theora - Dec 2009 - Mapping from granulepos to absolute time

I am trying to figure out how theora timestamps map to absolute time values.
This information would be needed to seek by 1 sec, 5 secs etc.
I understand the granulepos is interpreted as Absolute number of key frames
so far | number of frames since last key frame. E.g. 1000|1, 1000|2, ...
How does this map to absolute time in millisecs since the start, I can think
of the following formula:
Total number of frames = Total number of keyframes * Key frame interval +
prediction frames
Total number of millisecs = (Total number of frames / Frame rate) * 1000

Is this correct?
If so, where does the key frame interval come from, I don't think it is in
the Identification or the setup headers.
Thanks in advance.
pushkar
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2009/12/14 Pushkar Pradhan <pradhan.pushkar at
gmail.com>:
> I am trying to figure out how theora timestamps map to absolute time
values.
> This information would be needed to seek by 1 sec, 5 secs etc.
> I understand the granulepos is interpreted as Absolute number of key frames
> so far | number of frames since last key frame. E.g. 1000|1, 1000|2, ...
No, it is interpreted as:

frame number of most recent keyframe | number of frames since

so for the frame 1000|1, the frame number is simply 1000+1 = 1001.
> How does this map to absolute time in millisecs since the start, I can
think
> of the following formula:
> Total number of frames = Total number of keyframes * Key frame interval +
> prediction frames
> Total number of millisecs = (Total number of frames / Frame rate) * 1000
>
> Is this correct?
No, the keyframe interval is not necessarily constant.

so, just take the frame number that I described above, and divide by
the frame rate.

cheers,

Conrad.