[libav-api] FLV Variable Frame Rate

Ronald S. Bultje rsbultje at gmail.com
Fri May 27 16:05:24 CEST 2011

Hi Mark,

On Fri, May 27, 2011 at 9:56 AM, Mark Kenna
<mark.kenna at sureviewsystems.com> wrote:
> On 27/05/2011 14:51, Ronald S. Bultje wrote:
>> On Fri, May 27, 2011 at 9:49 AM, Mark Kenna
>> <mark.kenna at sureviewsystems.com>  wrote:
>>> I'm having a hard time getting my head around encoding variable frame
>>> rate
>>> containers in LibAV. I have taken the 'output_example.c' program and
>>> modified it to encode to FLV. What I would like to know is how I would
>>> modify the 'output_example.c' program to write variable time stamp values
>>> into the output file. For my project I will be taking a streaming input
>>> source (which will not have time-stamp information) and will need to
>>> create
>>> my own timestamp values (I guess from the delta value between the last
>>> received frame and the current frame?).
>>> I have read that you need to set the time base value to:
>>>        c->time_base.num=1;
>>>        c->time_base.den=1000;
>>> But I am not sure exactly what I should be setting the encoded packet's
>>> PTS
>>> values to?
>> This means "each pts value has a fractional unit of 1/1000th", or in
>> other words, millisecond precision. So if your timestamps are 10ms,
>> 22ms, 38ms, then pts would be 10, 22 and 38.
>> Ronald
> Wow! That was quick - thanks!
> So after encoding the frame and calculating what the pts value should be
> (based on the delay between the last frame and this one) I would do:
>            pkt.stream_index= st->index;
>            pkt.data= video_outbuf;
>            pkt.size= out_size;
>            pkt.pts= previousPacketPts + deltaDelay;
>            /* write the compressed frame in the media file */
>            ret = av_write_frame(oc, &pkt);
> If that makes any sense?

That works, if you're not using B-frames.

If you're using B-frames, I'd suggest using
AVCodecContext->coded_frame->pts as a way to set/get the PTS of
encoded frames before/after encoding. ffmpeg.c uses this also.


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