the Window

    You want to call the layout method any time the size of the layout container changes. If the size of the container varies depending on the size of the window you may need to do:

    var resizeTimeout;
    window.onresize = function() {
      resizeTimeout = setTimeout(function () {
      }, 20);

    This throttles calling layout so that it's not called over an over again when you're resizing the window.


    If you want to animate things you can use CSS3 transitions on the children. eg.

    .container > * {
      transition-property: all;
      transition-duration: 0.5s;

    This library adds an ot-layout className to elements once they have been layed out. This allows you to specify an initial transition for new elements being added like so:

    .container > * {
        width: 0;
        height: 0;
        opacity: 0;
    .container > *.ot-layout {
        opacity: 1;

    This will make the elements be size 0x0 with opacity 0 when they are first added. Then when they have the ot-layout className added they will grow to the right size and fade in. You can see this effect in the demo.

    Then if you want elements to fade out you can remove the ot-layout class from them before removing them from the DOM. Like so:

    session.on('streamDestroyed', (event) => {
      session.getSubscribersForStream( => {
        setTimeout(() => {
        }, 200);

    Big Elements

    If you add the bigClass to elements in the layout container they will be treated differently. They are essentially in a layout container of their own which takes up bigPercentage of the space and has it's own settings for ratios (bigMaxRatio, bigMinRatio, bigFixedRatio).

    You can have multiple elements which are treated as big elements which allow you to have all kinds of different layouts.

    To see how this works try the demo and double click on elements.

    Padding, Margins and Borders

    The Layout Container will take into account the padding, margins and borders on it's children. If you want spacing between elements simply give them a margin or padding and they will be spaced accordingly.


    npm i opentok-layout-js

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    • aullman