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

[Anssi Hannula]

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

-- 
Anssi Hannula