[gst-devel] nogotiating h264 packing formats

[Stefan Kost]

Wim Taymans schrieb:
> On Tue, 2008-12-02 at 10:06 +0200, Stefan Kost wrote:
>   
>> hi,
>>
>> h264 streams come in two flavours:
>> 1) length prefixed, each NALU is prefixed by a 32bit length field
>> (length of NALU)
>> 2) startcode delimmited, as described in annex b of h264 spec, NALUs are
>> separated by 24 bit value 0x000001.
>>
>> It looks like MP4/3GP/MOV and MKV use the AVC1 encoding
>> (length-prefix),while AVI, ES, and TS use
>> the Annex B encoding (startcodes).
>>
>> When demuxing videos that contain h264 streams, demuxer will just demux
>> what the file contains. decoders can check the presence of the
>> codec-data on the caps. If codec-data is given, stream is length
>> prefixed (1), otherwise its startedcode delimmited (where the SPS and
>> PPS are in the stream an not in codec-data).
>>
>> When muxing we have no mean to negotiate that right now. The x264enc
>> encoder has a byte-stream property to select the flavor, but that
>> requires manual setup to have it right. Doing t wrong usualy still
>> works, but then the file is not well playable.
>>
>> Should we add h264packing={length-prefixed,startcode-delimmited} to the
>> caps?
>>     
>
> codec_data on the caps currently signals the AVC1 encoding because you
> need to parse the codec data to get the size of the length prefix.
>
> The absense of codec_data signals an annex B stream.
>
> You would likely build your element to handle both cases. See
> rtph264pay/depay to see how this works.
>
> Wim
>   
Its quite clear for playback. I am concerned with the other way around.
h264pay has "scan-mode" property, but right now only seems to implement
startcode based packing. So how does h264pay tell the decoder to not
send a length-prefixed stream, but a startedcode delimmited one?

Stefan
>   
>> Stefan
>>
>>
>>
>>
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