[Libav-user] avcodec_decode_video2 missing the first 8 frames

[cyril poulet]

what av_read_frame does is take some data and fill your packet with it.
There is no guaranty that a full frame is in the packet, or even image data.
avcodec_decode_video2 is the only way to know if you have decoded an image
: you feed it your successive packets, it accumulates data, and tells you
when the data allowed him to successfully decode an image (through
frameFinished)
You should look online for basic examples of how to decode safely vids,
there is in particular a tutorial which introduces the structure SafePacket
(first version has been updated for latest versions of the libs)


2014-07-22 16:49 GMT+02:00 Matt Orlando <morlando616 at gmail.com>:

> Thanks for the response.  That's very interesting, though I wonder if it
> could actually be header related.  Based on the videos, I know for sure
> there are exactly 64 frames in the first movie, for instance.  My
> m_FrameRead is only increased if avcodec_decode_video2 says it was able to
> get a picture.  My thinking is that if it was a header that was read, that
> function wouldn't set frameFinished to "true".  So av_read_frame hits the
> EOF when my m_FrameRead counter is only 56.
>
> Are you saying that perhaps the av_read_frame is reading a bunch of data
> (header + frames) and my avcodec_decode_video2 says there's no picture
> because there's header data in there.  Then a few frames later (or maybe
> next frame), av_read_frame reads more data, now including the 9th 'frame'
> and finally avcodec_decode_video2 decodes and I increment my counter (which
> would be now 1 when I'd want it to be 9).
>
> Thanks for the help!
> -Matt
>
>
> On Tue, Jul 22, 2014 at 8:30 AM, cyril poulet <
> cyril.poulet at centraliens.net> wrote:
>
>> according to the doc, av_read_packet does not check wether the data in
>> the decoded frame is a valid image or not.
>> As it is always the 8 first "frames" missing, I would assume it is the
>> header that gets decoded first.
>> Have you noticed missing images when comparing the first real decoded
>> image and your stream in a player ?
>>
>>
>> 2014-07-21 18:37 GMT+02:00 Matt Orlando <morlando616 at gmail.com>:
>>
>>> Hi folks,
>>>
>>> I'm having some trouble with the decoding of my h.264 movies.  I've
>>> tested with different videos, one 64 frames long and another 52 frames
>>> long.  In both cases, the first 8 frames (exactly 8) weren't decoding.  In
>>> other words, the call to avcodec_decode_video2 returns 0 for the
>>> got_picture variable.  I track the frames read and received for our system
>>> and I never get notice that all the frames were read because my counter
>>> doesn't increase for the first 8...however all of the subsequent calls to
>>> av_read_frame return EOF.  To prevent any confusion (or perhaps just to add
>>> some context), I need the raw data from each decoded frame since I don't
>>> treat them as a frame to a movie but rather a frame of x,y,z positional
>>> data.
>>>
>>> I work in Unity but made a plugin to communicate with the libraries to
>>> read and decode the h.264 movies.  This is our update function:
>>>
>>> bool H264Stream::Update(int desiredFrame)
>>> {
>>>    if( desiredFrame <= m_FrameRead )
>>>       return true;
>>>
>>>    int frameFinished = 0;
>>>
>>>    if( av_read_frame( m_AVFormat, &m_Packet ) >= 0 )
>>>    {
>>>       if( m_Packet.stream_index == m_AVStream->index )
>>>       {
>>>          av_frame_unref( m_AVFrame );
>>>          avcodec_decode_video2( m_AVContext, m_AVFrame, &frameFinished,
>>> &m_Packet );
>>>
>>>          if( frameFinished != 0 )
>>>          {
>>>             sws_scale( m_SwsContext, m_AVFrame->data,
>>> m_AVFrame->linesize, 0, m_AVContext->height, m_Picture->data,
>>> m_Picture->linesize );
>>>
>>>             m_FrameRead++;
>>>
>>>             av_free_packet( &m_Packet );
>>>          }
>>>       }
>>>    }
>>>
>>>    return m_FrameRead >= desiredFrame;
>>> }
>>>
>>>
>>> Oh, quick note, this is my first time using these libraries so if
>>> there's something blatantly wrong with how I'm doing this, please let me
>>> know.  :)   Just keep in mind, it's the full frame of 'raw' data I use
>>> (post-decode) that I pass back into my Unity c# scripts.
>>>
>>> Thanks for the help everyone!
>>> -Matt
>>>
>>> _______________________________________________
>>> Libav-user mailing list
>>> Libav-user at ffmpeg.org
>>> http://ffmpeg.org/mailman/listinfo/libav-user
>>>
>>>
>>
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