Re: WebVTT Spec Question

On Sat, Mar 9, 2013 at 10:22 AM, Glenn Maynard <glenn@zewt.org> wrote:

> On Fri, Mar 8, 2013 at 3:10 PM, Silvia Pfeiffer <silviapfeiffer1@gmail.com
> > wrote:
>
>> No, there is no conflict. The first one is the current spec, the second
>> is the requirement on how to parse it so that the current spec can be
>> extended in the future.
>>
>
> Just to clarify (at least to my understanding): The first is the file
> format, and the second is the parser.  Both are part of the spec.  The file
> format tells you what's valid--what authors should be writing.  The parser
> tells you how to deal with every possible input, which includes error
> handling (inputs that don't follow the file format) and--as you
> said--forwards-compatibility.
>
> I've seen a couple people confused by this, leading to people trying to
> write implementations by looking at the file format, which is bad.  Browser
> vendors understand it, since that's how the HTML spec works, but since
> non-browser people without experience with that spec may be implementing
> this (more than most other parts of the web), maybe there should be a brief
> note at the top of the file format section explaining this.  ("If you're
> implementing WebVTT, you're in the wrong place!")
>

Agreed, we need to clarify this. I want to make sure to specify extension
points in the syntax specification to make sure that implementers are made
aware of this. Also, we should indeed add a notice like the one you're
suggesting. I've registered a bug for this:
https://www.w3.org/Bugs/Public/show_bug.cgi?id=21230 .


 We indeed have made use of header metadata lines in this spec:
>> https://dvcs.w3.org/hg/text-tracks/raw-file/default/608toVTT/608toVTT.html
>>
>
> This format doesn't seem to handle some important points we talked about
> before, like blank lines and the "-->" issue.  I'll do some digging to see
> what discussion there has been since then that I might have missed, review
> the old thread to recall what the various concerns were and start a
> separate thread with a summary so we can make sure we're not missing
> anything.
>

Indeed. I only did a rudimentary metadata header parsing for both the
608/708 transcoding spec and for the region extension. We need to clarify
this .Thanks for going back to that discussion!



> On Fri, Mar 8, 2013 at 3:23 PM, David Ronca <dronca@netflix.com> wrote:
> > Is implementation-specific metadata permissible (defined outside of any
> public spec)? If so, any parser that did not recognize the metadata should
> just ignore?
>
> No, implementations should not be inserting custom metadata.  The spec
> allows for metadata in order to leave room to add standard metadata in the
> future without breaking existing parsers, because the old parsers--as long
> as they follow the spec--understand how to skip past the headers, even
> though it doesn't know what they mean.
>
> If people start using this for their own purposes, it'll break things
> later and subvert the forward planning built into the parser.
>

David: it would indeed be better to pass any custom metadata that you want
to add by this group. However, if you just need to use something custom
between your WebVTT producer and your WebVTT consumer, then this mechanism
can be used for it. It's expected that that's the exception! Let's
standardise the headers that we need!


Cheers,
Silvia.