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    SubtitlesOctopus displays subtitles in .ass format and easily integrates with HTML5 videos. It supports all SSA/ASS features and fully compatible with libass.

    ONLINE DEMO / other examples with demo


    • Supports all SSA/ASS features
    • Supports any fonts (including woff2 fonts)
    • Works fast (because uses WebAssembly with fallback to asm.js if it's not available)
    • Uses Web Workers thus video and interface doesn't lag even on "heavy" subtitles (working in background)
    • Doesn't use DOM manipulations and render subtitles on single canvas
    • Fully compatible with libass
    • Easy to use - just connect it to video element

    Included Libraries

    • expat
    • fontconfig
    • freetype
    • fribidi
    • harfbuzz


    To start using SubtitlesOctopus you only need to instantiate a new instance of SubtitlesOctopus and specify its Options.

    var options = {
        video: document.getElementById('video'), // HTML5 video element
        subUrl: '/test/test.ass', // Link to subtitles
        fonts: ['/test/font-1.ttf', '/test/font-2.ttf'], // Links to fonts (not required, default font already included in build)
        workerUrl: '/libassjs-worker.js', // Link to WebAssembly-based file "libassjs-worker.js"
        legacyWorkerUrl: '/libassjs-worker-legacy.js' // Link to non-WebAssembly worker
    var instance = new SubtitlesOctopus(options);

    After that SubtitlesOctopus automatically "connects" to your video and it starts to display subtitles. You can use it with any HTML5 player.

    See other examples.

    Using only with canvas

    You're also able to use it without any video. However, that requires you to set the time the subtitles should render at yourself:

    var options = {
        canvas: document.getElementById('canvas'), // canvas element
        subUrl: '/test/test.ass', // Link to subtitles
        fonts: ['/test/font-1.ttf', '/test/font-2.ttf'], // Links to fonts (not required, default font already included in build)
        workerUrl: '/libassjs-worker.js' // Link to file "libassjs-worker.js"
    var instance = new SubtitlesOctopus(options);
    // And then...
    instance.setCurrentTime(15); // Render subtitles at 00:15 on your canvas

    Changing subtitles

    You're not limited to only display the subtitle file you referenced in your options. You're able to dynamically change subtitles on the fly. There's three methods that you can use for this specifically:

    • setTrackByUrl(url): works the same as the subUrl option. It will set the subtitle to display by its URL.
    • setTrack(content): works the same as the subContent option. It will set the subtitle to dispaly by its content.
    • freeTrack(): this simply removes the subtitles. You can use the two methods above to set a new subtitle file to be displayed.
    var instance = new SubtitlesOctopus(options);
    // ... we want to change the subtitles to the Railgun OP

    Cleaning up the object

    After you're finished with rendering the subtitles. You need to call the instance.dispose() method to correctly dispose of the object.

    var instance = new SubtitlesOctopus(options);
    // After you've finished using it...


    When creating an instance of SubtitleOctopus, you can set the following options:

    • video: The video element to attach listeners to. (Optional)
    • canvas: The canvas to render the subtitles to. If none is given it will create a new canvas and insert it as a sibling of the video element (only if the video element exists). (Optional)
    • subUrl: The URL of the subtitle file to play. (Require either subUrl or subContent to be specified)
    • subContent: The content of the subtitle file to play. (Require either subContent or subUrl to be specified)
    • workerUrl: The URL of the worker. (Default: libassjs-worker.js)
    • fonts: An array of links to the fonts used in the subtitle. (Optional)
    • availableFonts: Object with all available fonts - Key is font name in lower case, value is link: {"arial": "/font1.ttf"} (Optional)
    • timeOffset: The amount of time the subtitles should be offset from the video. (Default: 0)
    • onReady: Function that's called when SubtitlesOctopus is ready. (Optional)
    • onError: Function called in case of critical error meaning the subtitles wouldn't be shown and you should use an alternative method (for instance it occurs if browser doesn't support web workers). (Optional)
    • debug: Whether performance info is printed in the console. (Default: false)

    Fast Render Mode (Lossy) (EXPERIMENTAL)

    The Fast Render mode has been created by @no1d as a suggestion for fix browser freezing when rendering heavy subtitles (#46), it use createImageBitmap to render the bitmap in the Worker, using Promises instead of direct render on canvas in the Main Thread. When the browser start to hang, it will not lock main thread, instead will run Async, so if the function createImageBitmap fail, it will not stop the rendering process at all and may cause some bitmap loss or simply will not draw anything in canvas, mostly on low end devices.

    WARNING: Experimental, not stable and not working in Safari

    To enable this mode set the option lossyRender to true when creating an instance of SubtitleOctopus.

    Brotli Compressed Subtitles

    The SubtitleOctopus allow the use of compressed subtitles in brotli format, saving bandwidth and reducing library startup time

    To use, just run: brotli subFile.ass and use the .br result file with the subUrl option

    How to build?


    • git
    • emscripten (Configure the enviroment)
    • llvm
    • clang
    • ragel
    • make
    • autoconf
    • python3
    • pkgconfig
    • patch
    • libtool
    • itstool
    • automake
    • python-lxml
    • python-pip
    • python-html5lib
    • python-chardet
    • gperf

    Get the Source

    Run git clone --recursive

    Build with Docker

    1. Install Docker
    2. docker build -t dador/javascriptsubtitlesoctopus .
    3. docker run -it --rm -v ${PWD}:/code dador/javascriptsubtitlesoctopus:latest
    4. Artifacts are in /dist/js

    Why "Octopus"?

    How am I an Octopus? Ba da ba da ba!


    npm i libass-wasm

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