[FFmpeg-devel] Extracting NTP timestamp from RTCP

Michael Niedermayer michaelni at gmx.at
Thu Dec 5 01:08:21 CET 2013

On Wed, Dec 04, 2013 at 04:55:36PM -0700, Fred Rothganger wrote:
> On 12/04/2013 08:09 AM, Michael Niedermayer wrote:
> >diff --git a/libavformat/rtpenc.c b/libavformat/rtpenc.c
> >index 6fdc908..5e73661 100644
> >--- a/libavformat/rtpenc.c
> >+++ b/libavformat/rtpenc.c
> >@@ -119,7 +119,7 @@ static int rtp_write_header(AVFormatContext *s1)
> >          s->ssrc = av_get_random_seed();
> >      s->first_packet = 1;
> >      s->first_rtcp_ntp_time = ff_ntp_time();
> >-    if (s1->start_time_realtime)
> >+    if (s1->start_time_realtime != AV_NOPTS_VALUE)
> >this looks like it breaks ABI
> >a application that means "no value" would have used 0 before but now
> >0 is interpreted as a litteral 0
> >
> >
> I thought that was the whole point of the previous conversation.
> Reimar was concerned that we should have an explicit "not set" value
> that was different from 0. To be consistent in the use of the field,
> we should respect the "not set" value during encoding.
> Please clarify the nature of your objection. Are you saying that:
> 1) ABI changes are simply not allowed.
> 2) A more significant version number should be bumped to indicate
> the ABI change.
> 3) Use 0 to mean not set.
> 4) Give the field different semantics (however slight) between
> encoding and decoding.

change the meaning of 0 with the next ABI bump or introduce a new
field but that might be overkill, dunno

> In practical terms, someone will only set this field if they know
> the precise time that the first frame was captured. Otherwise, they
> will not touch it at all, and the default ("not set") will remain.

maybe, maybe not.
iam pretty sure some applications set some fields based on their
internal variables unconditionally


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Rewriting code that is poorly written but fully understood is good.
Rewriting code that one doesnt understand is a sign that one is less smart
then the original author, trying to rewrite it will not make it better.
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