setting the file size of output file

[pietrop]

Hello,
I was wondering if there is a way for setting the file size of output file?

I found that in ffmpeg is supposed to be ffmpeg -i input.avi -fs 10MB output.mp4 (althou this did not seem to work when I tried it).

Best

Pietro

[njoyard]

Did you try in on command line ? Did it work ?

[pietrop]

No doesn't work.

"The -fs option simply stops encoding once it reaches the -fs value. It does not make the whole thing fit to a particular size. To do that you will need to specify an appropriate bitrate for each stream. Using two-passes is recommended when you have a particular file size in mind. See https://trac.ffmpeg.org/wiki/Encode/H.264#twopass"

from http://ffmpeg.gusari.org/viewtopic.php?f=11&t=2141

Apparently first you need to calculate the bitrate for the size you are aiming for.

(50 MB * 8192 [converts MB to kilobits]) / 600 seconds = ~683 kilobits/s total bitrate
683k - 128k (desired audio bitrate) = 555k video bitrate

Then you need to do a two pass encoding

ffmpeg -y -i input.avi -c:v mpeg4 -vtag xvid -b:v 555k -pass 1 -an -f avi /dev/null
ffmpeg -i input.avi -c:v mpeg4 -vtag xvid -b:v 555k -pass 2 -c:a libmp3lame -b:a 128k output.avi

from https://trac.ffmpeg.org/wiki/Encode/MPEG-4

Once I've tested if this actually works, what would be the best way to use it as a node-fluent-ffmpeg function?

[njoyard]

You should do the first pass as usual first and then run the second pass from the 'end' event handler. Read the documentation to learn how to pass options to ffmpeg. You can use the 'outputOptions' method for any option that has no dedicated method.