[FFmpeg-cvslog] r13643 - in trunk: libavcodec/mpegvideo_parser.c tests/seek.regression.ref

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> On Tue, Jun 03, 2008 at 05:33:59AM +0100, M?ns Rullg?rd wrote:
>> michael <subversion at mplayerhq.hu> writes:
>> 
>> > Author: michael
>> > Date: Tue Jun  3 04:43:17 2008
>> > New Revision: 13643
>> >
>> > Log:
>> > In mpeg1/2 timestamps (against all logic) are associated with
>> > picture start codes and not with access units.
>> 
>> The change may be valid, but this description doesn't make sense.
>> This is what the spec says:
>> 
>>   For ITU-T Rec. H.262 | ISO/IEC 13818-2 video, if a PTS is present in
>>   a PES packet header, it shall refer to the access unit containing
>>   the first picture start code that commences in this PES packet. A
>>   picture start code commences in a PES packet if the first byte of
>>   the picture start code is present in the PES packet.
>> 
>> In other words, the access unit is defined as containing the picture
>> start code, and I can't think of any other sane way of defining it.
>
> H.264 in H.222 defines it as the first access unit that commences in the
> PES packet. (which is IMHO more sane)

MPEG2 defines a video access unit like this:

  In the case of video, an access unit includes all the coded data for
  a picture, and any stuffing that follows it, up to but not including
  the start of the next access unit. If a picture is not preceded by a
  group_start_code or a sequence_header_code, the access unit begins
  with the picture start code. If a picture is preceded by a
  group_start_code and/or a sequence_header_code, the access unit
  begins with the first byte of the first of these start codes. If it
  is the last picture preceding a sequence_end_code in the bitstream,
  all bytes between the last byte of the coded picture and the
  sequence_end_code (including the sequence_end_code) belong to the
  access unit.

H.264 defines an access unit like this:

  A set of NAL units always containing exactly one primary coded
  picture. In addition to the primary coded picture, an access unit
  may also contain one or more redundant coded pictures or other NAL
  units not containing slices or slice data partitions of a coded
  picture. The decoding of an access unit always results in a decoded
  picture.

For H.264 video, PTS is defined as:

  For ITU-T Rec. H.264 | ISO/IEC 14496-10 video, if a PTS is present
  in the PES packet header, it shall refer to the first AVC access
  unit that commences in this PES packet. An AVC access unit commences
  in a PES packet if the first byte of the AVC access unit is present
  in the PES packet.

The difference is how non-picture data, e.g. GOP headers and SPS, PPS,
SEI etc., are treated, and I really don't see how one or the other
would be significantly better.

> anyway ill try to improve the commit message tomorrow
>
> PS: i think our PS/TS muxers handle the timestamp association in the
> access unit (h.264) way instead of picture start code (mpeg1/2) way
> for mpeg1/2.

Do they ever start a new PES packet between the start of an access
unit and the first picture start code?

-- 
M?ns Rullg?rd
mans at mansr.com