[FFmpeg-trac] #1520(undetermined:new): allow mathematical expressions for -t and -ss

FFmpeg trac at avcodec.org
Tue Jul 10 19:20:48 CEST 2012

#1520: allow mathematical expressions for -t and -ss
             Reporter:  dericed      |                    Owner:
                 Type:  enhancement  |                   Status:  new
             Priority:  normal       |                Component:
              Version:  unspecified  |  undetermined
             Keywords:               |               Resolution:
             Blocking:               |               Blocked By:
Analyzed by developer:  0            |  Reproduced by developer:  0

Comment (by Cigaes):

 I see three separate issues here:

 First, the possibility of putting mathematical expressions in the -t
 option. In the example you show, you can just let the shell do the work:
 {{{./ffmpeg -t $(printf %.6f $[1001/30000.]) -f lavfi -i aevalsrc=0 -c:a
 pcm_s16le out.wav}}}
 works just fine (and we could easily do away with the printf for the
 rounding). Of course, it does not work if you want to use in the formula
 values that come from the input file. That is the reason for the formulas
 in most filters: you can make them depend on the input video size or

 Second, the accuracy of the time. It is completely unrelated to the way
 the time is parsed. Unless you are dealing with a recording with a sample
 rate beyond the megahertz, six decimal places is plenty enough.

 Third, your problems getting the exact duration you request. This is in
 fact a completely unrelated issue. If you examine your output file, you
 can notice that it has exactly 2048 samples. The reason is that
 {{{aevalsrc}}} always outputs samples 1024 at a time, and ffmpeg does not
 change the grouping to adjust for the -t option. The immediate solution
 for that is to select a frame size that will fit your selected number of
 samples. In this particular case, {{{n=1471}}} and {{{-t 0.033356}}} work
 perfectly. There is room for enhancement here too.

Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/1520#comment:1>
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