Barrier grid
The agamograph's closest relative, and the same trick one step further. An agamograph interlaces two images and uses a fold to choose between them. A barrier grid interlaces a whole loop and uses a mask — an opaque sheet cut with narrow slits — so sliding the mask by one slit advances one frame. Two states become motion.
Sold as Ombro-cinéma in the 1920s and rediscovered every couple of decades since, most recently as “scanimation”.
The loop
Using the house loop
Pick several images at once — they are ordered by filename, so name them 01, 02 and so on. Heavy, high-contrast artwork survives the sampling; photographs mostly do not.
- Sheet visible
- 17%
- Loop length
- 6 frames
- Mask period
- 24 px
Only one opening in every 6 is clear, so the piece returns 17% of the light falling on it, whichever geometry you pick. Raise the frame count and the motion smooths while the image goes dim and thin. That trade is the entire design problem, and it is why these were always printed on white stock with heavy, high-contrast artwork rather than photographs.
Two sheets: the picture on paper, the mask on transparency film. Lay the mask over the print and slide it.
Why it works
Output column x carries frame ⌊x / slit⌋ mod K. With the slits sitting over one frame's bands you see every K-th column of that frame and nothing else — a sampled sliver of a picture, which the eye assembles into a whole one by the same reflex that lets it read any interlaced image.
It is the same interlacing the Interlacer performs for a fold. Only the selector differs: a fold chooses by where you stand, a mask chooses by where it sits.