[FFmpeg-user] Taking screenshots before and after re-encoding

Martin Cracauer cracauer at cons.org
Mon Feb 15 16:05:11 CET 2016

Carl Eugen Hoyos wrote on Sat, Feb 13, 2016 at 07:11:37PM +0000: 
> Martin Cracauer <cracauer <at> cons.org> writes:
> > Out of curiosity, is there ever a need for a second 
> > filter_complex?
> I don't understand:
> I don't know if there is a need but only one filter_complex 
> can be supplied.

You understood and answered all my questions perfectly.  Thanks again,
you've been a great help.

I think the core bit I was missing is this:

My mental model as a beginner with ffmpeg is that of universal pipes
of "video/audio/subtitles" that can be passed around.  

FFmpeg's mental model is that of pipes of raw format data that can be
connected to decoding and encoding at the outer ends of the entire
program, and all the internal piping is limited to the raw flow in

(below quote left for reference for the next person)


> > Is there anything that ffmpeg can do that you cannot 
> > put into a filter_complex 
> Subtitles!
> (I suspect this is not the answer you were looking for, I 
> fear I don't understand the question though.)
> > so that you then have to have a second one after the
> > non-filter-complex statement?
> I still don't understand;-(
> Let me add the following to my answer to your first email:
> FFmpeg allows you to specify an input (often a file). 
> This input will be decoded by FFmpeg (to a raw video 
> format in case of video), the decoded data can be fed 
> into a filter chain, the output of the filter chain 
> is then sent to an encoder that outputs a different format 
> (that you cannot feed into a filter chain because it is 
> not the required raw format) which is accepted by a 
> muxer.
> FFmpeg does not allow to send your encoded data to a 
> decoder to allow a second filter chain: You need two 
> instances of FFmpeg to do that.
> (this should have been "ffmpeg" all over the paragraph)
> Carl Eugen
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Martin Cracauer <cracauer at cons.org>   http://www.cons.org/cracauer/

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