RTMP live stream directly from NVENC encoder - ffmpeg

Question

I am trying to create a live RTMP stream containing the animation generated with NVIDIA OptiX. The stream is to be received by nginx + rtmp module and broadcasted in MPEG-DASH format. Full chain up to dash.js player is working if the video is first saved to .flv file and then I send it with ffmpeg without any reformatting using command:
ffmpeg -re -i my_video.flv -c:v copy -f flv rtmp://x.x.x.x:1935/dash/test
But I want to stream directly from the code. And with this I am failng... Nginx logs an error "dash: invalid avcc received (2: No such file or directory)". Then it seems to receive the stream correctly (segments are rolling, dash manifest is there), however the stream is not possible to play in the browser.
I can see only one difference in the manifest between direct stream and stream from file. Codecs attribute of the representation in the direct stream is missed: codecs="avcc1.000000" instead of "avc1.640028" which I get when streaming from file.
My code opens the stream:
av_register_all();
AVOutputFormat* fmt = av_guess_format("flv",
file_name, nullptr);
fmt->video_codec = AV_CODEC_ID_H264;
AVFormatContext* _oc;
avformat_alloc_output_context2(&_oc, fmt, nullptr, "rtmp://x.x.x.x:1935/dash/test");
AVStream* _vs = avformat_new_stream(_oc, nullptr);
_vs->id = 0;
_vs->time_base = AVRational { 1, 25 };
_vs->avg_frame_rate = AVRational{ 25, 1 };
AVCodecParameters *vpar = _vs->codecpar;
vpar->codec_id = fmt->video_codec;
vpar->codec_type = AVMEDIA_TYPE_VIDEO;
vpar->format = AV_PIX_FMT_YUV420P;
vpar->profile = FF_PROFILE_H264_HIGH;
vpar->level = _level;
vpar->width = _width;
vpar->height = _height;
vpar->bit_rate = _avg_bitrate;
avio_open(&_oc->pb, _oc->filename, AVIO_FLAG_WRITE);
avformat_write_header(_oc, nullptr);
Width, height, bitrate, level and profile I get from NVENC encoder settings. I also do the error checking, ommited here. Then I have a loop writing each encoded packets, with IDR frames etc all prepared on the fly with NVENC. The loop body is:
auto & pkt_data = _packets[i];
AVPacket pkt = { 0 };
av_init_packet(&pkt);
pkt.pts = av_rescale_q(_n_frames++, AVRational{ 1, 25 }, _vs->time_base);
pkt.duration = av_rescale_q(1, AVRational{ 1, 25 }, _vs->time_base);
pkt.dts = pkt.pts;
pkt.stream_index = _vs->index;
pkt.data = pkt_data.data();
pkt.size = (int)pkt_data.size();
if (!memcmp(pkt_data.data(), "\x00\x00\x00\x01\x67", 5))
{
pkt.flags |= AV_PKT_FLAG_KEY;
}
av_write_frame(_oc, &pkt);
Obviously ffmpeg is writing avcc code somewhere... I have no clue where to add this code so the RTMP server can recognize it. Or I am missing something else?
Any hint greatly appreciated, folks!

Answer 1

Thanks to Gyan's comment I was able to solve the issue. Following the AV_CODEC_FLAG_GLOBAL_HEADER flag in the wrapper one can see how the global header is added, which was missing in my case. You can use directly the NVENC API function nvEncGetSequenceParams, but since I am anyway using SDK, it is a bit cleaner.
So I had to attach the header to AVCodecParameters::extradata:
std::vector<uint8_t> payload;
_encoder->GetSequenceParams(payload);
vpar->extradata_size = payload.size();
vpar->extradata = (uint8_t*)av_mallocz(payload.size() + AV_INPUT_BUFFER_PADDING_SIZE);
memcpy(vpar->extradata, payload.data(), payload.size());
_encoder is my instance of NvEncoder from SDK.
The wrapper is doing the same thing, however using deprecated struct AVCodecContext.