Which frame does ffmpeg get when reducing the frame rate

Question

I have some video that I would like to convert to images of frames every 2 sec.

E.g. If the video is 7 seconds long at 15 fps I would like to get frames 1, 31, 61, 91.

The command:

ffmpeg -i foo.mp4 -r 0.5  -f image2 -vcodec mjpeg foo%d.jpg

appears to do what I want, but which frame does it get? 1, 31, 61, 91 or 30, 60, 90 or 13, 43, 73, 103?

Answer 1

The first image will be from the very first frame.

Note that you very well may get an image or two more that you expect. I believe this is because of rounding and/or that ffmpeg creates a final images. E.g.: Is your video really 7s long? Or is it 7.63s long?

Answer 2

I ended up doing the following largely borrowed from the ffmpeg tutorial:

ffmpeg -v 3 -vsync 0 -sameq -i movie.mpr  -f image2 "images-%03d.jpeg"

This gives me each frame of the movie as a JPEG numbered 1 to the end of the movie. I then filtered these files using a scripting language, knowing the frame rate was 30fps, to grab every 60th frame.