[FFmpeg-user] Replacing Specific Frames with Specific Other Frames

[Sam Logan]

I have a video file "Foobar.mp4" (stream 0 is a video stream, stream 1
is an audio stream).

My goal is to replace video frames 56-98 (a total of exactly 43
frames) of the video with a back-and-forth loop of frames 46-55 (a
total of 10 frames). In other words, I want to replace frames 56, 57,
58, ..., 98 in the video with this exact sequence of 43 frames:

55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 47, 48, 49, 50, 51, 52, 53,
54, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 47, 48, 49, 50, 51, 52,
53, 54, 55, 54, 53, 52, 51, 50, 49

(If the pattern is not clear, it first goes down to frame 46, then
goes up to frame 55, and then repeats until we get to 43 frames.)

Does anyone know how to write a filtergraph command that will
accomplish this? Below, I detail my failed attempt.

****

Anyway, my failed attempt: I tried using the overlay filter as
follows. First, I extracted frames to PNGs:

ffmpeg -t 00:00:10 -i "Foobar.mp4" "Frame%04d.png"

Then I manually copied/renamed the PNGs for frames 46-55 so that they
were in the order I gave above, giving them new filenames:

Frame0001.png = frame 55
Frame0002.png = frame 54
...
Frame0043.png = frame 49

and then deleted all other PNGs except for these 43 frames. Then I did:

ffmpeg -i "Foobar.mp4" -i "Frame%04d.png" -filter_complex
"[0:v][1:v]overlay=enable='between(n,56,98)'[out]" -acodec copy -map
"[out]" -map 0:1 "FoobarEdited.mp4"

But this did not work: frames 56-98 of "FoobarEdited.mp4" were indeed
replaced, but all 43 frames were replaced with 43 copies of
"Frame0043.png" instead of with what I was expecting: "Frame0001.png",
"Frame0002.png", "Frame0003.png", etc.

Does again, anyone know how to write a filtergraph command that will
accomplish my goal? Thanks.