[x264-devel] Behaviour of Annex B encoding: bug or not?

[Alex Giladi]

RTP payload (at RFC 3984) uses RBSP. Maybe it's worth adding "rtp" as
an output option?
--ag

On Tue, Jan 5, 2010 at 2:04 PM, Jason Garrett-Glaser
<darkshikari at gmail.com> wrote:
> On Tue, Jan 5, 2010 at 1:41 PM, Philip Spencer
> <pspencer at fields.utoronto.ca> wrote:
>> First a disclaimer: I know very little about the H.264 spec, and am just
>> trying to resolve an interoperability issue we are having, so please bear
>> with me if I misstate or misunderstand something.
>>
>> In the routine x264_nal_encode (in common/common.c), the flag b_annexb
>> controls whether or not to add a NAL start code (00 00 00 01) to the
>> beginning of the packet, but does not control whether or not to do the
>> escaping of sequences of the form 00 00 00/1/2/3 by inserting a 03 byte into
>> the third position -- that escaping is ALWAYS done, even if b_annexb is not
>> set.
>>
>> Is this a bug, or is it meant to be that way? (I don't have access to the
>> text of Annex B).
>
> We didn't make it an option because we didn't know of any devices that
> expected non-escaped bytestreams.  All containers we knew that didn't
> want Annex-B startcodes still expected escaped NAL units.
>
>> It certainly breaks interoperability with several devices. In particular,
>> x264 cannot be used with the Ekiga softphone application to communicate with
>> the "LifeSize Room" brand of videoconferencing equipment: that device
>> expects non-Annex-B packets over RTP, and cannot handle the extra 03 byte
>> that is inserted. The result is that the session parameters (like stream
>> resolution and other such settings, which often contain multiple zero bytes
>> in a row) get completely garbled because of the Annex-B bytestream encoding.
>>
>> Also, if one attempts to sniff the network traffic with software such as
>> WireShark, it too chokes on the unexpected 03 bytes in RTP packets.
>>
>> It would seem to me that this is a bug: if Annex B bytestream encoding is
>> not desired, such as for an RTP packet, then the extra escape bytes should
>> not be inserted.
>>
>> On our system, I have applied the patch below to common.c and then
>> H.264 connectivity to the LifeSize Room videoconferencing equipment works
>> just fine.
>>
>> On the other hand, from a quick glance at the source code of the reference
>> encoder/decoder, it seems that it behaves the same way as x264: always
>> inserts the escape byte. Is this a bug in the reference encoder/decoder too,
>> or does the text of Annex B specify that ALL H.264 streams should have the
>> extra bytes inserted, even when bytestream encoding is not being used?
>>
>> In the latter case, then obviously the LifeSize brand videoconference units
>> are buggy, but since I know they interperate will over H.264 with a wide
>> range of units from other manufacturers there must be a lot of buggy devices
>> out there -- would it be worth adding an extra flag to x264 that says "do
>> bytestream encoding only in Annex B mode, for compatibility with
>> devices that cannot handle it in RTP mode"?
>
> The best solution here is probably to add another parameter to control
> the escaping.  b_escaped_nals or similar.  In the case of
> b_escaped_nals == 0, a straight memcpy would be used.
>
> Dark Shikari
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