[ogg-dev] handling multitrack Ogg

[Frank Barchard]

It sounds like you're on the right track.  There are 2 places I would start
looking - DVD's, and Quicktime.
Quicktime is mature and the container is the basis for mp4 and 3gp.
Can you find a way to transcode to Ogg maintaining the track information?

As someone who makes videos, its beyond me to translate audio to other
languages, but I would be interested in doing commentary tracks.  Right now
I do a second version of the video for that.
A nice feature in iMovie is that it does 'ducking', so the commentary pushes
the audio levels of other tracks down as needed.

Re
if (video.tracks[1].lang == "fr")          video.tracks[1].enabled = true;

It would help for W3C to standardize language codes.  Quicktime uses 3
characters, Ogg uses 2 characters?
There are also variations of the same language.  Can a script find a closest
match?

In addition to scripting, a system level setting could be used to select
preferred tracks.
What ever you do, should equally apply to subtitles.

On Tue, Feb 2, 2010 at 3:01 AM, Silvia Pfeiffer
<silviapfeiffer1 at gmail.com>wrote:

> (cc-ed to theora-dev because that will reach a larger audience -
> please reply to only one mailing list.)
>
> Hi all,
>
> In discussions with the video accessibility subgroup of the W3C HTML
> working group, we are currently looking at how to deal with multitrack
> video, e.g. such video that has a main video and audio track, plus
> e.g. a sign language video track, an audio description audio track, a
> caption track and several subtitle tracks in different languages.
> (i.e. several theora, several vorbis and several kate tracks)
>
> We are in the process of developing a JavaScript API for extracting
> information from such files and thus be able to control them from
> JavaScript, e.g. turn them on/off, find out what is there and what is
> relevant etc. In this JavaScript API, it is necessary to address the
> tracks.
>
> Now, I don't think there is an explicit track order in an Ogg file -
> tracks are just regarded as parallel entities and differentiated by
> serial number, right?
>
> I found out that the MPEG/QuickTime container format has an explicit
> "track ID" that numbers its tracks.
>
> The track order also determines the display order. "Lower numbered
> layers are shown on top of higher-numbered layers. This layering order
> is important mainly when multiple video tracks are combined though
> graphics modes." (see
> http://developer.apple.com/quicktime/qttutorial/movies.html) There is
> also an explicit rendering mode, e.g. to alphablend, dither, blend or
> copy a layer on top of another one.
>
> Further, MPEG/QuickTime has an explicit "alternate group ID". This ID
> defines tracks that belong to the same group and can thus only be
> displayed alternately of each other, e.g. subtitle tracks, audio dubs,
> or video tracks with differing video quality.
>
> I now wonder whether we need to introduce an explicit "track ID" into
> Ogg, which will define a fixed order independent of the parsing system
> and will allow us in JavaScript to directly address tracks.
>
> For example, the current draft looks something like this:
>
> interface MediaTrack {
>  readonly attribute DOMString title;
>  readonly attribute DOMString type;
>  readonly attribute DOMString role;
>  readonly attribute DOMString lang;
>           attribute boolean enabled;
>  ...
> };
> interface MediaTracks {
>  readonly attribute unsigned long length;
>  getter MediaTrack item(in unsigned long index);
>  ...
> };
> interface HTMLMediaElement : HTMLElement {
>  ...
>  readonly attribute MediaTracks tracks;
>  ...
> };
>
> Which could be used for something like this:
>
> if (video.tracks[1].role == "caption") video.tracks[1].enbled = true;
> if (video.tracks[1].lang == "fr")          video.tracks[1].enbled = true;
>
>
> As you can see, there is track index that runs through all the tracks.
> If we want to keep them in the same order in every system, we have to
> come up with an ordering scheme.
>
> Right now, I can see two different systems: the order in which their
> BOS pages are given in the Ogg header part - or the order in which the
> serial numbers go, when ordered. Alternatively, we can introduce an
> explicit track ID and order by that number.
>
> I'm curious what others think.
>
> Cheers,
> Silvia.
> _______________________________________________
> ogg-dev mailing list
> ogg-dev at xiph.org
> http://lists.xiph.org/mailman/listinfo/ogg-dev
>
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