[Vorbis-dev] Muxing vorbis in WebM

[Chris Cunningham]

Hey all, friendly ping :) Any help is much appreciated.

On Thu, Jun 18, 2015 at 6:56 PM, Chris Cunningham <chcunningham at chromium.org
> wrote:

> Hi Vorbis-Dev,
>
> I'm investigating various WebM/Vorbis bugs in chromium. AFAIK muxing
> Vorbis inside of WebM does not have an official specification, so I'm using
> ffmpeg's implementation to try to answer 2 questions:
>
>    1. Under what circumstances is it valid to find WebM Blocks containing
>    Vorbis data with zero duration? (This would mean the next Block in the
>    Cluster has the exact same timecode).
>
>    2. FFmpeg seems to use granulepos for presentation timestamp - is this
>    correct? See here:
>
>    https://github.com/FFmpeg/FFmpeg/blob/master/libavcodec/libvorbisenc.c#L345l
>
> To me it seems the breakdown of ffmpeg's libvorbis_encode_frame is:
>
>    - get an ogg_packet from libvorbis (I think this contains a *single*
>    vorbis block, right?)
>    - store just the data from that packet (no header) in ffmpeg's own
>    AVPacket struct
>    - eventually this AVPacket data will be inserted as the contents of a
>    WebM (Matroska) block:
>
>    https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/matroskaenc.c#L1584
>
> Starting with question 1, some important context comes from this excerpt
> of the vorbis spec:
>
>
> *Data is not returned from the first frame; it must be used to ’prime’ the
> decode engine. The encoder accounts for this priming when calculating PCM
> offsets; after the first frame, the proper PCM output offset is ’0’
> - http://www.xiph.org/vorbis/doc/Vorbis_I_spec.html#x1-190001.3.1
> <http://www.xiph.org/vorbis/doc/Vorbis_I_spec.html#x1-190001.3.1>*
>
> Am I right that "frame" and "block" are interchangable in the vorbis spec?
> If so, I would then expect the granulepos of the first ogg_packet processed
> by ffmpeg to be 0. I'm using this definition of granulepos:
>
> *This is the last sample, frame or other unit of information ('granule')
> that can be completely decoded from this packet
> - https://xiph.org/ogg/doc/libogg/ogg_packet.html
> <https://xiph.org/ogg/doc/libogg/ogg_packet.html>*
>
> IIUC, we can derive duration from granulepos by simply scaling the count
> of samples up by the number of samples / time unit. So my answer to
> question 1 would be: the first block could / should have 0 duration. And my
> answer to question 2 would be: they should not use granulepos for
> presentation time - they will always be off by the duration of a packet.
>
> What do you guys think? What am I missing?
>
> Thanks!
> Chris
>
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