[FFmpeg-devel] [PATCH 06/11] avformat/dashenc: addition of @availabilityTimeOffset in MPD

[Jeyapal, Karthick]

On 3/23/18 10:57 AM, vdixit at akamai.com wrote:
> From: Vishwanath Dixit <vdixit at akamai.com>
>
> availability time of Nth segment = availabilityStartTime + (N*segment duration) - availabilityTimeOffset.
> This field helps to reduce the latency by about a segment duration in streaming mode.
> ---
>  libavformat/dashenc.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/libavformat/dashenc.c b/libavformat/dashenc.c
> index b62cb3e..d20bdba 100644
> --- a/libavformat/dashenc.c
> +++ b/libavformat/dashenc.c
> @@ -85,6 +85,7 @@ typedef struct OutputStream {
>      char filename[1024];
>      char full_path[1024];
>      char temp_path[1024];
> +    int64_t chunk_duration;
Remove this variable chunk_duration, and instead create a variable directly for availabilityTimeOffset.
>  } OutputStream;
>  
>  typedef struct DASHContext {
> @@ -343,8 +344,12 @@ static void output_segment_list(OutputStream *os, AVIOContext *out, AVFormatCont
>      if (c->use_template) {
>          int timescale = c->use_timeline ? os->ctx->streams[0]->time_base.den : AV_TIME_BASE;
>          avio_printf(out, "\t\t\t\t<SegmentTemplate timescale=\"%d\" ", timescale);
> -        if (!c->use_timeline)
> +        if (!c->use_timeline) {
>              avio_printf(out, "duration=\"%d\" ", c->seg_duration);
> +            if (c->streaming && os->chunk_duration)
> +                avio_printf(out, "availabilityTimeOffset=\"%.3f\" ",
> +                            ((double) c->seg_duration - os->chunk_duration) / AV_TIME_BASE);
> +        }
>          avio_printf(out, "initialization=\"%s\" media=\"%s\" startNumber=\"%d\">\n", c->init_seg_name, c->media_seg_name, c->use_timeline ? start_number : 1);
>          if (c->use_timeline) {
>              int64_t cur_time = 0;
> @@ -1283,6 +1288,10 @@ static int dash_write_packet(AVFormatContext *s, AVPacket *pkt)
>          format_date_now(c->availability_start_time,
>                          sizeof(c->availability_start_time));
>  
> +    if (!os->chunk_duration && pkt->duration)
> +        os->chunk_duration = av_rescale_q(pkt->duration, st->time_base,
> +                                          AV_TIME_BASE_Q);
Don’t use the term ‘chunk_duration’ here, as it is not clearly conceptualized in this file. 
Instead use the term ‘frame_duration’(local variable) to avoid confusion.
> +
>      if (c->use_template && !c->use_timeline) {
>          elapsed_duration = pkt->pts - os->first_pts;
>          seg_end_duration = (int64_t) os->segment_index * c->seg_duration;