In mp4, there are multiple types of frames. An I-frame is an entire frame: other types record changes relative to neighbouring frames.

See [here](https://superuser.com/questions/669716/how-to-extract-all-key-frames-from-a-video-clip):
```bash
ffmpeg -i 2.flv -vf "select=eq(pict_type\,I),scale=73x41" -vsync vfr -qscale:v 2 thumbnails-%02d.jpeg
```

Getting time offset of I-frames. See [here](https://stackoverflow.com/questions/27529930/get-video-frames-information-with-ffmpeg)
```bash
ffmpeg -show_frames input.mkv
```