[FFmpeg-user] Why are PTS values different from what's expected?

[Mark Filipak (ffmpeg)]

On 2021-04-01 13:06, pdr0 wrote:
> Mark Filipak (ffmpeg) wrote
>> On 2021-04-01 11:41, pdr0 wrote:
>>> Mark Filipak (ffmpeg) wrote
>>>> On 2021-04-01 07:13, Mark Filipak (ffmpeg) wrote:
>>>>> The source is MKV. MKV has a 1/1000 TB, so any PTS variance should be
>>>>> less than 0.1%.
>>>>>
>>>>> The filter complex is thinned down to just this:
>>>>> settb=1/720000,showinfo
>>>>>
>>>>> Here is selected lines from the showinfo report (with   ...comments):
>>>>>
>>>>> [Parsed_showinfo_1 @ 00000247d719ef00] config in time_base: 1/720000,
>>>>> frame_rate: 24000/1001
>>>>>       ...So, deltaPTS (calculated: 1/TB/FR) should be 30030.
>>>>> [Parsed_showinfo_1 @ 00000247d719ef00] n:   1 pts:  30240   ...should
>>>>> be
>>>>> 30030
>>>>> [Parsed_showinfo_1 @ 00000247d719ef00] n:   2 pts:  59760   ...should
>>>>> be
>>>>> 60060
>>>>> [Parsed_showinfo_1 @ 00000247d719ef00] n:   3 pts:  90000   ...should
>>>>> be
>>>>> 90090
>>>>> [Parsed_showinfo_1 @ 00000247d719ef00] n:   4 pts: 120240   ...should
>>>>> be
>>>>> 120120
>>>>>
>>>>> The PTS variance is 0.7%.
>>>>>
>>>>> Why are PTS values different from what's expected?
>>>>>
>>>>> Note: If I force deltaPTS via setpts=N*30030, then of course I get
>>>>> what's
>>>>> expected.
>>>>>
>>>>> Thanks. This is critical and your explanation is greatly appreciated!
>>>>> Mark.
>>>>
>>>> UPDATE
>>>>
>>>> If I change the filter complex to this:
>>>>
>>>> settb=1/720000,setpts=N*30030,fps=fps=48000/1001,showinfo
>>>>
>>>> all my follow-on processing goes straight into the toilet.
>>>>
>>>> Explanation of the factors in the filter complex:
>>>> settb=1/720000   ...mandate 1.3[8..] ms time resolution
>>>> setpts=N*30030   ...force the input to exactly 24000/1001fps cfr
>>>> fps=fps=48000/1001   ...frame double
>>>>
>>>> However, fps=fps=48000/1001 does more than just frame double. It resets
>>>> TB
>>>> to 20.8541[6..] ms time
>>>> resolution. Look:
>>>>
>>>> [Parsed_showinfo_3 @ 000001413bf0ef00] config in time_base: 1001/48000,
>>>> frame_rate: 48000/1001
>>>> [Parsed_showinfo_3 @ 000001413bf0ef00] n:   0 pts:      0
>>>> [Parsed_showinfo_3 @ 000001413bf0ef00] n:   1 pts:      1
>>>> [Parsed_showinfo_3 @ 000001413bf0ef00] n:   2 pts:      2
>>>> [Parsed_showinfo_3 @ 000001413bf0ef00] n:   3 pts:      3
>>>>
>>>> Gee, I wish the fps filter documention said that it changes TB and sets
>>>> deltaPTS to '1'.
>>>>
>>>> My follow-on frame processing can't tolerate 20.8541[6..] ms time
>>>> resolution -- that explains why my
>>>> mechanical frame gynmastics have been failing!
>>>>
>>>> Explanation: My follow-on processing does fractional frame adjustment
>>>> that
>>>> requires at least
>>>> 8.341[6..] ms resolution.
>>>>
>>>> Workaround: I can frame double by another method that's somewhat ugly
>>>> but
>>>> that I know works and
>>>> doesn't trash time resolution.
>>>
>>> Did you try changing the order? ie. -vf fps first ?
>>
>> Before the 'settb=1/720000,setpts=N*30030'? That wouldn't be appropriate
>> because I need to guarantee
>> that the input is forced to 24000/1001fps cfr, first. Only then will
>> fps=fps=48000/1001 actually
>> double each frame without dropping any -- without such assurance, if any
>> particular frame happens to
>> have a PTS that's 'faster' than 24000/1001fps, then the shift to
>> 48000/1001fps would drop it because
>> the fps filter works solely at the frame level.
> 
> <sorry I was using a web client, sometimes message not posted correctly >
> 
> 
> That's what -vf fps=24000/1001 does. It forces 24000/1001 CFR. Use it first
> 
> I'm sure it was mentioned in one of your other threads

Is this another documentation problem?

https://ffmpeg.org/ffmpeg-filters.html#fps-1
"11.88 fps
Convert the video to specified constant frame rate by duplicating or dropping frames as necessary."

I want to duplicate (specifically, double and only double) all frames. And I want to avoid any 
dropping. I guess the key is: What does 'as neccessary' mean?

Like so much of the documentation, it's vague.

That 'said', I've seen fps drop frames that had slightly 'late' PTSs.