[Libav-user] avcodec_decode_video2

Alex Cohn alexcohn at netvision.net.il
Thu Dec 29 17:24:00 CET 2011

On Wed, Dec 28, 2011 at 23:55, Neil Menne <nhmenne42 at students.tntech.edu> wrote:
> I'll start from the beginning to see if we can find the problem: I do the
> register all; I do a find_decoder (which is currently looking for
> "CODEC_ID_H264"); if that's successful, I allocate the context (using
> alloc_context3); I then do an avcodec_open2 with arguments of my context, my
> codec, and NULL for the dictionary; if this succeeds, I allocate the frame;
> next, I initialize a packet and set the packets data to the buffer I
> received which contains the H264 video; if that all works, i do a
> decode_video2.
> The return is a -1 as i save it to a unique variable and print the result
> immediately; it also states that whole "no frame!" prior to my printout of
> the result.

It's natural to receive -1 from decode_video2() for the first few
frames -- the number depends on your stream (how many b-frames are
allowed). The question about -1 pops up from time to time. The problem
is, some H264 streams or files have non-zero b-frame count in the
header, even if they actually carry no b-frames. It is not
unacceptable to edit the header before it is interpreted by the H264
decoder, to reset the flag to zero.

> I feel like it's tied into either the AVCodecContext or the AVPacket
> (something I'm not populating in the packet besides supplying the buffer);
> but that's just a guess at this point.
> Thanks for taking time to look at this problem.
> -Neil
> P.S. If it matters, I'm using Live555 (because I need direct access to some
> of the RTP related data) to pull down my RTSP stream and passing along the
> buffer it returns.
> On Wed, Dec 28, 2011 at 4:39 AM, Alex Cohn <alexcohn at netvision.net.il>
> wrote:
>> On Wed, Dec 28, 2011 at 01:19, Neil Menne <nhmenne42 at students.tntech.edu>
>> wrote:
>> > How do I pass a raw buffer into this function?
>> >
>> > I find the appropriate codec and codec context, allocate the AVFrame,
>> > and
>> > initialize the packet. When I try and set the packet's data to the
>> > buffer:
>> > AVPacket packet;
>> > av_init_packet(&packet);
>> > packet.data = buf;
>> >
>> > where buf is the buffer received; I'm getting an error that seems to be
>> > unrelated. It says "no frame!" and returns -1.
>> Are you sure you receive -1 from the code above? maybe, some important
>> line of code is missing in your post?
>> -Alex
>> > If it helps, my AVFrame is initialized with:
>> > codecFrame = avcodec_alloc_frame();
>> >
>> > -Neil

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