W3C home > Mailing lists > Public > public-html-a11y@w3.org > February 2010

Re: timing model of the media resource in HTML5

From: Philip Jägenstedt <philipj@opera.com>
Date: Tue, 02 Feb 2010 17:19:59 +0100
To: "Silvia Pfeiffer" <silviapfeiffer1@gmail.com>
Cc: "Eric Carlson" <eric.carlson@apple.com>, "HTML Accessibility Task Force" <public-html-a11y@w3.org>, "Ken Harrenstien" <klh@google.com>
Message-ID: <op.u7iczik1atwj1d@sisko.linkoping.osa>
On Tue, 02 Feb 2010 14:21:20 +0100, Silvia Pfeiffer  
<silviapfeiffer1@gmail.com> wrote:

> On Wed, Feb 3, 2010 at 12:17 AM, Philip Jägenstedt <philipj@opera.com>  
> wrote:
>> On Tue, 02 Feb 2010 14:08:36 +0100, Silvia Pfeiffer
>> <silviapfeiffer1@gmail.com> wrote:
>>> On Wed, Feb 3, 2010 at 12:03 AM, Silvia Pfeiffer
>>> <silviapfeiffer1@gmail.com> wrote:
>>>> On Tue, Feb 2, 2010 at 7:19 PM, Philip Jägenstedt <philipj@opera.com>
>>>> wrote:
>>>>> On Mon, 01 Feb 2010 23:30:19 +0100, Silvia Pfeiffer
>>>>> <silviapfeiffer1@gmail.com> wrote:
>>>>>> On Tue, Feb 2, 2010 at 3:59 AM, Eric Carlson  
>>>>>> <eric.carlson@apple.com>
>>>>>> wrote:
>>>>>>> On Feb 1, 2010, at 4:19 AM, Silvia Pfeiffer wrote:
>>>>>>> On Fri, Jan 29, 2010 at 12:39 AM, Philip Jägenstedt
>>>>>>> <philipj@opera.com>
>>>>>>> wrote:
>>>>>>> On Wed, 27 Jan 2010 12:57:51 +0100, Silvia Pfeiffer
>>>>>>> If we buried the track information in a javascript API, we would
>>>>>>> introduce an additional dependency and we would remove the ability  
>>>>>>> to
>>>>>>> simply parse the Web page to get at such information. For example,  
>>>>>>> a
>>>>>>> crawler would not be able to find out that there is a resource with
>>>>>>> captions and would probably not bother requesting the resource for  
>>>>>>> its
>>>>>>> captions (or other text tracks).
>>>>>>> Surely, robots would just index the resources themselves?
>>>>>>> Why download binary data of indeterminate length when you can  
>>>>>>> already
>>>>>>> get it out of the text of the Web page? Surely, robots would  
>>>>>>> prefer to
>>>>>>> get that information directly out of the Webpage and not have to go
>>>>>>> and download gazillions of binary media files that they have to  
>>>>>>> decode
>>>>>>> to get information about them.
>>>>>>> Right now, everybody who sees a video element in a HTML5 page  
>>>>>>> simply
>>>>>>> assumes that it consists of a video and a audio track and has no  
>>>>>>> other
>>>>>>> information in it. This is fine in the default case and in the  
>>>>>>> default
>>>>>>> case no extra resource description is probably necessary. But when  
>>>>>>> we
>>>>>>> actually do have a richer source, we need to expose that.
>>>>>>>  This argument leads down a very slippery slope. If it is crucial  
>>>>>>> to
>>>>>>> include caption information in markup for spiders, what about other
>>>>>>> media
>>>>>>> file metadata that a crawler might want to read - intrinsic width  
>>>>>>> and
>>>>>>> height, duration, encoding format, file size, bit rate, frame rate,
>>>>>>> etc,
>>>>>>> etc, etc? Robots may prefer to have all of this in the page do they
>>>>>>> don't
>>>>>>> have to load and parse the file, but I don't think it is necessary  
>>>>>>> or
>>>>>>> appropriate.
>>>>>> Not quite.
>>>>>> It is a difference if you are a web crawler that wants to collect
>>>>>> captions or one that wants to collect such file metadata. For file
>>>>>> metadata, you are bound to always be successful when parsing the
>>>>>> header of a binary file. So, I agree there with you.
>>>>>> But if you are only keen on captions, you are bound to often parse
>>>>>> useless information if you have to download the media file header. A
>>>>>> hint inside the markup that there are captions/subtitles there and
>>>>>> that it is useful to parse the file - and then parse it fully - is
>>>>>> very relevant.
>>>>> Even if all browser vendors should agree that this is useful and
>>>>> implemented
>>>>> the suggested track markup, it will only be used by authors in very  
>>>>> rare
>>>>> situations -- when they want to populate the browser's context menu
>>>>> before
>>>>> HAVE_METADATA. As most videos that have multiple audio/video/text  
>>>>> tracks
>>>>> won't be marked up as such in HTML, robots will still have to  
>>>>> download
>>>>> the
>>>>> headers of all videos to see if they have captions. If they want to
>>>>> index
>>>>> the captions (not just the fact that they exist), they'll also have  
>>>>> to
>>>>> download the whole file.
>>>> I still believe it's useful to expose the tracks in a media file to
>>>> the browser and to automated tools without having to use javascript to
>>>> get to them or having to download the media data and decode the
>>>> headers.
>>>> But I don't think any browser vendors will want to implement it at
>>>> this stage, so I concede.
>>>> Let's instead focus on getting the JavaScript API right and get to a
>>>> state where we can at least make use of such multitrack media files.
>>>> I have put Eric's proposal with some slight changes (replace "type"
>>>> with "role" in the examples, added a "role" attribute, added a "name"
>>>> attribute, added a namedItem accessor:
>>>> http://www.w3.org/WAI/PF/HTML/wiki/Media_MultitrackAPI
>>>> I'd say everyone should free to edit that page as they see fit, but
>>>> leave a comment on the mailing list as to why the changes were
>>>> necessary.
>>> Philip: you mentioned
>>> http://www.w3.org/TR/mediaont-api-1.0/#webidl-for-api . Do you think
>>> the track elements should have some of these characteristics, too, and
>>> expose them?
>> I quite like Eric's suggestion of exposing this interface on both on the
>> media element and on each track. The interface isn't as good as it  
>> could be
>> yet (e.g. throwing NoValue isn't going to fly, just return undefined)  
>> but I
>> do think we can reuse this and implement as much of it as possible.
>> I've already sent some feedback to the Media Annotations WG, but a lot  
>> more
>> is needed if we want to use this.
> Is there a similar API for images that we could compare it with to  
> evaluate?

Not that I know of, for images no metadata at all is exposed except the  
width and height.

Philip Jägenstedt
Core Developer
Opera Software
Received on Tuesday, 2 February 2010 16:20:59 UTC

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