[FFmpeg-user] Creating video from every i'th picture

[anders70]

Hello,
I regularly have to create videos from a huge number of pictures 
(high-speed-camera). In order to avoid unnecessary huge video-files, I 
want to be able to downsample the (existing) list of input-pictures 
without deleting, copying or renaming them. These pictures are either 
consecutively numbered files (e.g. pic_000.tif, pic_001.tif, 
pic_002.tif, ..., pic_xxx.tif) or "presampled" numbered files (e.g. 
pic_000.tif, pic_003.tif, pic_006.tif, pic_009.tif, ..., pic_xxx.tif).
Is there a way to tell ffmpeg to only use every i'th picture in a 
specific list or to tell ffmpeg to only use pictures which number is a 
multiple of "i"?
So far, I've been able to tell ffmpeg with which picture to start 
("-start_number ${START}" as an input option) and with which picture to 
end ("-frames $(((${STOP}-${START}+1)/${STEP}))" as an output option).

I hope somebody could help me here.
Greetings, Sten