[FFmpeg-devel] [PATCH] lavf: inspect more frames when the time base is unreliable

[Måns Rullgård]

Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:

> On Sat, Jan 29, 2011 at 02:35:04PM +0200, Anssi Hannula wrote:
>> On 29.01.2011 12:28, Reimar D?ffinger wrote:
>> > On Sat, Jan 29, 2011 at 02:39:38AM +0200, Anssi Hannula wrote:
>> >> For the record, here you can see the progression of the cumulative error
>> >> of the two competing rates when detecting the frame rate:
>> >>
>> >> 0.042000
>> >> for 24.000000, at 0.042000, error 0.000333, cumulative 0.000000111
>> >> for 23.976024, at 0.042000, error 0.000292, cumulative 0.000000085
>> >> 0.041000
>> >> for 24.000000, at 0.041000, error -0.000667, cumulative 0.000000444
>> >> for 23.976024, at 0.041000, error -0.000708, cumulative 0.000000502
>> >> 0.083000
>> >> for 24.000000, at 0.083000, error -0.000333, cumulative 0.000000556
>> >> for 23.976024, at 0.083000, error -0.000417, cumulative 0.000000675
>> >> 0.125000
>> >> for 24.000000, at 0.125000, error 0.000000, cumulative 0.000000556
>> >> for 23.976024, at 0.125000, error -0.000125, cumulative 0.000000691
>> >> 0.167000
>> >> for 24.000000, at 0.167000, error 0.000333, cumulative 0.000000667
>> >> for 23.976024, at 0.167000, error 0.000167, cumulative 0.000000719
>> >> 0.208000
>> >> for 24.000000, at 0.208000, error -0.000333, cumulative 0.000000778
>> >> for 23.976024, at 0.208000, error -0.000542, cumulative 0.000001012
>> >> 0.250000
>> >> for 24.000000, at 0.250000, error 0.000000, cumulative 0.000000778
>> >> for 23.976024, at 0.250000, error -0.000250, cumulative 0.000001075
>> >> 0.292000
>> >> for 24.000000, at 0.292000, error 0.000333, cumulative 0.000000889
>> >> for 23.976024, at 0.292000, error 0.000042, cumulative 0.000001076
>> >> 0.333000
>> > 
>> > Well, the problem is that the time stamps _do_ match 24 fps better
>> > than 23.97..., and it is pure chance that parsing more changes this.
>> 
>> I don't think so. The timestamps have millisecond precision and they
>> thus have a pattern of 41ms and 42ms differences.
>
> Sorry, failure to use calculator properly.
> What is the time_base, is FFmpeg aware the time stamps only have
> millisecond precision? Because I thought there was code to ignore
> only off-by-one differences.
>
>> > But it might be possibly to go for whatever is stored in the MKV
>> > file (if anything) in such an unclear case.
>> 
>> The mkv file contains a default frame duration of 0.041708332ms, which
>> is an approximation of 24000/1001 (23.976fps).
>
> That could/should probably be used in some way then.
> Note that I do not know if it might also be that 24000/1001
> causes an issue, that is a non-standard frame-rate, whereas
> 24 fps is standard and I think some parts of FFmpeg prefer
> standard values.

The track DefaultDuration is by definition the frame duration in
nanoseconds.  If this is set, there is no need to attempt guessing
from timestamps.

-- 
M?ns Rullg?rd
mans at mansr.com