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

[Gil Pedersen]

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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>> 
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