[FFmpeg-user] Use cropdetect only every xth frame

[Gerion Entrup]

Hello,

is it possible to say the cropdetect filter, that it should analyse the picture 
only every 10th frame (or something similar)?

The background is:

I want to find the correct values for the crop and volume filters, the simplest 
way, I know, is:
ffmpeg -i inputfile -filter:v cropdetect -filter:a volumedetect -map 0 -f null 
/dev/null
This command takes a long time.

Another way would be:
ffmpeg -i inputfile -filter:a volumedetect -map 0:a -f null /dev/null
ffmpeg -ss 20 -i inputfile -filter:v cropdetect -t 2 -map 0:v -f null /dev/null
or something like this. The two commands need much less time than the first. My 
Question is: Is there one command that do the things of the two commands in 
one step, e.g. to decode the videotrack only every x seconds for one second 
and analyse with cropdetect or decode only every xth I-Frame and analyse this 
but decode in the same time every audio frame?

Regards,
Gerion