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Accessibility/Video Codecs: Difference between revisions
Accessibility/Video Codecs (view source)
Revision as of 06:42, 21 September 2008
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As an aside: captions (or subtitles) can also be created for videos by "burning" them into the video. However, to re-access such captions, e.g. in order to do searches, requires OCR. In modern digital times, that is not a preferred way of dealing with captions and will not be regarded here. | As an aside: captions (or subtitles) can also be created for videos by "burning" them into the video. However, to re-access such captions, e.g. in order to do searches, requires OCR. In modern digital times, that is not a preferred way of dealing with captions and will not be regarded here. | ||
== Software support == | |||
Desktop environments support video codecs through [http://en.wikipedia.org/wiki/Multimedia_framework media frameworks]. A media framework is a piece of software that provides media decoding (and sometimes encoding) and rendering support to application programmers. A media framework typically consists of a set of filters that are put together into a filter network to perform the task. | |||
For example, to decode a Ogg Theora/Vorbis video file, you would need filters to read the file, strip off the Ogg encapsulation, de-multiplex the codec tracks, decode the Theora video, decode the Vorbis audio, send the decoded video to a given display area, and the decoded audio to the sound device, and make sure the synchronisation is still given. | |||
Media frameworks are usually very flexible in their codec and container format support, because filters are built such that they can easily connect to each other. Also, it is easy to extend support for a new container format or codec by adding the libraries for these formats to the system and adding a plugin to the framework that adds the filters. | |||
It is through the [http://www.xiph.org/dshow/ OggCodecs filters] that the Microsoft Windows [http://en.wikipedia.org/wiki/DirectShow DirectShow] media framework receives support for Ogg Theora. | |||
It is through the [http://www.xiph.org/quicktime/ XiphQT components] that the Apple http://en.wikipedia.org/wiki/QuickTime QuickTime] media framework receives support for Ogg Theora. | |||
{| summary="Frameworks by platform" border="1" | |||
! Desktop | |||
! Framework | |||
|- | |||
| Windows | |||
| DirectShow | |||
|- | |||
| Mac OS X | |||
| QuickTime | |||
|- | |||
| Gnome | |||
| GStreamer | |||
|- | |||
| KDE | |||
| Phonon | |||
|} | |||
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Instead, video codecs tends to have a conventional native container and the choice of video codec dictates the container and audio codec to use. By choosing Ogg Theora as a baseline codec, the native container format is Ogg and the native audio codecs are one of Vorbis, Speex, or FLAC. | Instead, video codecs tends to have a conventional native container and the choice of video codec dictates the container and audio codec to use. By choosing Ogg Theora as a baseline codec, the native container format is Ogg and the native audio codecs are one of Vorbis, Speex, or FLAC. | ||
{| summary="Map of video technology for the web" border="1" | |||
! Container | |||
! Codecs | |||
! Authoring tools | |||
! Natural captioning format | |||
|- | |||
| Ogg | |||
| | |||
* Theora (video) | |||
* Vorbis (audio) | |||
| | |||
| CMML, Kate | |||
|- | |||
| MP4 | |||
| H.264 | |||
| | |||
| 3GPP Timed Text | |||
|- | |||
| .flv | |||
| VP6, H.264 | |||
| | |||
| W3C TT | |||
|- | |||
| WMV | |||
| WMV9/VC-1 | |||
| | |||
| | |||
|} | |||
The Flash plug-in effectively provides a specialized playback framework for .flv and .mp4 containers. Gecko embeds an Ogg-specific playback framework called liboggplay. It only supports the Ogg container format. | The Flash plug-in effectively provides a specialized playback framework for .flv and .mp4 containers. Gecko embeds an Ogg-specific playback framework called liboggplay. It only supports the Ogg container format. | ||