What it does
Each pixel shows a different moment in time — a gradient or control map melts the image across recent frames for slit-scan, rolling-shutter, and time-displacement looks.
The original instrument is disarmingly simple: a camera exposing film through a narrow slit while the film moves past it. One axis of the resulting photograph is space, the other is time. Photo finishes at racetracks are slit-scans, which is why every horse in them crosses the same line: the image records when things arrived, not where they stood. Douglas Trumbull industrialized the idea for 2001, dragging huge backlit artwork past a slit during long single-frame exposures, and the streaking corridors of the stargate are what motion looks like when a photograph is made of time.
The digital version generalizes the slit into a map. Keep the last several seconds of frames in memory, then let a field decide, pixel by pixel, how far back to read: a vertical ramp reproduces the classic rolling scan, concentric rings ripple outward as rings of time, a sine wave slices the frame into alternating past and present bands. The map can even be the image itself, its own brightness deciding the delay so highlights and shadows live at different moments, which is the trick behind the melting, liquid body distortions the technique is loved for.
It is an effect that demands motion. On a still frame every moment is identical and the effect vanishes; feed it a walking figure, drifting smoke, or a handheld camera and the temporal differences become geometry. The quality of the melt comes from how finely time is sliced, more stored frames giving a smoother gradient of past, and the reach of the melt is set in seconds of history. The map animates too, so the boundary between now and then can sweep, pulse, or orbit through the frame.
Signature looks
Photo finish
A hard linear ramp across the frame. Moving subjects shear into ribbons while the background holds, the racetrack camera pointed at everyday life.
Time ripple
Concentric rings of delay spreading from a point, the present at the center aging toward the edges. Motion arrives in waves.
Liquid melt
The image’s own brightness driving the delay, so a moving figure drips and smears through itself like wet paint.
Try it on your own footage
This effect runs live in the Modulate editor, with layers, effect chains, masks, and keyframes. Free, no signup, nothing to install.
Common questions
What is slit-scan photography?
A technique where the camera records the scene through a narrow slit over time, either by moving film past the slit or moving the slit across the scene. One axis of the image becomes a time axis, so stationary things render normally while moving things stretch and shear. Racetrack photo-finish cameras, some scientific instruments, and a long tradition of experimental film all use it.
How was the stargate sequence in 2001 made?
Douglas Trumbull built a slit-scan rig: a camera on a track shooting long exposures through a slit while large backlit artwork moved behind it. Each pass produced one frame of smeared light, and the machine ground through the sequence frame by frame. The technique defined the psychedelic corridor and was reused for years afterward, including the warp-drive stretch in Star Trek.
What is time displacement?
The digital generalization of slit-scan. Instead of a physical slit, a stored history of recent frames is sampled per pixel: a grayscale map decides how far into the past each pixel reads, brighter meaning one end of the window and darker the other. A simple gradient map recreates classic slit-scan, and any other image, noise, or the footage itself can drive the delay instead.
Why does slit-scan need moving video?
Because the effect displaces pixels in time, not space. If nothing changes between frames, reading the past gives exactly the same picture and the effect is invisible. The melt appears where motion creates differences between moments, so it thrives on gesture, traffic, water, camera drift, anything alive, and it needs the footage to play forward so a history can accumulate.
Can you do slit-scan in the browser?
Yes. Modulate keeps a rolling buffer of recent frames on the GPU and runs the time displacement live, with gradient shapes you can drag around the frame or any layer acting as the delay map. It plays in real time on video in the browser, no plugin or render queue involved.
Related effects
Datamoshing
Smears moving parts of the image into blocky trails, like a corrupted video that never gets a clean frame.
Echo effect
Leaves a decaying trail of recent frames behind moving content.
Wave effect
Bends the image with moving distortion: swirling turbulence, rolling directional waves, or rings spreading from a point.
Pixel sorting
Sorts runs of pixels into glitchy streaks by brightness, hue, or saturation. The classic pixel sorting look.