How to Reverse a Video With FFmpeg (and Prove the Last Frame Became the First)

#ffmpeg #python #video #testing

Reversing a clip in FFmpeg looks like a one-liner until you ask a sharper question: did the last decoded frame actually become the first frame of the output? Stream copy cannot answer that. A lossy H.264 round-trip can look right while failing a byte-level check. This article shows both a PNG-sequence proof that is exact at the ends, and the everyday reverse filter path with a PSNR sanity check.

If you need a brand-new forward clip instead of reversing someone else's footage, generate one on Grok AI Video Generator on Voor.

What we are proving

Source clip used in the tests: forward.mp4, 1280×720, 24 fps, 72 frames, 3.000 s.

Pass criteria:

Geometry preserved after reverse.
sha256(source[0]) == sha256(reversed[-1]) on PNG frames.
sha256(source[-1]) == sha256(reversed[0]) on PNG frames.
Filter-based H.264 reverse stays visually close to the PNG-path reverse (PSNR average ≥ 30; we measured ~47).

Method A — PNG sequence (exact ends)

# 1) explode
ffmpeg -i forward.mp4 -fps_mode passthrough f-%04d.png

# 2) rebuild in reverse order (copy/rename into rev/f-0001.png …)
ffmpeg -framerate 24 -i rev/f-%04d.png \
  -c:v libx264 -pix_fmt yuv420p -r 24 reversed.mp4

In Python the same idea is: extract → list(reversed(pngs)) → rebuild. That is what reverse_via_pngs() in the companion module does.

Method B — filter reverse (delivery one-liner)

ffmpeg -i forward.mp4 -vf reverse -an \
  -c:v libx264 -pix_fmt yuv420p reversed_filter.mp4

Notes:

reverse is a filter, so -c copy is not an option.
Audio is dropped here on purpose (-an). To reverse audio too, add areverse in a filter graph.
Re-encoding changes pixels; do not expect SHA equality against the source.

Minimal helper

def reverse_filter_h264(src, dest, ffmpeg="ffmpeg"):
    subprocess.run(
        [ffmpeg, "-y", "-i", str(src), "-vf", "reverse", "-an",
         "-c:v", "libx264", "-pix_fmt", "yuv420p", str(dest)],
        check=True,
    )

Pytest results (2026-11-06)

....                                                                     [100%]
4 passed in 1.36s

The suite checks geometry, exact PNG end-swap, PSNR between PNG-path and filter-path outputs, and a non-empty delivery file.

When not to reverse

Copyrighted music videos you do not have rights to alter.
Clips where reversed motion implies a false real-world event.
Any case where you actually need a new forward shot — generate one instead of reversing.

Voor alternative

Open Grok AI Video Generator, write a forward brief (or attach a still), and generate a new clip rather than reversing a source you should not republish.

Disclosure

This article was drafted with an autonomous assistant. All FFmpeg commands and pytest checks were run locally on the sample clip before publishing.

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