[Ffmpeg-devel] problem decoding frames when first frame is B

[Måns Rullgård]

"rkmr.em at gmail.com" <rkmr.em at gmail.com> writes:

> On 4/23/07, Michel Bardiaux <mbardiaux at mediaxim.be> wrote:
>>
>> M?ns Rullg?rd wrote:
>> > Michel Bardiaux wrote:
>> >> rkmr.em at gmail.com wrote:
>> >>> On 4/20/07, M?ns Rullg?rd <mans at mansr.com> wrote:
>> >>>> "rkmr.em at gmail.com" <rkmr.em at gmail.com> writes:
>> >>>>> I have a valid MPEG-2 video stream in which the first two frames are
>> >>>>> B-frames. ffmpeg is not able to decode those two frames:
>> >>>>>
>> >>>>>                        avcodec_decode_video( m_vDec,
>> m_decodedPicture,
>> >>>>> &got_picture, (uint8_t*) samplePtr, sampleLength );
>> >>>>>
>> >>>>> got_picture is 0 for the first two frames. Is this a know problem?
>> >>>> Upload a sample, please.
>> >>>
>> >>> can you tell how to do this? it is a 49MB file.
>> >>>
>> >>>
>> >>> Do you mean that the first frame is a B-frame with only intra coded
>> >>>> blocks, or that the first frames in display order are B-frames
>> >>>> predicted against a following (display order) I-frame, which is the
>> >>>> first coded frame?
>> >>>
>> >>> the display order is like this:
>> >>> BBIBBP..
>> >>> I am not sure how it is actually coded.
>> >> I have seen MPEG (1, not 2) streams coming from hardware encoders and
>> >> having that structure. The full GOP structure was
>> >>
>> >> BBIBBPBBPBBP|BBIBBPBBPBBP|...
>
> This is exactly the frame strucuture and is a hardware encoder that is
> producing this stream!!
>
>>> the 2 leading B-frames being coded against the last P-frame of the
>> >> preceding GOP. A typical not-closed GOP. And a nightmare to decode.
>> >
>> > The only problem is that you don't have the reference frame for
>> > the first couple of frames.  Nothing to worry about once you've
>> > got the first two non-B frames.  Open GOP makes some sense when
>> > random access is impossible, such as broadcast, which is where it
>> > is primarily found.  DVD uses closed
>
> So it is not possible to decode the first two frames?  Is this true
> even with the latest svn version of ffmpeg?

The first frame produced when the decoder is switched on must be
possible to decode somehow.  If you jump into the stream at a later
point you'll always have a couple of initial B-frames with one
reference before your starting point.  Not even FFmpeg can invent this
missing data.  The decoder copes with it just fine though, so there's
no big deal.

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