[FFmpeg-devel] h264_annexbtomp4 filter ideas, was: Collection of patches

[Baptiste Coudurier]

Thorsten Jordan wrote:
> Thorsten Jordan schrieb:
>> Baptiste Coudurier schrieb:
>>> Hi,
>>>
>>> Thorsten Jordan wrote:
>>>> Thorsten Jordan schrieb:
>>>>> Michael Niedermayer schrieb:
>>>>>> On Wed, Apr 23, 2008 at 09:15:40AM +0200, Thorsten Jordan wrote:
>>>>>> [...]
>>> No, mp4 muxer use a "trailer" and therefore computes atom when all
>>> frames have been muxed. However, this might be a problem when stream
>>> copying and using fragmented muxing, but we will see when somebody will
>>> try to implement it.
>> yes i saw this in the code too, so it would work, but the issue remains
>> when multiple SPS/PPS are in the stream that are not the same. This
>> could be ignored for a first version though...
> 
> Thinking about possible implementation, there is a problem if the data
> given to the filter() function would not start with a start code. Since
> in MP4 format each NALU must have its length stored before it, we can
> compute the length only, if every data chunk given to filter() consists
> of complete NALUs. This should be the case though.

?? annexbtomp4 filter will search for starcodes...

> So the filter context would hold memory for SPS and PPS, that is filled
> while parsing. SPS/PPS from stream data is ripped out and the first of
> each sort are stored in the filter context. Beside that every startcode
> is replaced by the length value (32bit). On filter_close() the global
> extradata is compiled by writing the 6 lead bytes, then SPS+PPS like in
> ff_isom_write_avcc(). That should do it...

I think this should work.

> Such a filter would need code like libavformat/avc.c to find startcodes
> and parse NALUs, but it would be stored in libavcodec/. To avoid code
> duplication, some functions need to be moved thus. Here someone with
> more SVN knowledge than me has to take some action :/

Yes, once patches are sent and approved, someone will apply them.

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