[Libav-user] Getting a Frame question

[Charles]

On 09/04/2016 09:01 PM, M N wrote:
> That was my question, how can I read a frame from a specific stream? And what does av_read_frame really reads?
>

I don't think you can, you will need to read all frames and keep the packets that match your stream.
See https://github.com/FFmpeg/FFmpeg/blob/master/doc/examples/transcoding.c
avformat_find_stream_info around line 60

You can fprobe the file for stream info too
Stream #0:0: Video: mpeg2video (Main), yuv420p(tv, bt709), 1280x720 [SAR 1:1 DAR 16:9], 29.97 fps, 29.97 tbr, 1200k tbn, 59.94 tbc
That is the stream number
Find the one you want and only pass data that is part of that stream, that is the demuxing.

See https://github.com/FFmpeg/FFmpeg/blob/master/doc/examples/transcoding.c
Line 498 could be
    if ( packet.stream_index == mystreamid ) then avcodec_send_packet

Compile transcoding.c and make small changes to it until it does what you want.

Thanks
cco


> 05.09.2016, 04:01, "Christopher Snowhill" <kode54 at gmail.com>:
>> On 9/4/16 5:27 PM, Charles wrote:
>>>  On 09/04/2016 10:52 AM, M N wrote:
>>>>  Gonzalo then how should I continue to read packets from input format
>>>>  context and write them to output format context?
>>>>
>>>>  04.09.2016, 05:14, "Charles" <linux2 at orion15.org>:
>>>>>  On 09/03/2016 08:34 PM, Gonzalo Garramuño wrote:
>>>>>>   El 03/09/2016 a las 17:55, M J escribió:
>>>>>>>   Hi
>>>
>>>  MJ,
>>>  I probably wasn't clear, each call is dependent on the return value of
>>>  the previous call.
>>>  i.e. if you did not get return 0 or EAGIN from send_packet you should
>>>  probably not call receive_frame
>>>
>>>>>>>   while(av_read_frame(input_ctx, pkts) == 0)
>>>>>>>   {
>>>>>>>            //decoding
>>>>>>>            int ret1 = avcodec_send_packet(input_codecCtx, pkts);
>>>>>>>            int ret2 = avcodec_receive_frame(input_codecCtx, rawFrame);
>>>
>>>  Why don't you just print the return values. SEE av_strerror
>>>  https://ffmpeg.org/doxygen/3.1/group__lavu__error.html#ga5792b4a2d18d7d9cb0efbcfc335dce24
>>>
>>>>>>>            //encoding
>>>>>>>            avcodec_send_frame(output_codecCtx, rawFrame);
>>>>>>>            avcodec_receive_packet(output_codecCtx, pktr);
>>>
>>>  These have return values also.
>>>
>>>>>>>            int ret = av_interleaved_write_frame(output_ctx, pktr);
>>>>>>>   }
>>>>>>>   *************************************************
>>>>>>   Hi, MJ.
>>>
>>>  Assuming everything is setup correctly, and 1 frame -> 1 packet && 1
>>>  packet -> 1 frame
>>>
>>>  while ( av_read_frame == 0 )
>>>     if avcodec_send_packet == 0
>>>        if avcodec_receive_frame == 0
>>>           if avcodec_send_frame == 0
>>>              if avcodec_receive_packet == 0
>>>                 av_interleaved_write_frame
>>>
>>>  But, each of those IF's have multiple return values
>>>  What are you going to do if the return is AVERROR(EAGAIN) or AVERROR_EOF
>>>  What about the frame delay on the encoder?
>>>
>>>  Thanks
>>>  cco
>>
>> Don't forget, maybe he's parsing an interleaved stream, and isn't paying
>> attention to which stream each packet belongs to.