FFMPEG streaming RTP: time base not set

Question

I'm trying to create a small demo to get a feeling for streaming programmatically with ffmpeg. I'm using the code from this question as a basis. I can compile my code, but when I try to run it I always get this error:

[rtp @ 0xbeb480] time base not set

The thing is, I have set the time base parameters. I even tried setting them for the stream (and the codec associated with the stream) as well, even though this should not be necessary as far as I understand it. This is the relevant section in my code:

AVCodec* codec = avcodec_find_encoder(AV_CODEC_ID_H264);
AVCodecContext* c = avcodec_alloc_context3(codec);
c->pix_fmt = AV_PIX_FMT_YUV420P;
c->flags = CODEC_FLAG_GLOBAL_HEADER;
c->width = WIDTH;
c->height = HEIGHT;
c->time_base.den = FPS;
c->time_base.num = 1;
c->gop_size = FPS;
c->bit_rate = BITRATE;

avcodec_open2(c, codec, NULL);
struct AVStream* stream = avformat_new_stream(avctx, codec);

// TODO: causes an error
avformat_write_header(avctx, NULL);

The error occurs when calling "avformat_write_header" near the end. All methods that can fail (like avcodec_open2) are checked, I just removed the checks to make the code more readable.

Digging through google and the ffmpeg source code didn't yield any useful results. I think it's really basic, but I'm stuck. Who can help me?

Answer 1

You are making settings in a wrong codec context.

The streams created by avformat_new_stream() have their own internal codec contexts, the one you created with avcodec_alloc_context3() is unnecessary and has no effect on the workings of avformat_write_header().

To set the variables correctly, set them this way:

AVCodec* codec = avcodec_find_encoder(AV_CODEC_ID_H264);
struct AVStream* stream = avformat_new_stream(avctx, codec);
stream->codec->pix_fmt = AV_PIX_FMT_YUV420P;
stream->codec->flags = CODEC_FLAG_GLOBAL_HEADER;
stream->codec->width = WIDTH;
stream->codec->height = HEIGHT;
stream->codec->time_base = (AVRational){1,FPS};
stream->codec->gop_size = FPS;
stream->codec->bit_rate = BITRATE;

That solved this particular problem for me, I added the other answer given here as well, as that's how I have it set, though your method of setting the time_base probably could have worked too, if you had been talking to the correct codec context.