[FFmpeg-user] Get exact duration of an mpeg1 file

[Carl Eugen Hoyos]

2018-12-20 14:22 GMT+01:00, Robert Krüger <krueger at lesspain.software>:
> On Tue, Dec 18, 2018 at 11:40 PM Carl Eugen Hoyos <ceffmpeg at gmail.com>
> wrote:
>
>> 2018-12-18 21:49 GMT+01:00, Robert Krüger <krueger at lesspain.software>:
>> > On Fri, Dec 14, 2018 at 5:28 PM Carl Eugen Hoyos <ceffmpeg at gmail.com>
>> wrote:
>> >
>> >> 2018-12-14 17:19 GMT+01:00, Robert Krüger <krueger at lesspain.software>:
>> >> > On Fri, Dec 14, 2018 at 3:03 AM Carl Eugen Hoyos <ceffmpeg at gmail.com>
>> >> wrote:
>> >> >
>> >> >> 2018-12-13 17:10 GMT+01:00, Robert Krüger <krueger at lesspain.software
>> >:
>> >> >>
>> >> >> > is there a way to get the exact duration of a raw mpeg1 file using
>> >> >> > the
>> >> >> > ffmpeg command line? For testing I created a 30 minute test file
>> >> >> > using
>> >> >> > ffmpeg (using -c:v mpeg1video -f mpeg1video), then verified the
>> >> duration
>> >> >> > using VLC and mediainfo and it is indeed exactly 30 minutes long
>> but
>> >> >> > ffmpeg -i shows the file's duration as 00:00:04.98 after
>> >> >> > displaying
>> >> this
>> >> >> > warning:
>> >> >> >
>> >> >> > [mpegvideo @ 0x7fe212806000] Estimating duration from bitrate,
>> >> >> > this
>> >> >> > may be inaccurate
>> >> >> >
>> >> >> > Are there command line parameters to make it compute the exact
>> >> >> > size or is this a design or implementation limitation?
>> >> >>
>> >> >> Just the mpeg specification;-)
>> >> >>
>> >> > Yes, sure, having to count packets/frames is unavoidable for some
>> >> formats.
>> >> > My question is rather whether ffmpeg's (or rather libavformat's)
>> >> > duration
>> >> > computation code can be made to do this internally without these
>> >> > workarounds.
>> >> >
>> >> >> $ ffmpeg -i input -c copy -f null -
>> >> >
>> >> > Thanks for the command line.
>> >> >
>> >> > Just a comparison: This approach gives me the duration information
>> for a
>> >> > 180 minute file in 1.8 seconds while mediainfo does it in 0.25
>> >> > seconds
>> >>
>> >> (Does your input have timestamp discontinuities? You could try to
>> >> concatenate a few parts - or cut away something - from the input file.)
>> >>
>> >
>> > Very unlikely since I generated the sample using an ffmpeg command line
>> > using testsrc.
>>
>> But you should test with a file with timestamp discontinuities to
>> test your claim that it can be done faster than with FFmpeg.
>
> I begin to understand. You are saying that it is possible that Mediainfo is
> more or less "cheating" to obtain its results.

I meant that it is possible to calculate a duration in an mpeg stream
by looking at the first and the last timestamp, MPlayer does it iirc.

Carl Eugen