[FFmpeg-devel] A change in r_frame_rate values after upgrade to FFmpeg 6.1

[Antoni Bizoń]

I understand that the r_frame_rate is the lowest framerate with which all timestamps can be represented accurately. And I know it is just a guess. But why did the logic behind the calculation change?
Changes between 6.0 and 6.1:
--- a/libavformat/demux.c
+++ b/libavformat/demux.c
@@ -2864,15 +2872,12 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
-      AVRational time_base = avctx->framerate.num ? av_inv_q(av_mul_q(avctx->framerate,
-                                                            (AVRational){avctx->ticks_per_frame, 1}))
-                                                   /* NOHEADER check added to not break existing behavior */
-                                                   : ((ic->ctx_flags & AVFMTCTX_NOHEADER) ? (AVRational){0, 1}
-                                                                                           : st->time_base);
-      if (   time_base.den * (int64_t) st->time_base.num
-           <= time_base.num * (uint64_t)avctx->ticks_per_frame * st->time_base.den) {
-          av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den,
-                    time_base.den, (int64_t)time_base.num * avctx->ticks_per_frame, INT_MAX);
+      const AVCodecDescriptor *desc = sti->codec_desc;
+      AVRational mul = (AVRational){ desc && (desc->props & AV_CODEC_PROP_FIELDS) ? 2 : 1, 1 };
+      AVRational  fr = av_mul_q(avctx->framerate, mul);
+      if (fr.num && fr.den && av_cmp_q(st->time_base, av_inv_q(fr)) <= 0) {
+           st->r_frame_rate = fr;
         } else {
               st->r_frame_rate.num = st->time_base.den;
               st->r_frame_rate.den = st->time_base.num;

Proposed changes:
diff --git a/libavformat/demux.c b/libavformat/demux.c
index 6f640b92b1..9d58b440a9 100644
--- a/libavformat/demux.c
+++ b/libavformat/demux.c
@@ -2876,7 +2876,8 @@ int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options)
                 AVRational  fr = av_mul_q(avctx->framerate, mul);

                 if (fr.num && fr.den && av_cmp_q(st->time_base, av_inv_q(fr)) <= 0) {
-                    st->r_frame_rate = fr;
+                    av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den,
+                              fr.num, (int64_t)fr.den * mul.num, INT_MAX);
                 } else {
                     st->r_frame_rate.num = st->time_base.den;
                     st->r_frame_rate.den = st->time_base.num;


Antoni Bizoń

________________________________
From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> on behalf of Anton Khirnov <anton at khirnov.net>
Sent: Sunday, September 15, 2024 2:41 PM
To: ffmpeg-devel at ffmpeg.org <ffmpeg-devel at ffmpeg.org>
Subject: Re: [FFmpeg-devel] A change in r_frame_rate values after upgrade to FFmpeg 6.1

Quoting Antoni Bizoń (2024-09-13 15:20:16)
> Hello,
> I recently upgraded FFmpeg to version 6.1 in my web application and
> encountered a change while running my app's tests. After the upgrade,
> ffprobe reported an r_frame_rate of 60/1 instead of the expected 30/1
> for two videos in my test suite.

r_frame_rate does not mean what you probably think it means, and is
actually NOT guaranteed to be a frame rate of anything. See if
avg_frame_rate doesn't work better.

--
Anton Khirnov
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