[Libav-user] avcodec_decode_video2 missing the first 8 frames
Matt Orlando
morlando616 at gmail.com
Tue Jul 22 17:14:46 CEST 2014
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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>>
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