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

[Gil Pedersen]

On 06/01/2010, at 01.36, Alex Giladi wrote:

> Gil,
> Quoting section 3 of the RFC:  "This payload specification can only be
> used to carry the "naked" H.264 NAL unit stream over RTP, and not the
> bitstream format discussed in Annex B of H.264". Escaping is defined
> in Annex B (section B.1).
> Haven't played with this (I work with plain old MPEG-2 TS), but this
> is my interpretation of the specs.

You're reading it wrong then. Escaping (by inserting emulation prevention bytes) is defined for NAL units in section 7.3.1. Annex B only defines further encapsulation of these bits but doesn't change the actual representation.
/Gil

> Also, there is no "de-facto standard": there is only compliance or
> lack of compliance.
> Alex.
> 
> On Tue, Jan 5, 2010 at 5:44 PM, Gil Pedersen <gil at cmi.aau.dk> wrote:
>> On 05/01/2010, at 20.49, Alex Giladi wrote:
>> 
>>> RTP payload (at RFC 3984) uses RBSP. Maybe it's worth adding "rtp" as
>>> an output option?
>>> --ag
>> 
>> Is this true? I see no mention of RBSP in the RFC. The RFC 3984 payload is based on NAL units, which according to the H.264 spec consists of a 1-byte header followed by RBSP _and_ the necessary emulation bytes. Further, as mentioned, the H.264 reference encoder seems to output RTP packets with the emulation bytes.
>> 
>> As far as I can tell any decoder that expects raw RBSP + 1 byte header is broken and x264 should never allow this to be generated unless it can be proven that it's the de-facto standard.
>> 
>> /Gil
>> 
>>> 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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