[Libav-user] avcodec_decode_video2 missing the first 8 frames

cyril poulet cyril.poulet at centraliens.net
Tue Jul 22 17:22:43 CEST 2014


I think "*For video, the packet contains exactly one frame.*" is not true
for all codecs, but is for most of the usual ones.
try this one :
http://blog.tomaka17.com/2012/03/libavcodeclibavformat-tutorial/
It may be that avcodec_decode_video2 recognizes that the first packet data
is header and discards it as irrelevant for image decoding...


2014-07-22 17:14 GMT+02:00 Matt Orlando <morlando616 at gmail.com>:

> The documentation
> <https://www.ffmpeg.org/doxygen/trunk/group__lavf__decoding.html#ga4fdb3084415a82e3810de6ee60e46a61> says
> "*For video, the packet contains exactly one frame.*"   I used this
> <http://dranger.com/ffmpeg/> tutorial to understand the flow of what
> needs to be done.  Some of the calls were deprecated so a few google
> searches got me the replacements.  What appears to be happening is those
> lost frames are just gone...the data isn't being accumulated.  Is it
> possible my av_free_packet call is in a bad spot?
>
>
> On Tue, Jul 22, 2014 at 10:58 AM, cyril poulet <
> cyril.poulet at centraliens.net> wrote:
>
>> 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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