[gst-devel] nogotiating h264 packing formats

Wim Taymans wim.taymans at gmail.com
Tue Dec 2 10:07:19 CET 2008

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.


> Stefan
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