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

[Michael Niedermayer]

On Mon, Feb 14, 2011 at 05:00:25PM +0200, Anssi Hannula wrote:
> On 14.02.2011 14:24, Michael Niedermayer wrote:
> > On Sun, Feb 13, 2011 at 12:52:35PM +0200, Anssi Hannula wrote:
> >> On 06.02.2011 08:32, Anssi Hannula wrote:
> >>> Anssi Hannula wrote:
> >>>> 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?
> >>>
> >>> Ping. Also, I noticed an extra added newline in the patch, now removed.
> >>
> >> This patch is indeed not enough. The h264 decoder may not discover the
> >> time base immediately from the first few frames, so st->codec->time_base
> >> may still be stream time base. So the code will notice from the first 4
> >> frames that the stream time base can accurately represent timestamps
> >> (which is always true), and sets codec_tb_matches_dts = 1.
> >> Since the mpegts timebase is 1/90000 (i.e. too fine to be fps),
> >> tb_unreliable() says (correctly) false despite codec_tb_matches_dts==1,
> >> so the detection code continues to inspect frames.
> >>
> >> When the h264 decoder finally sets the codec timebase (e.g. 1/25), it is
> >> assumed correct due to codec_tb_matches_dts==1, even if it is not able
> >> accurately represent timestamps at all (e.g. 1/50 intervals).
> >>
> >> At least this sample shows the above issue:
> >> 
> >>
> >> So some checks need to be added to the patch to guard against above. Or
> >> use some entirely different/better approach :)
> >>
> >> I'd really appreciate someone more experienced looking at this issue,
> >> but I'll take a further look at this myself later as well.
> > 
> > Could you explain elaborately what issue you are trying to fix?
> 
> OK, here's a recap of this thread :)
> 
> - normally lavf assumes that the (1 / codec->time_base /
>   codec->ticks_per_frame) is the fps (r_frame_rate), unless it would be
>   too high to be represented by the st->time_base, in which case
>   (1 / st->time_base) is taken instead (utils.c 2431-2440)
> 
> - if ((1/5) > codec->time_base >= (1/101)) is false, the
>   codec->time_base is considered unreliable and instead a custom
>   fps probing code is used (tb_unreliable()) which reads the timestamps
>   of the first 20 packets
> 
> - some H.264 (and I guess MPEG2) streams have a specific interlacing
>   mode that causes there to be 2x more packets (as output from the
>   demuxer) than codec->time_base and codec->ticks_per_frame would
>   indicate (well, I guess technically they are correct, if the packets
>   actually contain half-frames due to interlacing), e.g. [1]
> 
> - due to the above, H264 and MPEG2 are always assumed to have unreliable
>   timebase and the fps probing code is always used
> 
> - mkv tracks have generally millisecond precision for timestamps
> 
> - 23.976fps therefore requires a pattern of 41ms and 42ms frames, that
>   add up to 1.001s in 24 frames
> 
> - the fps probing code doesn't detect the difference between 24fps and
>   23.976fps from just 20 frames, it would need more than that (25-30)
>   => 23.976fps files are wrongly detected as 24fps
> 
> - 23.976fps files are progressive (unless insane), so the codec timebase
>   as got from the decoder would actually be reliable and show an exact
>   rate of 24/1.001.
> 
> - mkv tracks also contain a default_duration field that contains the
>   length of frames in nanosecond precision. In the file I checked it
>   was accurate to within 1.5ns (some rounding issue I guess), which
>   corresponds to an error of about 0.000001 fps.
> 

> So the issue is that 23.976 h264 mkv files are detected as having wrong
> r_frame_rate of 24.

i need a sample to look into this
(and sorry if a url was posted and i missed it)

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

You can kill me, but you cannot change the truth.
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