Datamosh Effect
Datamoshing online, for a video or even a still photo: old pixels smear along blocky motion until an I-frame clears them. No codec tricks or file hacking.
What is datamoshing?
Datamoshing is a glitch that comes out of video compression. A codec only stores a complete frame every so often, called an I-frame or keyframe. The frames in between are described as blocks of the previous picture moved along motion vectors.
Take the I-frames out and the decoder keeps shoving old pixels around with new motion, so one shot melts into the next. Takeshi Murata and Sven König were among the artists who pioneered it, and Nabil Elderkin used it in the video for Kanye West's Welcome to Heartbreak.
How do you datamosh a video online?
Load a clip or a photo and the page starts on the Bleed preset. Flow is how far pixels drag each frame, and Block size is the size of the macroblocks that move together. Persistence decides how much of the smeared past survives from one frame to the next.
Bleed-through is a threshold: changes in the picture below it stay hidden under the smear. I-frame every sets the seconds between clean frames, and 0 means it never clears. Vector changes controls how often the motion picks a new direction.
Melt pours slowly and never resets. Pulse clears once a second, Big blocks moves coarse macroblocks, and Rush drags hard and changes direction a lot.
Real datamosh or a simulation: which do you need?
A real datamosh deletes I-frames from the file, so the decoder pushes old pixels along your clip's own motion. When a dancer moves, the smear goes with her. This tool draws the smear itself. On each frame it drags its own previous output along a grid of motion vectors chosen by the Seed, and only strong changes in the new frame break through. So the smear moves on its own, no matter what the footage is doing.
Persistence sets how long a trail hangs around. At 0.9 a dragged pixel keeps 90 percent each frame and is mostly gone after 20 to 30 frames; at 0.97 it lasts about three times longer, and at 0.99 ten times. Wherever a strong change breaks through, that hold gets cut in half.
If the smear has to follow the movement in your footage, go with the file method. For the melt on anything at all, still photos included, and the same result on every export, use this one. Signal Loss Effect splits a feed into dropped bands with no smear, and Glitch Effect knocks blocks sideways for a few frames at a time.
Settings for a melting cut, a beat pulse and a liquid still photo
Melting one shot into the next: I-frame every 0, Persistence 0.95, Bleed-through 0.8 and Vector changes 2. With Bleed-through up that high, even a hard cut registers as a small change. The new shot stays buried under the old pixels and only comes up slowly as they drag.
Pulsing on the beat: I-frame every 0.5 clears on each beat at 120 BPM, and 1 does it at 60 BPM. The clean frames are counted from the start of the clip, so that's where the first beat needs to be.
Liquid still photo: load a picture, choose Melt (Persistence 0.97, Flow 3, no I-frames) and record at least ten seconds. The smear builds up the longer it runs.
Why didn't my cut melt into the next shot?
Start with the clean frames. They fall at fixed times from the start of the clip, at 3, 6 and 9 seconds with the default setting, and a cut that lands on one of them starts over clean. Set I-frame every to 0, or move the cut.
Next, look at Bleed-through. A low value lets the new shot show through anywhere it differs a lot from the smear, and a cut differs a lot almost everywhere. Raise it to 0.7 or more. Also, with a low Persistence the past is wiped within a few frames whatever else you do.
Why does a datamosh vanish or stutter after export?
With the file method, it happens a lot. We read 21 forum and issue threads about datamoshing, and 9 of them described a mosh that never showed up, played back choppy or disappeared after the file was exported and reopened. The answers blame the encoder, which keeps writing fresh I-frames of its own. One poster found them every 10 or so frames, and Avidemux 2.7 renamed the setting that controls them to Gop Size.
That can't happen here. The smear is drawn into each frame before encoding, so the MP4 stores it like any other picture and the file plays and edits normally. The Keyframe option in the export (every 1, 2, 5 or 10 seconds) only affects how cleanly the file seeks. Clean frames in the picture itself come from I-frame every and nothing else.
Moshed footage has moving detail all over, and that's hard to compress, so choose High quality to stop fine smears from going blocky. In a GIF, long smooth smears band into steps of color, so leave its Dither on.