[FFmpeg-user] How to drop every Nth frame?

L. Lee llee040 at sbcglobal.net
Fri Sep 27 23:35:47 CEST 2013

On 9/25/13 10:33 AM, "Andrey Aleksandrovich"
<andrey.aleksandrovich at googlemail.com> wrote:

>Oh, thanks so much to all of You.
>All is working now.
>On 9/24/13, Stefano Sabatini <stefasab at gmail.com> wrote:
>> On date Monday 2013-09-23 13:36:25 +0300, Andrey Aleksandrovich wrote:
>>> I also try:
>>> -filter:v select=mod'(n,372)',crop=in_w-20::11:,scale=320:240
>>> but it's the same
>>> [Parsed_select_0 @ 0x543d35fc90] [Eval @ 0x3bd5b30b370] Missing ')' or
>>> too many args in 'mod(n'
>>> [Parsed_select_0 @ 0x543d35fc90] Error while parsing expression 'mod(n'
>>> [AVFilterGraph @ 0x543d2dfed0] Error initializing filter 'select' with
>>> args 'mod(n'
>> Please read and understand this:
>> http://ffmpeg.org/ffmpeg-filters.html#Notes-on-filtergraph-escaping
>> In this case you have to choose:
>> -filter:v "select='mod(n,372)',..."
>> or
>> -filter:v "select=mod(n\,372),..."
>> Also note the embedding double quotes required by the shell.
>> Also your crop syntax doesn't make sense (you're setting empty
>> expressions, please read crop documentation if you have doubts).
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This interested me because I am trying inverse telecine on 30 fps source
(I normally work on 60 fps telecined 1080i sources). This was Carl Eugen's
first reply to the discussion:

"Both the select and the fps filter allow this.
If this is actually an inverse telecine process (for pal
input), consider using pullup."

It lead me to try both select and fps. I had already used the following
combination along with "-tune film" to get 24 fps in which every 5th frame
duplicated every 4th frame, so frame rate was incorrect:


The problem appeared to have been corrected by using:


Why doesn't "fieldmatch=order=tff:combmatch=none,select='mod(n\,5)'" also
correct the problem?

Another unsuccessful attempt (which normally works well with my typical
1080i sources) consists of "fieldmatch=order=tff:combmatch=none,decimate".


Laine Lee

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