[theora] Indexing Ogg files for faster seeking

[Conrad Parker]

2009/10/14 Chris Pearce <chris at pearce.org.nz>:
> On 10/14/2009 3:21 PM, Conrad Parker wrote:
>> Players should at least use the presentation time, ie. not display
>> earlier frames.
>>
>> The BOS page you have specified is mostly the same as the BOS page for
>> Skeleton anyway. I think it'd make more sense to rev Skeleton to
>> include a presentation end-time than to introduce a whole new BOS
>> page.
>>
>> Presentation end-time would also be useful for players when the file
>> is a chopped section, to say that after this point you may not have
>> all required audio+video.
>
> The skeleton's "presentation time" isn't the presentation time of the
> first frame in the media? Just the time at which you want to start
> playing? I didn't realise that. So it's valid to have the "presentation
> time" set to be half way through the physical media, and by doing so
> ensure that a valid decoder will download, decode and discard half the
> media? Now why would you want to do that?
>
> Chopping media I can understand, but why ship data which you're not
> decoding? That seems silly to me.

It's defined like that as it's simply stating "this is an offset from
which the content is valid". The main use case is chopping, eg.
presentation time corresponds to a frame halfway between two
keyframes, and the audio is only from there.

> Why would I want to include the index in the skeleton? That would
> require players to implement its more controversial features such as
> this in order to get the benefit of the index.

Yes players should implement presentation time.

I'll ignore your claim that it's controversial.

> The index should index the entire media, not the "presentation window"
> as specified by the skeleton track, as if a player chooses to ignore the
> skeleton's presentation window we still want the index to be as useful
> as possible.
>
> The index belongs in its own track. Best to not confuse the skeleton's
> presentation window with the actual medias boundaries.

So include separate fields.

Conrad.