How to reverse a video so it plays backwards

To reverse a video, every frame of the section has to be decoded first, then written out again in the opposite order. That is why reversing takes longer than playing, and why tools limit how much they reverse at once. The reverse video tool does it on your device for clips of up to 20 seconds at a time; here is how it works and how to get a good result.

Why reversing is harder than it looks

Compressed video is not a stack of independent pictures. Codecs such as H.264 store a complete keyframe every so often and, in between, frames that describe only changes from earlier or later frames. To show any given frame, a player must start at the previous keyframe and decode forward. Playing forwards that work is shared from frame to frame; playing backwards it is not, because each step back may mean decoding a run of frames again. The practical answer is to decode the whole section once, keep every frame, and play the stored frames in reverse. How video compression works: bitrate, keyframes and codecs explains keyframes in more detail.

How the reverse tool works

• Reading frames. The tool seeks the video to each frame time in turn, for example every 1/24 of a second, waits for the browser to finish decoding, and copies the picture to a canvas.
• Storing them. Each frame is saved as a high-quality JPEG rather than raw pixels. Raw 720p frames take nearly 3.7 MB each, so ten seconds at 24 fps would need almost 900 MB of memory; JPEG keeps that manageable.
• Recording. The frames are drawn onto a canvas in reverse order at the chosen frame rate, and the canvas is recorded with the browser's MediaRecorder at about 5 Mbps.

Reading is as fast as your device can seek; recording runs in real time, so a ten-second result takes ten seconds. Keep the tab visible during recording, because browsers slow hidden tabs. The output is MP4 where the browser can record it (Safari and recent Chrome and Edge) and WebM otherwise (Firefox); WebM to MP4 converts if you need MP4.

Section, frame rate and size

Choose a start and end of up to 20 seconds. The cap exists because every frame is held in memory. The frame rate can be 15, 24 or 30 fps: 30 is smoothest, 15 processes twice as fast and suits slow movement. Size can be capped at 480, 720 or 1,080 pixels on the longer side; 720 is the default and enough for social posts. If your source is long, cut the moment out with the video trimmer first. A tick box plays the clip forwards and then backwards in one file, without repeating the turning frame, which is the basis of a boomerang; How to make a boomerang video from any clip covers that style.

What happens to the sound

The reversed video is silent. Reversed speech and music are rarely what anyone wants, and the browser recording approach captures the picture only. If you do want a backwards soundtrack for effect, extract the audio with video to audio, reverse it with the audio reverse tool and combine them in an editor. To remove the sound from a video without reversing it, the video muter does that.

Shots that look good backwards

Reverse motion works best when the backwards version breaks a physical rule people recognise instantly:

• pouring a drink, so liquid leaps back into the bottle;
• jumping into water, so the swimmer flies out feet first;
• smoke, steam, falling leaves or thrown confetti;
• knocking over a stack of blocks, which rebuilds itself;
• someone walking, which instantly looks uncanny.

Use a steady camera, ideally a tripod, so the only motion is the subject's. Avoid visible text and people talking, both of which give the trick away. Shoot a few seconds more than you need on either side of the action, so you can choose the cleanest start and end points when you set the section to reverse. Good light reduces motion blur, which looks odd in reverse. To slow the reversed clip down or speed it up, run it through the video speed tool afterwards. Nothing you load is uploaded.