EBML

EBML stands for Extensible Binary Meta-Language and is somewhat of a binary version of XML. It's used for container formats like WebM or MKV.

Note

This is for version 3.0.0 and up, which has undergone a massive rewrite and now builds with RollupJS.

Version 2.2.4 is the last version to have guaranteed legacy semantics.

Install

Install via NPM or Yarn:

npm install ebml --save
# or
yarn add ebml

Usage

The Decoder() class is implemented as a Node Transform stream. As input it takes EBML. As output it emits a sequence of chunks: two-element arrays looking like this example.

[ "tag",
  {
    name: "TimecodeScale",
    type: "u",
    value: 1000000
   }
 ]

The first element of the array is a short text string. For tags containing values, like this example, the string is 'tag'. ebml also has nesting tags. The opening of those tags has the string 'start' and the closing has the string 'end'. Integers stored in 6 bytes or less are represented as numbers, and longer integers are represented as hexadecimal text strings.

The second element of the array is an object with these members, among others:

name is the Matroska Element Name.
type is the data type.
- u: unsigned integer. Some of these are UIDs, coded as 128-bit numbers.
- i: signed integer.
- f: IEEE-754 floating point number.
- s: printable ASCII text string.
- 8: printable utf-8 Unicode text string.
- d: a 64-bit signed timestamp, in nanoseconds after (or before) 2001-01-01T00:00UTC.
- b binary data, otherwise uninterpreted.
value is the value of the data in the element, represented as a number or a string.
data is the binary data of the entire element stored in a Uint8Array.

Elements with the Block and SimpleBlock types get special treatment. They have these additional members:

payload is the coded information in the element, stored in a Uint8Array.
track is an unsigned integer indicating the payload's track.
keyframe is a Boolean value set to true if the payload starts an I frame (SimpleBlocks only).
discardable is a Boolean value showing the value of the element's Discardable flag. (SimpleBlocks only).

And the value member shows the block's Timecode value.

Examples

This example reads a media file into memory and decodes it. The decoder invokes its data event for each Element.

const fs = require('fs');
const { Decoder } = require('./lib/ebml.js');

const decoder = new Decoder();

decoder.on('data', chunk => console.log(chunk));

fs.readFile('media/test.webm', (err, data) => {
    if (err) {
        throw err;
    }
    decoder.write(data);
});

This example does the same thing, but by piping the file stream into the decoder (a Transform stream).

const { Decoder } = require('./lib/ebml.js');

const ebmlDecoder = new Decoder();
const counts = {};

require('fs')
    .createReadStream('media/test.webm')
    .pipe(ebmlDecoder)
    .on('data', chunk => {
        const { name } = chunk[1];
        if (!counts[name]) {
            counts[name] = 0;
        }
        counts[name] += 1;
    })
    .on('finish', () => console.log(counts));