/node-webvtt

Parse WebVTT files, segments and generates HLS playlists for them

Primary LanguageJavaScriptMIT LicenseMIT

WebVTT compiler parser and segmenter

Compiles, parses WebVTT files, segments and generates HLS playlists for them.

CircleCI

Usage

For a WebVTT file:

WEBVTT

00:00:00.000 --> 00:00:01.000
Hello world!

00:00:30.000 --> 00:00:31.000 align:start line:0%
This is a subtitle

00:01:00.000 --> 00:01:01.000
Foo

00:01:50.000 --> 00:01:51.000
Bar

We can parse, segment and create HLS playlists, and compile back to WebVTT format:

const webvtt = require('node-webvtt');

const segmentDuration = 10; // default to 10
const startOffset = 0; // Starting MPEG TS offset to be used in timestamp map, default 900000

const parsed = webvtt.parse(input);
const compile = webvtt.compile(input);
const segmented = webvtt.parse(input, segmentDuration);
const playlist = webvtt.hls.hlsSegmentPlaylist(input, segmentDuration);
const segments = webvtt.hls.hlsSegment(input, segmentDuration, startOffset);

Parsing

Parses the WebVTT file and returns an object with valid === true if parsed correctly and an array of cues parsed.

Each cue can have:

  • identifier - Id, if any of the cue
  • start - Start time of cue in seconds
  • end - End time of cue in seconds
  • text - Text of the subtitle
  • styles - If any of the cue

If the WebVTT file is invalid, the parser will throw a ParserError exception. So for safety, calls to parse should be in try catch.

For the above example we'd get:

{
   "valid":true,
   "cues":[
      {
         "identifier":"",
         "start":0,
         "end":1,
         "text":"Hello world!",
         "styles":""
      },
      {
         "identifier":"",
         "start":30,
         "end":31,
         "text":"This is a subtitle",
         "styles":"align:start line:0%"
      },
      {
         "identifier":"",
         "start":60,
         "end":61,
         "text":"Foo",
         "styles":""
      },
      {
         "identifier":"",
         "start":110,
         "end":111,
         "text":"Bar",
         "styles":""
      }
   ]
}

By default the parser is strict. It will throw errors if:

  • Header is incorrect, i.e. does not start with WEBVTT
  • If any cue is malformed in any way

Setting the option parameter of strict to false will allow files with malformed cues to be parsed. The resulting object will have valid === false and all errors in an errors array.

If strict is set to false, the parser will also not categorize it as an error if a cue starts and ends at the same time. This might be the correct behaviour but changing would introduce a breaking change in version 1.x.

const input = `WEBVTT

MALFORMEDCUE -->
This text is from a malformed cue. It should not be processed.

1
00:00.000 --> 00:00.001
test`;

const result = parse(input, { strict: false });

/*
result = {
  valid: false,
  strict: false,
  cues: [ { identifier: '1', start: 0, end: 0.001, text: 'test', styles: '' } ],
  errors: [ { Error: Invalid cue timestamp (cue #0) message: 'Invalid cue timestamp (cue #0)', error: undefined } ]
}
*/

Metadata

Some WebVTT strings may also contain lines of metadata after the initial WEBVTT line, for example:

WEBVTT
Kind: captions
Language: en

00:00:00.000 --> 00:00:01.000
Hello world!

By passing { meta: true } to the parse method, these metadata will be returned as an object called meta. For example, parsing the above example:

parse(webvtt, { meta: true });

would return the following:

{
   "valid":true,
   "meta":{
      "Kind": "captions",
      "Language": "en"
   },
   "cues":[
      {
         "identifier":"",
         "start":0,
         "end":1,
         "text":"Hello world!",
         "styles":""
      }
   ]
}

If no metadata is available, meta will be set to null in the result if the option is specified.

Compiling

Compiles JSON from the above format back into a WebVTT string.

If the object is missing any attributes, the compiler will throw a CompilerError exception. So for safety, calls to compile should be in try catch.

Segmenting

Segments a subtitle according to how it should be segmented for HLS subtitles.

  • Does a one pass of the cues for segmenting, this might have been a good idea or bad, only time will tell
  • The One and Only Source of Truth is Apple's mediasubtitlesegmenter CLI

For the above example:

[
    { duration: 10, cues: [ [Object] ] },
    { duration: 30, cues: [ [Object] ] },
    { duration: 30, cues: [ [Object] ] },
    { duration: 41, cues: [ [Object] ] }
]

HLS playlist

Creates a subtitle playlist. For the above:

#EXTM3U
#EXT-X-TARGETDURATION:41
#EXT-X-VERSION:3
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-PLAYLIST-TYPE:VOD
#EXTINF:10.00000,
0.vtt
#EXTINF:30.00000,
1.vtt
#EXTINF:30.00000,
2.vtt
#EXTINF:41.00000,
3.vtt
#EXT-X-ENDLIST

HLS Segments

Creates a list of HLS segments for the subtitles, returning an array of them with filename and content.

[
   {
      "filename":"0.vtt",
      "content":"WEBVTT\nX-TIMESTAMP-MAP=MPEGTS:900000,LOCAL:00:00:00.000\n\n00:00:00.000 --> 00:00:01.000\nHello world!\n"
   },
   {
      "filename":"1.vtt",
      "content":"WEBVTT\nX-TIMESTAMP-MAP=MPEGTS:900000,LOCAL:00:00:00.000\n\n00:00:30.000 --> 00:00:31.000 align:start line:0%\nThis is a subtitle\n"
   },
   {
      "filename":"2.vtt",
      "content":"WEBVTT\nX-TIMESTAMP-MAP=MPEGTS:900000,LOCAL:00:00:00.000\n\n00:01:00.000 --> 00:01:01.000\nFoo\n"
   },
   {
      "filename":"3.vtt",
      "content":"WEBVTT\nX-TIMESTAMP-MAP=MPEGTS:900000,LOCAL:00:00:00.000\n\n00:01:50.000 --> 00:01:51.000\nBar\n"
   }
]

CLI

For segmenting a WebVTT file quickly, you can use the included CLI tool:

./webvtt-segment.js -v --target-duration 10 -o ./subs subs.vtt
$ ./webvtt-segment.js --help

  Usage: webvtt-segment [options] <webvtt file>

  Options:

    -h, --help                        output usage information
    -V, --version                     output the version number
    -t, --target-duration [duration]  Target duration for each segment in secods, defaults to 10
    -o, --output-directory [dir]      Output directory for segments and playlist
    -v, --verbose                     Chatty output
    -s, --silent                      No output

Development

This has been written with TDD so we've got a good coverage of the features.

npm install
npm test
mocha -w
<write failing test>
<write passing code>
<lather, rinse, repeat>

References