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adhere-lib

Javascript library for processing AD profile TTML2 and presenting with an HTML video element

adhere-lib is a reference implementation for the TTML2 Spec work it is to showcase the use of audio attributes that can be controlled from a TTML2 document.

The library is designed as a module to be incorporated into package-based builds e.g. with webpack.

For details of the TTML2 Dubbing and Audio Description profiles of TTML2 (DAPT), see https://www.w3.org/TR/dapt/ document format, which replaces the older ADPT at https://w3c.github.io/adpt/

A web page showcasing adhere-lib is available at https://bbc.github.io/Adhere/

Prerequisites

Basic build instructions

Setup

nvm install 14.16.0
nvm use 14.16.0

cd to project dir

npm install

Build and lint

npm run-script build # builds the project once.
npm run-script lint # checks for linting issues.

The build artefacts are placed into the dist folder. Import the module into your project either using a local file path in your package.json or by referencing bbc/adhere-lib, for example:

    "dependencies": {
        "adhere-lib": "file:../adhere-lib"
    }

Usage Documentation

... is something we ought to write more properly! In the meantime:

There are three main objects:

  • Logger
  • TextPresenter
  • VideoAudioHook
import { VideoAudioHook, TextPresenter, Logger } from "adhere-lib";

Logger

Log messages will be sent to the console if true is passed to Logger.enableBrowserLogging(). To do something else with them register a callback handler.

For example to prepend new log messages to a <textarea> HTML element whose id="logarea", and log everything to the browser:

        const logarea = document.getElementById("logarea");
        Logger.addCallback((str) => {
            logarea.textContent = `${str}\r\n${logarea.textContent}`;
        });
        Logger.enableBrowserLogging(true);

To prepend each log message with the video playback time, tell the Logger which video element is being used:

        videoplayer = document.querySelector("video");
        Logger.setVideoPlayer(videoplayer);

TextPresenter

To process the text of audio descriptions when they are played, add a callback handler to TextPresenter.

For example to set the text of an HTML <p> element whose id="textpresentationarea":

        const textarea = document.getElementById("textpresentationarea");
        TextPresenter.addCallback((str) => {
            textarea.style.display = "none";
            textarea.textContent = str;
            textarea.style.display = "block";
        });

Ny using role and aria-live attributes on the p you can expose this text to assistive technology such as a screen reader. Changing the display style property to block after setting the text can trigger the screen reader to examine the element.

VideoAudioHook

VideoAudioHook is the main object that processes the TTML2 audio description file.

When a TTML file is loaded, it creates Text Track Cues and associates them with the video. During playback, the video's audio output is processed using Web Audio, if gain or pan changes are made. During playback, if text to speech is indicated in the TTML file, the Web Speech API is used.

Associate with the <video> element

        videoplayer = document.querySelector("video");
        vah = new VideoAudioHook(videoplayer);
        vah.setup();

Load a TTML file

Three mechanisms are provided for loading a TTML file:

  1. Via an XMLHttpRequest using VideoAudioHook.getHttpTTML(ttmlUrl)
  2. From a local file using VideoAudioHook.attachTTMLfromFile(ttmlFile) passing in a file object
  3. From a URL using fetch using VideoAudioHook.attachTTMLfromURL(ttmlURL)

Play video

Use normal video controls to manage video playback.

Reset

If you need to do this explicitly:

        vah.clearTree();

Development

Please raise pull requests with changes in rather than committing directly to main.

Linting

Please make sure that all code is linted before it is committed to the repo otherwise it will fail the Continuous delivery tests on the GOCD server. Linting happens as part of the build script.

Testing

Cucumber tests exist but the scripts to run them no longer exist.

Unit tests can be run using:

npm run-script test

Continuous Delivery

BBC provides a GoCD PR builder for this project.

For those with access, information about the previous builds can be found here MPAS Go CD

Versioning

See VERSIONING.md

We use Semantic Versioning for this repository

Contributing

See CONTRIBUTING.md

Please ensure you have run the test suite before submitting a Pull Request, and include a version bump in line with our Versioning policy.

License

See LICENSE.md

This is licensed under the Apache 2.0 License.

Copyright

Copyright (c) 2021 BBC