[FFmpeg-user] Using expressions in "FADE" filter

[Shlomo Morosow]

Thanks Moritz,
The reason I need to use mathematical values is because I'd like to make a
preset command to fade out the last second of any given video and since the
fade filter needs to be instructed on when to begin in relation to the
start of the video I need to be able to tell it to start at duration minus
one second.

Regarding the documentation on using expressions in filters, this can be
found in the the text-expansion section of the ffmpeg filters documentation:

https://www.ffmpeg.org/ffmpeg-utils.html#Expression-Evaluation


To which can be applied the the expressions-evaluation found in the ffmpeg
expression-evaluation documentation:



On Thu, Apr 30, 2015 at 3:50 AM, Moritz Barsnick <barsnick at gmx.net> wrote:

> On Wed, Apr 29, 2015 at 17:57:43 -0400, Shlomo Morosow wrote:
> > I need the ability to input mathematical values for setting the start
> time
> > (st)  on the "fade" filter, I've attempted to use an expression and this
> > particular filter does not seem to work with expressions, nor simple math
> > as "10*1".
>
> As far as I can tell, you are right in saying that this filter (or at
> least the "st" argument) does not support expressions.
>
> For "st", it also doesn't make much sense. Expressions are used not for
> convenient calculation, but rather for expressing something which
> changes over the course of a media stream, i.e. depending on frame
> count, timestamp or the likes. "st" can only ever express a constant
> value.
>
> If you need to do math, you can let your interpreter (on Unix: the
> shell) do the math for you. I'm not sure how to do this on Windows (and
> usually avoid it, even professionally). ;-)
>
> > ffmpeg.exe -y -ss "0:00:20" -t "0:00:10" -i input.avi -filter_complex
> > "fade=out:st=%{expr\\\: (10-1)}:d=1" -f dvd -target ntsc-dvd -r 29.97
> > -aspect 4:3 -b:v 4500k -mbd rd -trellis 1 -flags +mv0 -cmp 0 -subcmp 2
> -b:a
> > 128k output.vob
>
> Where is it documented that an expression is expressed as such?
>
> Moritz
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