[Libav-user] Saving a range of frames from a network camera (RTSP) to a mp4 file

[Dídac Pérez]

Hi all,

I am quite confused about how to save a video stream into a mp4 file. Let
me explain the problem:

1. I connect to a network camera via RTSP (H.264 stream)
with avformat_open_input(), avformat_find_stream_info(), av_read_play(),
and I get frames with av_read_frame().
2. Each time I get a frame with av_read_frame(), I store the corresponding
AVPacket in a circular buffer.
3. In some points in my application, a range of this circular buffer is
selected. I find a key frame used to start from.
4. Once I have a list of AVPacket's starting from a key frame, I write
header, frames, and tail, as I described below in the code.

The problem is that the resulting mp4 video has artifacts if I try to watch
it using VLC, Windows Media Player, or another one.

I have also realized that the pts of that packets are not continuous, while
dts are continuous. I know about B frames, but is this a problem in my case?

// Prepare the output
AVFormatContext* oc = avformat_alloc_context();
oc->oformat = av_guess_format(NULL, "video.mp4", NULL);

// Must write header, packets, and trailing
avio_open2(&oc->pb, "video.mp4", AVIO_FLAG_WRITE, NULL, NULL);

// Write header
AVStream* stream = avformat_new_stream(oc, (AVCodec*)
context->streams[video_stream_index]->codec->codec);
avcodec_copy_context(stream->codec,
context->streams[video_stream_index]->codec);
stream->sample_aspect_ratio =
context->streams[video_stream_index]->codec->sample_aspect_ratio;
avformat_write_header(oc, NULL);

// FOR EACH FRAME...
... av_write_frame(oc, circular[k]); ...

// Write trailer and close the file
av_write_trailer(oc);
avcodec_close(stream->codec);
avio_close(oc->pb);
avformat_free_context(oc);

Thank you so much,

--
Dídac Pérez
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