[FFmpeg-devel] [PATCH] lavf: accept time base from untrusted codecs if it matches timings

[Anssi Hannula]

If the first 4 frames contain timestamps that closely match the codec
time base, accept the codec time base in tb_unreliable() even if the
codec is blacklisted (currently MPEG-2 and H.264).

---

Here's a new patch that checks the timestamps of the first 4 frames
(using the same method which is used in the guess-framerate code) and
uses codec time base for r_frame_rate if the timestamps fit to it.

I tested also with several other files, including H.264 PAFF, MVE
(ipmovie.c), and spotted no regressions.

What do you think?


 libavformat/avformat.h |    2 ++
 libavformat/utils.c    |   38 +++++++++++++++++++++++++++-----------
 2 files changed, 29 insertions(+), 11 deletions(-)

diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index f9f9be5..67e0da9 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -627,6 +627,8 @@ typedef struct AVStream {
         int64_t last_dts;
         int64_t duration_gcd;
         int duration_count;
+        int codec_tb_matches_dts;
+        double codec_tb_dur_error;
         double duration_error[MAX_STD_TIMEBASES];
         int64_t codec_info_duration;
     } *info;
diff --git a/libavformat/utils.c b/libavformat/utils.c
index 4f51c26..61633f9 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -2174,15 +2174,20 @@ static int get_std_framerate(int i){
  * MPEG-2 commonly misuses field repeat flags to store different framerates.
  * And there are "variable" fps files this needs to detect as well.
  */
-static int tb_unreliable(AVCodecContext *c){
-    if(   c->time_base.den >= 101L*c->time_base.num
-       || c->time_base.den <    5L*c->time_base.num
-/*       || c->codec_tag == AV_RL32("DIVX")
-       || c->codec_tag == AV_RL32("XVID")*/
-       || c->codec_id == CODEC_ID_MPEG2VIDEO
-       || c->codec_id == CODEC_ID_H264
-       )
+static int tb_unreliable(AVStream *st){
+    if(   st->codec->time_base.den >= 101L*st->codec->time_base.num
+       || st->codec->time_base.den <    5L*st->codec->time_base.num)
         return 1;
+
+    if(st->info->codec_tb_matches_dts)
+        return 0;
+
+    if(/* st->codec->codec_tag == AV_RL32("DIVX")
+       || st->codec->codec_tag == AV_RL32("XVID") ||*/
+          st->codec->codec_id == CODEC_ID_MPEG2VIDEO
+       || st->codec->codec_id == CODEC_ID_H264)
+        return 1;
+
     return 0;
 }
 
@@ -2256,7 +2261,7 @@ int av_find_stream_info(AVFormatContext *ic)
             if (!has_codec_parameters(st->codec))
                 break;
             /* variable fps and no guess at the real fps */
-            if(   tb_unreliable(st->codec) && !(st->r_frame_rate.num && st->avg_frame_rate.num)
+            if(   tb_unreliable(st) && !(st->r_frame_rate.num && st->avg_frame_rate.num)
                && st->info->duration_count<20 && st->codec->codec_type == AVMEDIA_TYPE_VIDEO)
                 break;
             if(st->parser && st->parser->parser->split && !st->codec->extradata)
@@ -2336,6 +2341,16 @@ int av_find_stream_info(AVFormatContext *ic)
                     st->info->duration_error[i] += error*error;
                 }
                 st->info->duration_count++;
+
+                if (st->info->duration_count <= 4) {
+                    double codec_tb_rate = st->codec->time_base.den / (double)st->codec->time_base.num * st->codec->ticks_per_frame;
+                    double error = dur - lrintf(dur * codec_tb_rate) / codec_tb_rate;
+                    st->info->codec_tb_dur_error += error*error;
+
+                    if (st->info->duration_count == 4 && st->info->codec_tb_dur_error * codec_tb_rate < 0.0001)
+                        st->info->codec_tb_matches_dts = 1;
+                }
+
                 // ignore the first 4 values, they might have some random jitter
                 if (st->info->duration_count > 3)
                     st->info->duration_gcd = av_gcd(st->info->duration_gcd, duration);
@@ -2383,10 +2398,11 @@ int av_find_stream_info(AVFormatContext *ic)
             // the check for tb_unreliable() is not completely correct, since this is not about handling
             // a unreliable/inexact time base, but a time base that is finer than necessary, as e.g.
             // ipmovie.c produces.
-            if (tb_unreliable(st->codec) && st->info->duration_count > 15 && st->info->duration_gcd > 1 && !st->r_frame_rate.num)
+            if (tb_unreliable(st) && st->info->duration_count > 15 && st->info->duration_gcd > 1 && !st->r_frame_rate.num)
                 av_reduce(&st->r_frame_rate.num, &st->r_frame_rate.den, st->time_base.den, st->time_base.num * st->info->duration_gcd, INT_MAX);
+
             if (st->info->duration_count && !st->r_frame_rate.num
-               && tb_unreliable(st->codec) /*&&
+               && tb_unreliable(st) /*&&
                //FIXME we should not special-case MPEG-2, but this needs testing with non-MPEG-2 ...
                st->time_base.num*duration_sum[i]/st->info->duration_count*101LL > st->time_base.den*/){
                 int num = 0;
-- 
1.7.3