HTML5 videoにキャプションと字幕をつける方法

キャプション付きのビデオの例

この記事では、私達はVキャプション付きのvideoに関して説明します。この例ではBlender Foundationが作成したSintel open movieを使用します。

HTML5 and Video Captions

どうやってビデオにキャプション機能をつけるかについて説明する前に、私達が知っておくべきいくつかのことについて説明します。

キャプション V.S. 字幕

Captions and subtitles are not the same thing: they have significantly different audiences, and convey different information, and it is recommended that you read up on the differences if you are not sure what they are. They are however implemented in the same way technically, so the material in this article will apply to both.

For this article we will refer to the text tracks displayed as captions, as their content is aimed at hearing people who have difficulty understanding the language of the film, rather than deaf or hard-of-hearing people.

 <track> タグ

 <track> タグを使用することでHTML5ではビデオにキャプションをつけることができます。 The various attributes of this element allow us to specify such things as the type of content that we're adding, the language it's in, and of course a reference to the text file that contains the actual caption information.

WebVTT

The files that contain the actual caption data are simple text files that follow a specified format, in this case the Web Video Text Tracks (WebVTT) format. The WebVTT specification is still being worked on, but major parts of it are stable so we can use it today.

Video providers (such as the Blender Foundation) provide captions and subtitles in a text format with their videos, but they're usually in the SubRip Text (SRT) format. These can be easily converted to WebVTT using an online converter such as srt2vtt.

Modifications to the HTML and CSS

This section summarises the modifications made to the previous article's code in order to facilitate the addition of subtitles to the video. If you are not interested in thism and just want to get straight into the JavaScript and more relevant CSS, skip to the Caption implementation section.

In this example we are using a different video, Sintel, as it actually has some speech in it and therefore is better for illustrating how captions work!

HTML Markup

As mentioned above, we need to make use of the new HTML5 <track> element to add our caption files to the HTML5 video. We actually have our captions in three different languages — English, German, and Spanish — so we will reference all three of the relevant VTT files by adding <track> elements inside our HTML5 <video> element:

<video id="video" controls preload="metadata">
   <source src="video/sintel-short.mp4" type="video/mp4">
   <source src="video/sintel-short.webm" type="video/webm">
   <track label="English" kind="captions" srclang="en" src="captions/vtt/sintel-en.vtt" default>
   <track label="Deutsch" kind="captions" srclang="de" src="captions/vtt/sintel-de.vtt">
   <track label="Español" kind="captions" srclang="es" src="captions/vtt/sintel-es.vtt">
</video>

As you can see, each <track> element has the following attributes set:

• kind is given a value of captions, indicating the type of content the files contain
• labelにはキャプションが何語なのかの情報が入ります。例えば、 English や Deutsch です。これらの Labels は画面上に表示されユーザーは簡単に言語を選択することができます。
• src is assigned a valid URL pointing to the relevant WebVTT caption file in each case.
• srclang indicates what language each captions files' contents are in.
• The default attribute is set on the English <track> element, indicating to the browser that this is the default caption file definition to use when captions have been turned on and the user has not made a specific selection.

In addition to adding the <track> elements, we have also added a new button to control the captions menu that we will build. As a consequence, the video controls now look as follows:

<div id="video-controls" class="controls" data-state="hidden">
   <button id="playpause" type="button" data-state="play">Play/Pause</button>
   <button id="stop" type="button" data-state="stop">Stop</button>
   <div class="progress">
      <progress id="progress" value="0" min="0">
         <span id="progress-bar"></span>
      </progress>
   </div>
   <button id="mute" type="button" data-state="mute">Mute/Unmute</button>
   <button id="volinc" type="button" data-state="volup">Vol+</button>
   <button id="voldec" type="button" data-state="voldown">Vol-</button>
   <button id="fs" type="button" data-state="go-fullscreen">Fullscreen</button>
   <button id="captions" type="button" data-state="captions">CC</button>
</div>

CSSの変更

The video controls have undergone some minor changes in order to make space for the extra button, but these are relatively straightforward.

No image is used for the captions button, so it is simply styled as:

.controls button[data-state="captions"] {
    height:85%;
    text-indent:0;
    font-size:16px;
    font-size:1rem;
    font-weight:bold;
    color:#666;
    background:#000;
    -moz-border-radius:2px;
    -webkit-border-radius:2px;
    border-radius:2px;
}

There are also other CSS changes that are specific to some extra JavaScript implementation, but these will be mentioned at the appropriate place below.

Caption implementation

A lot of what we do to access the video captions revolves around JavaScript. Similar to the video controls, if a browser supports HTML5 video captions, there will be a button provided within the native control set to access them. However, since we have defined our own video controls, this button is hidden, and we need to define our own.

Browsers do vary as to what they support, so we will be attempting to bring a more unified UI to each browser where possible. There's more on browser compatibility issues later on.

Initial setup

As with all the other buttons, one of the first things we need to do is store a handle to the captions' button:

var captions = document.getElementById('captions');

We also initially turn off all captions, in case the browser turns any of them on by default:

for (var i = 0; i < video.textTracks.length; i++) {
   video.textTracks[i].mode = 'hidden';
}

The video.textTracks property contains an array of all the text tracks attached to the video. We loop through each one and set its mode to hidden.

Note: The WebVTT API gives us access to all the text tracks that are defined for an HTML5 video using the <track> element.

Building a caption menu

Our aim is to use the captions button we added earlier to display a menu that allows users to choose which language they want the captions displayed in, or to turn them off entirely.

We have added the button, but before we make it do anything, we need to build the menu that goes with it. This menu is built dynamically, so that languages can be added or removed later by simply editing the <track> elements within the video's markup.

All we need to do is to go through the video's textTracks, reading their properties and building the menu up from there:

var captionsMenu;
if (video.textTracks) {
   var df = document.createDocumentFragment();
   var captionsMenu = df.appendChild(document.createElement('ul'));
   captionsMenu.className = 'captions-menu';
   captionsMenu.appendChild(createMenuItem('captions-off', '', 'Off'));
   for (var i = 0; i < video.textTracks.length; i++) {
      captionsMenu.appendChild(createMenuItem('captions-' + video.textTracks[i].language, video.textTracks[i].language,         video.textTracks[i].label));
   }
   videoContainer.appendChild(captionsMenu);
}

This code creates a documentFragment, which is used to hold an unordered list containing our captions menu. First of all an option is added to allow the user to switch all captions off, and then buttons are added for each text track, reading the language and label from each one.

The creation of each list item and button is done by the createMenuItem() function, which is defined as follows:

var captionMenuButtons = [];
var createMenuItem = function(id, lang, label) {
   var listItem = document.createElement('li');
   var button = listItem.appendChild(document.createElement('button'));
   button.setAttribute('id', id);
   button.className = 'captions-button';
   if (lang.length > 0) button.setAttribute('lang', lang);
   button.value = label;
   button.setAttribute('data-state', 'inactive');
   button.appendChild(document.createTextNode(label));
   button.addEventListener('click', function(e) {
      // Set all buttons to inactive
      captionMenuButtons.map(function(v, i, a) {
         captionMenuButtons[i].setAttribute('data-state', 'inactive');
      });
      // Find the language to activate
      var lang = this.getAttribute('lang');
      for (var i = 0; i < video.textTracks.length; i++) {
         // For the 'captions-off' button, the first condition will never match so all will captions be turned off
         if (video.textTracks[i].language == lang) {
            video.textTracks[i].mode = 'showing';
            this.setAttribute('data-state', 'active');
         }
         else {
            video.textTracks[i].mode = 'hidden';
         }
      }
      captionsMenu.style.display = 'none';
   });
   captionMenuButtons.push(button);
   return listItem;
}

This function builds the required <li> and <button> elements, and returns them so they can be added to the captions menu list. It also sets up the required event listeners on the button to toggle the relevant caption set on or off. This is done by simply setting the required caption's mode attribute to showing, and setting the others to hidden.

Once the menu is built, it is then inserted into the DOM at the bottom of the videoContainer.

Initially the menu is hidden by default, so an event listener needs to be added to our captions button to toggle it:

captions.addEventListener('click', function(e) {
   if (captionsMenu) {
      captionsMenu.style.display = (captionsMenu.style.display == 'block' ? 'none' : 'block');
   }
});

Caption menu CSS

We also added some rudimentary styling for the newly created captions menu:

.captions-menu {
    display:none;
    position:absolute;
    bottom:14.8%;
    right:20px;
    background:#666;
    list-style-type:none;
    margin:0;
    padding:0;
    width:100px;
    padding:10px;
}

.captions-menu li {
    padding:0;
    text-align:center;
}

.captions-menu li button {
    border:none;
    background:#000;
    color:#fff;
    cursor:pointer;
    width:90%;
    padding:2px 5px;
    -moz-border-radius:2px;
    -webkit-border-radius:2px;
    border-radius:2px;
}

Styling the displayed captions

One of the less well known about and supported features of WebVTT is the ability to style the individual captions (something called text cues) via CSS Extensions.

The ::cue pseudo-element is the key to targetting individual text track cues for styling, as it matches any defined cue. There are only a handful of CSS properties that can be applied to a text cue:

• color
• opacity
• visibility
• text-decoration
• text-shadow
• background shorthand properties
• outline shorthand properties
• font shorthand properties, including line-height
• white-space

For example, to change the text colour of the text track cues you can write:

::cue {
   color:#ccc;
}

If the WebVTT file uses voice spans, which allow cues to be defined as having a particular "voice":

0
00:00:00.000 --> 00:00:12.000
<v Test>[Test]</v>

Then this specific 'voice' will be stylable lik so:

::cue(v[voice='Test']) {
   color:#fff;
   background:#0095dd;
}

ブラウザ サポート

Browser support for WebVTT and the <track> element is fairly good, although some browsers differ slightly in their implementation.

Internet Explorer

Since Internet Explorer 10+ captions are enabled by default, and the default controls contain a button and a menu that offers the same functionality as the menu we just built. The default attribute is also supported.

Safari

Safari 6.1+ has similar support to Internet Explorer 11, displaying a menu with the different available options, with the addition of an "Auto" option, which allows the browser to choose.

Chrome and Opera

These browsers have similar implementations again: captions are enabled by default and the default control set contains a 'cc' button that turns captions on and off. Chrome and Opera ignore the default attribute on the <track> element and will instead try to match the browser's language to the caption's language.

Firefox

Firefox's implementation was completely broken due to a bug, leading to Mozilla turning off WebVTT support by default (you can turn it on via the media.webvtt.enabled flag.) However, this bug looks to have been fixed and WebVTT support re-enabled as of Gecko 31, so this will not be a problem for Firefox final release users for much longer (on Gecko 29 as of the time of this writing.)

プラグイン

If, after reading through this article you decide that you can't be bothered to do all of this and want someone else to do it for you, there are plenty of plugins out there that offer caption and subtitle support that you can use.

playr

This small plugin implements subtitles, captions, and chapters as well as both WebVTT and SRT file formats.

jwplayer

This video player is very extensive and does a lot more than simply support video captions. It supports WebVTT, SRT, and DFXP file formats.

MediaElement.js

Another complete video player that also support video captions, albeit only in SRT format.

LeanBack Player

Yet another video player that supports WebVTT captions as well as providing other standard player functionality.

SublimeVideo

This player also supports captions through WebVTT and SRT files.

Video.js

Supports WebVTT video subtitles.