[whatwg] Thoughts on video accessibility

Silvia Pfeiffer silviapfeiffer1 at gmail.com
Thu Jul 16 06:40:37 PDT 2009

On Thu, Jul 16, 2009 at 6:28 PM, Philip Jägenstedt<philipj at opera.com> wrote:
> On Thu, 16 Jul 2009 07:58:30 +0200, Silvia Pfeiffer
> <silviapfeiffer1 at gmail.com> wrote:
>>>> > 3. Timed text stored in a separate file, which is then parsed by the
>>>> > user agent and rendered as part of the video automatically by the
>>>> > browser.
>>>> >
>>>> Maybe we should consider solving this differently. Either we could
>>>> encapsulate into the video container upon download. Or we could create a
>>>> zip-file or tarball upon download. I'd just find it a big mistake to
>>>> ignore the majority use case in the standard, which is why I proposed
>>>> the <text> elements inside the <video> tag.
>>> If browser vendors are willing to merge subtitles and video files when
>>> saving them, that would be great. Is this easy to do?
>> My suggestion was really about doing this server-side, which we have
>> already implemented years ago in the Annodex project for Ogg
>> Theora/Vorbis.
>> However, it is also possible to do this in the browser: in the case of
>> Ogg, the browser just needs to have a multiplexing library installed
>> as well as a means to encode the subtitle file (which I like to call a
>> "text codec"). Since it's text, it's nowhere near as complex as
>> encoding audio or video and just consists of light-weight packaging
>> code. So, yes, it is totally possible to have the browsers create a
>> binary video file that has the subtitles encapsulated that were
>> previously only accessible as referenced text files behind a separate
>> URL.
>> The only issue I see is the baseline codec issue: every browser that
>> wants to support multiple media formats has to implement this
>> multiplexing and text encoding for every media encapsulation format
>> differently, which is annoying and increases complexity. It's however
>> generally a small amount of complexity compared to the complexity
>> created by having to support multiple codecs.
> I disagree, remuxing files would be much more of an implementation burden
> than supporting multiple codecs, at least if a format-agnostic media
> framework is used (be that internal or external to the browser). Remuxing
> would require you to support/create parts of the media framework that you
> otherwise aren't using, i.e. parsers, muxers, file writers and plugging of
> these together (which unlike decoding isn't automatic in any framework I've
> seen).

The point that I was trying to make is that if one had to only
implement it for one encapsulation format, it would be simple and a
small piece of dedicated code. However, if one has to be
format-agnostic, it indeed requires supporting parts of a media
framework that is not needed for demuxing and decoding. So, yes, I
agree with you: in the general case it might create extraneous
complexity in a browser.

> Anything is doable of course, but I think this is really something that is
> best done server-side using specialized tools.

I agree with this. This can be a special service that some servers
would offer who want to allow their users to share single video files
that contain their timed text within.

>>> It would be interesting to hear back from the browser vendors about how
>>> easily the subtitles could be kept with the video in a way that survives
>>> reuse in other contexts.
> I think that in the case of external subtitles the browser could simply save
> it alongside with the video. It is my experience that is media players have
> much more robust support for external subtitles (like SRT) than for internal
> subtitles, so this is my preferred option (plus it's easier).

Agreed: this would be the fallback for content downloaded from servers
that do no offer the special muxing capability.

In fact, such a separate handling of composed content through multiple
files is nothing new to HTML: all Web pages that I download from the
Internet require me to download each component of the Web page
separately: the images, the text, the css, the javascript. (Worse even
if the text is created e.g. through a database query.) I agree with
Philip that the separate handling of subtitle files and media files is
not as much of an issue as it may seem.


More information about the whatwg mailing list