Reverse video

Reversed video is a creative effect used in music videos, product marketing, and social content. A water splash played backward, a jump cut that returns the subject to their starting position, a product assembling itself from components. The effect also has practical uses: reverse motion is used in sports analysis to trace where a ball was before a bounce or where a fault occurred in a machinery failure.

Reversing is distinct from looping: a reversed clip plays the footage backward once. Combining reverse and loop (by merging the original clip with its reversed version) creates a back-and-forth boomerang effect popular on Instagram and TikTok.

The reverse filter must buffer the entire video in memory before it can output a single frame. This is fundamentally different from filters like crop or scale that process one frame at a time. FFmpeg reads all frames into memory, then writes them out in reverse order. This is why the progress bar shows no movement during the initial buffering phase: the code explicitly notes that "the reverse filter buffers the entire video before outputting frames, so there will be no progress events during the buffering phase."

Audio reversal uses the areverse filter, which has the same full-buffer requirement for the audio stream. If audio reversal is disabled, -an strips the audio entirely. The output uses libx264 at the ultrafast preset with +faststart.

The full-buffer requirement means memory use scales with video length and resolution. A 30-second 1080p clip requires roughly 500 MB to 1 GB of WebAssembly memory to buffer all frames. Most browsers allocate 2 to 4 GB for WebAssembly, so clips up to about 30 to 45 seconds at 1080p are usually safe. Longer clips or higher resolutions may exhaust browser memory and produce an out-of-memory error. If this happens, trim the clip first.

The lack of progress during buffering is expected. The browser is not frozen: FFmpeg is reading and buffering frames as fast as it can. Progress events begin appearing once the reversed output starts writing.

Desktop editors reverse video by reading frames from the end of the file backward, which requires the entire file to be in RAM or a fast disk cache. The memory constraint is the same as in the browser. Command-line FFmpeg has the same reverse filter with the same memory behavior. This tool runs it locally with no setup, with an explicit out-of-memory error message and a recommendation to try a shorter clip.