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

Måns Rullgård mans
Mon Apr 23 16:37:14 CEST 2007

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:
>> 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
> 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

M?ns Rullg?rd
mans at mansr.com

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