Tag Archives: Ogg video

Accessibility support in Ogg and liboggplay

At the recent FOMS/LCA in Wellington, New Zealand, we talked a lot about how Ogg could support accessibility. Technically, this means support for multiple text tracks (subtitles/captions), multiple audio tracks (audio descriptions parallel to main audio track), and multiple video tracks (sign language video parallel to main video track).

Creating multitrack Ogg files
The creation of multitrack Ogg files is already possible using one of the muxing applications, e.g. oggz-merge. For example, I have my own little collection of multitrack Ogg files at http://annodex.net/~silvia/itext/elephants_dream/multitrack/. But then you are stranded with files that no player will play back.

Multitrack Ogg in Players
As Ogg is now being used in multiple Web browsers in the new HTML5 media formats, there are in particular requirements for accessibility support for the hard-of-hearing and vision-impaired. Either multitrack Ogg needs to become more of a common case, or the association of external media files that provide synchronised accessibility data (captions, audio descriptions, sign language) to the main media file needs to become a standard in HTML5.

As it turn out, both these approaches are being considered and worked on in the W3C. Accessibility data that are audio or video tracks will in the near future have to come out of the media resource itself, but captions and other text tracks will also be available from external associated elements.

The availability of internal accessibility tracks in Ogg is a new use case – something Ogg has been ready to do, but has not gone into common usage. MPEG files on the other hand have for a long time been used with internal accessibility tracks and thus frameworks and players are in place to decode such tracks and do something sensible with them. This is not so much the case for Ogg.

For example, a current VLC build installed on Windows will display captions, because Ogg Kate support is activated. A current VLC build on any other platform, however, has Ogg Kate support deactivated in the build, so captions won’t display. This will hopefully change soon, but we have to look also beyond players and into media frameworks – in particular those that are being used by the browser vendors to provide Ogg support.

Multitrack Ogg in Browsers
Hopefully gstreamer (which is what Opera uses for Ogg support) and ffmpeg (which is what Chrome uses for Ogg support) will expose all available tracks to the browser so they can expose them to the user for turning on and off. Incidentally, a multitrack media JavaScript API is in development in the W3C HTML5 Accessibility Task Force for allowing such control.

The current version of Firefox uses liboggplay for Ogg support, but liboggplay’s multitrack support has been sketchy this far. So, Viktor Gal – the liboggplay maintainer – and I sat down at FOMS/LCA to discuss this and Viktor developed some patches to make the demo player in the liboggplay package, the glut-player, support the accessibility use cases.

I applied Viktor’s patch to my local copy of liboggplay and I am very excited to show you the screencast of glut-player playing back a video file with an audio description track and an English caption track all in sync:


Further developments
There are still important questions open: for example, how will a player know that an audio description track is to be played together with the main audio track, but a dub track (e.g. a German dub for an English video) is to be played as an alternative. Such metadata for the tracks is something that Ogg is still missing, but that Ogg can be extended with fairly easily through the use of the Skeleton track. It is something the Xiph community is now working on.

This is great progress towards accessibility support in Ogg and therefore in Web browsers. And there is more to come soon.

Cool HTML5 video demos

I’ve always thought that the most compelling reason to go with HTML5 Ogg video over Flash are the cool things it enables you to do with video within the webpage.

I’ve previously collected the following videos and demos:

First there was a demo of a potential javascript interface to playing Ogg video inside the Web browser, which was developed by CSIRO. The library in use later became the library that Mozilla used in Firefox 3.5:

Then there were Michael Dale’s demos of Metavidwiki with its direct search, access and reuse of video segments, even a little web-based video editor:

Then there was Chris Double’s video SVG demo with cool moving, resizing and reshaping of video:

and Chris kept them coming:

Then Chris Blizzard also made a cool demo for showing synchronised video and graph updates as well as a motion detector:

And now we have Firefox Director Mike Belitzer show off the latest and coolest to TechCrunch, the dynamic content injection bit of which you can try out yourself here:

It just keeps getting better!

UPDATE: Here are some more I’ve come across:

Sites with Ogg in HTML5 video tag

Yesterday, somebody mentioned that the HTML5 video tag with Ogg Theora/Vorbis can be played back in Safari if you have XiphQT installed (btw: the 0.1.9 release of XiphQT is upcoming). So, today I thought I should give it a quick test. It indeed works straight through the QuickTime framework, so the player looks like a QuickTime player. So, by now, Firefox 3.5, Chrome, Safari with XiphQT, and experimental builds of Opera support Ogg Theora/Vorbis inside the HTML5 video tag. Now we just need somebody to write some ActiveX controls for the Xiph DirectShow Filters and it might even work in IE.

While doing my testing, I needed to go to some sites that actually use Ogg Theora/Vorbis in HTML5 video tags. Here is a list that I came up with in no particular order:

I’m sure there’s a lot more out there – feel free to post links in the comments.