Here’s how to do it step by step:

1. Extract the frames of the GIF:

   ffmpeg -i input.gif frame_%04d.png
   

   This command will extract all frames and save them as separate PNG files (frame_0001.png, frame_0002.png, etc.).

2. Shift the frames:
   Let's say your GIF has 20 frames, and you want to start at frame 10. You would need to rename the frames so that the 10th frame becomes the first, followed by the rest of the frames, and then looping the first part back.

   You can use a simple script (e.g., in Python) to rename the files, or manually if it's a small set of frames. For example:

   • Rename frame_0010.png to frame_0001.png,
   • frame_0011.png to frame_0002.png,
   • and so on until you loop back to the earlier frames (frame_0001.png becomes the last).