SRT parser, compiler, and segmenter with HLS support
npm install node-srttext
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 Subrip format:
`javascript
const subrip = require('node-srt');
const segmentDuration = 10; // default to 10
const startOffset = 0; // Starting MPEG TS offset to be used in timestamp map, default 900000
const parsed = subrip.parse(input);
const compile = subrip.compile(input);
const segmented = subrip.parse(input, segmentDuration);
const playlist = subrip.hls.hlsSegmentPlaylist(input, segmentDuration);
const segments = subrip.hls.hlsSegment(input, segmentDuration, startOffset);
`
$3
Parses the Subrip 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 Subrip 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:
`json
{
"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.
`javascript
const input = WEBVTT
;
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 } ]
}
*/
`
$3
Some Subrip strings may also contain lines of metadata after the initial WEBVTT line, for example:
`text
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:
`javascript
parse(subrip, { meta: true });
`
would return the following:
`json
{
"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.
$3
Compiles JSON from the above format back into a Subrip string. If a meta key is in the input,
it will be compiled as well. The meta value must be an object and each key and value must be a 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.
`javascript
const input = {
meta: {
Kind: 'captions',
Language: 'en'
},
cues: [{
end: 140,
identifier: '1',
start: 135.001,
text: 'Hello world',
styles: ''
}],
valid: true
};
const result = compile(input);
/*
1
00:02:15.001 --> 00:02:20.000
Hello world
*/
`
$3
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:
`javascript
[
{ duration: 10, cues: [ [Object] ] },
{ duration: 30, cues: [ [Object] ] },
{ duration: 30, cues: [ [Object] ] },
{ duration: 41, cues: [ [Object] ] }
]
`
$3
Creates a subtitle playlist. For the above:
`text
#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
`
$3
Creates a list of HLS segments for the subtitles, returning an array of them with filename and content.
`json
[
{
"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 Subrip file quickly, you can use the included CLI tool:
`bash
./subrip-segment.js -v --target-duration 10 -o ./subs subs.vtt
`
`bash
$ ./subrip-segment.js --help
Usage: subrip-segment [options]
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.
`bash
npm install
npm test
mocha -w
`
References
* Anne van Kesteren's WebVTT validator
* Live validator
* WebVTT Ruby parser and segmenter
* mediasubtitlesegmenter` from Apple