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In-Depth Information
hold other metadata files too, such as subtitles. WebM is a con-
tainer (based on the open standard Matroska Media Container
format) that holds VP8 video and Ogg Vorbis audio. Currently,
WebM doesn't support subtitles, as Google is waiting for the
Working Groups to specify the HTML5 format: “WHATWG/W3C
RFC will release guidance on subtitles and other overlays in
HTML5
<video>
in the near future. WebM intends to follow that
guidance”. (Of course, even if the container can contain addi-
tional metadata, it's still up to the media player or browser to
expose that information to the user.)
Out-of-band text alternatives are those that aren't inside the
media container but are held in a separate file and associated
with the media file with a child <track> element:
<video controls>
<source src=movie.webm>
<source src=movie.mp4>
<track src=english.vtt kind=captions srclang=en>
<track src=french.vtt kind=captions srclang=fr>
<p>Fallback content here with links to download video
¬ files</p>
</video>
This example associates two caption tracks with the video, one
in English and one in French. Browsers will have some UI mech-
anism to allow the user to select the one she wants (listing any
in-band tracks, too).
The
<track>
element doesn't presuppose any particular format,
but the browsers will probably begin by implementing the new
WebVTT format (previously known as WebSRT, as it's based on
the SRT format)
(
www.whatwg.org/specs/web-apps/current-work/
This format is still in development by WHATWG, with lots of
feedback from people who really know, such as the BBC, Netflix,
and Google (the organisation with probably the most experience
of subtitling web-delivered video via YouTube). Because it's still
in flux, we won't look in-depth at syntax here, as it will probably
be slightly different by the time you read this.
WebVTT is just a UTF-8 encoded text file, which looks like this
at its simplest:
WEBVTT
00:00:11.000 --> 00:00:13.000
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