Freeze a slit and scan it across the frame
Slit-scan photography exposes film through a narrow opening while the film or the subject moves. Each column of the negative records a different moment, so one frame holds a span of time instead of an instant. Photo finish cameras at a racetrack work this way, and so does the Star Gate sequence in 2001: A Space Odyssey.
Applied to a still, there is no time axis to sample, so the scan borrows from space instead. Pick a slit position, and every pixel past it is pulled from that one line rather than from its own location.
Slit position and stretch
The slit is a line: vertical in horizontal mode, horizontal in vertical mode, a circle in radial mode. Nothing before it is touched.
Stretch decides how hard the freeze bites. At 100% every scanned pixel samples the slit line exactly, which gives the clean streaked look. Lower values sample a compressed version of the real image, so detail survives but is squeezed toward the slit. Around 40% the frame reads as a heavy motion smear rather than a scan.
Drift and wave
A frozen line repeated across a frame is a flat streak, which is not what a real slit-scan looks like. Drift offsets each scanned line perpendicular to the scan, growing as the scan travels, so the streak leans. Negative values lean the other way.
Wave adds a sine on top of that offset, and Wave cycles sets how many times it crosses the scanned region. Two or three cycles read as a slow ripple; ten or more break the smear into ribbons.
In radial mode both act on the angle instead of the offset, which produces a spiral rather than a lean.
Example: a portrait against a wall
Put the subject on the left of the frame. Set horizontal mode, slit position 40%, stretch 100%, drift 18%, wave 12%, cycles 3. The face stays intact and the wall behind it stretches into a leaning ribbon of its own colours.
If the smear looks like a flat block, raise drift or wave. If the subject is being eaten, raise the slit position until the slit sits past them.
Radial mode
The slit becomes a circle centred on the image, and the scan runs outward. Everything inside the circle is untouched; everything outside is pulled back toward the circle’s edge and rotated by the drift.
Small slit positions leave a tiny undisturbed disc in the middle and turn the rest into a vortex. Larger values keep more of the picture and give the corners a swirled border.
Sampling and resolution
Every scanned pixel is bilinearly interpolated from the source, and the drift and wave amounts are percentages of the frame rather than pixel counts. The preview runs at a reduced size for speed, but because nothing is measured in absolute pixels the full-resolution download matches what the preview showed.
Pixels sampled from outside the frame clamp to the nearest edge, so the scan never introduces transparency and the export keeps the source format.