[Libav-user] avcodec_decode_video2 missing the first 8 frames

Matt Orlando morlando616 at gmail.com
Tue Jul 22 20:59:45 CEST 2014


Thank you very much, I'll take some time and look at this tutorial.


On Tue, Jul 22, 2014 at 11:22 AM, cyril poulet <cyril.poulet at centraliens.net
> wrote:

> 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
>>>>>>
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>>>>>>
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