Find another method for efficiently sampling frames from video?

[brimoor]

eta.core.video.sample_select_frames(..., fast=True) uses an ffmpeg command of the form

ffmpeg -i video.mp4 -vf select='eq(n\,5)+eq(n\,15)+eq(n\,25)' -vsync 0 frames-%03d.png

to sample specific frames from the video. Unfortunately, for long videos with lots of frames to sample, we get a bash(?) error indicating that the ffmpeg command is too long...

Traceback (most recent call last):
  File "/engine/theta/eta/eta/modules/sample_videos.py", line 177, in <module>
    run(*sys.argv[1:])
  File "/engine/theta/eta/eta/modules/sample_videos.py", line 173, in run
    _sample_videos(config)
  File "/engine/theta/eta/eta/modules/sample_videos.py", line 107, in _sample_videos
    _process_video(data.input_path, data.output_frames_dir, parameters)
  File "/engine/theta/eta/eta/modules/sample_videos.py", line 161, in _process_video
    input_path, frames, output_patt=output_patt, size=size, fast=True)
  File "/engine/theta/eta/eta/core/video.py", line 1872, in sample_select_frames
    ffmpeg.run(video_path, tmp_patt)
  File "/engine/theta/eta/eta/core/video.py", line 3020, in run
    self._p = Popen(self._args, stdin=PIPE, stdout=PIPE, stderr=PIPE)
  File "/usr/lib/python3.6/subprocess.py", line 729, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1364, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
OSError: [Errno 7] Argument list too long: 'ffmpeg'

One can always use fast=False in this method, but one needs a constant-command-sized method to efficiently sample frames and naming them according to their frame number in the original video.