[FFmpeg-devel] H264: mp4toannexb BSF and streaming

Luca Abeni lucabe72
Wed Aug 18 14:47:41 CEST 2010

On 08/18/2010 02:42 PM, Alexandre Ferrieux wrote:
> Hi,
> I'm in a (n admittedly suboptimal) situation where I'm receving a
> 3GP/H264 video file in streaming (over a TCP socket, yes...).
> I would like to transcode it on-the-fly without waiting for the full 3GP
> file to be received. I know that the 3GP format is not AT ALL designed
> for that, and that even the first chunks will be invalid since the
> 3GP-writer application does a seek() back to the beginning to fill the
> header, etc. In short, 3GP is utterly non-streamable, but I need to do
> something with it nevertheless :/
> What I've come up with so far, is to detect the beginning of the 'mdat'
> chunk, which comes quickly in the stream, since it contains the bulk of
> the bitstream (in AVC format).
> Now, I know that this AVC bitstream is not suitable for input by ffmpeg,
> who is expecting annex B. I also know of the mp4toannexb BSF, but I can
> see in its code that it is relying on extradata coming from the
> container (in other chunks, coming at the end), which it will convert
> and interleave between the annex B NALUs it will spit out.
> Hence, I conclude that mp4toannexb cannot work on a "pure" AVC bitstream
> without extradata.

As far as I know "AVC" always has extradata, and is not decodable without
it (I think the extradata contain the SPS and PPS NALs, which are needed
for decoding the H.264 stream). So, I think "pure AVC" without extradata
does not exist (or is not usable).


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