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

[Anssi Hannula]

On 15.02.2011 07:40, Anssi Hannula wrote:
> On 15.02.2011 01:54, Michael Niedermayer wrote:
>> 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)
> 
> Well, here's one:
> http://onse.fi/files/h264-23.976-detected-as-24.mkv
> 

As there has been no activity, I added this to roundup to make sure it
will not be forgotten:
https://roundup.ffmpeg.org/issue2629

-- 
Anssi Hannula