Remove duplicate frames with ffmpeg, and keep the result playable.

Why the frames are there

Recorders such as OBS write a constant frame rate. When the game has not finished drawing a new frame by the next tick, the recorder repeats the last one. The dropped-frames counter stays at zero because every slot got a frame. The file looks like A B C C C D E E F instead of A B C D E F G H I. The longer explanation covers how to confirm that this is what your file has.

What mpdecimate does

mpdecimate compares each frame with the previous kept frame, block by block. If no 8×8 block differs by more than hi, and the number of blocks differing by more than lo is below the fraction frac, the frame is considered a duplicate and dropped. Defaults are hi=768, lo=320, frac=0.33, tuned for telecined film. Games are different: a late-game BTD6 frame has thousands of moving sprites, so when the game really did move, almost every block changes, and when it stalled, almost none do. The defaults usually work; the knobs below are for when they don't.

The full workflow

1. Count first. ffmpeg -i in.mp4 -vf mpdecimate -f null - prints how many frames survive. Compare that to the total frame count (ffprobe -count_frames -select_streams v -show_entries stream=nb_read_frames in.mp4). If 70% of frames are dropped, you have a lag recording; if 2% are, the stutter is something else.
2. Drop and re-time. Dropping frames alone leaves gaps in the timestamps, and most players show that as a stutter. setpts=N/FRAME_RATE/TB re-numbers the kept frames evenly. -fps_mode cfr -r 60 (older ffmpeg: -vsync cfr) writes a constant frame rate container. Result: the game appears to run smoothly at the pace of the frames that were actually drawn.
3. Choose a pace. The command above plays kept frames at 60 per second, which is a fast-forward if the game was really drawing 20. For real-time pacing instead, keep the original timestamps: -vf mpdecimate -fps_mode vfr and skip the setpts. For a specific speed-up, use setpts=PTS/3 after the decimate instead of the N-based form.
4. Deal with audio. Once frames are removed the video is shorter than the audio, so the two cannot stay in sync. Use -an and lay music or commentary back in your editor. If you need the original audio under a real-time (vfr) cut, it will drift at every removed stall; there is no clean fix for that.
5. Check the output. Step through frame by frame in a player and watch the game clock. If it never freezes, the pass worked.

Tuning for games

That last row is the limit of the approach. Duplicates are a repetition problem and mpdecimate solves it; pause menus and loading screens are a content problem, and need either a manual cut list or a classifier trained on the game. Cutting dead time from gameplay video covers both.

If you'd rather not run ffmpeg

LagCut does the duplicate pass in the browser with a per-game frame model (it compares only the map viewport in They Are Billions, so tooltips and webcams don't count), detects BTD6 pause menus, shows you the cut list on a timeline to accept or reject, and exports a constant-frame-rate MP4 at 1× or 3×. Nothing is uploaded. It processes the first 30 seconds of a clip, so trim first; for a two-hour VOD, ffmpeg is still the right tool.

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