The Weird World of Slit-Scan Photography
The Slit-Scan Movie Maker lets you simulate the slit-scan photography technique with a movie file. The app opens the movie file, then combines one row or column of pixels from a series of frames into a single new frame. (Each row or column of pixels simulates the effect of a slit passing over that part of the image.)
Each slit-scanned frame captures a period of time moving in the direction the slit travels. For example, if the slit travels from top to bottom, portions of the image on the bottom are exposed later than portions of the image on the top.
In standard photography, you open a shutter for some period of time and then close it. The entire image gets exposed in one pop. Anything that's stationary looks normal. Moving objects may be blurred depending on the length of the exposure.
In slit-scan photography (aka strip photography), rather than opening and closing a shutter, you drag a narrow slit across the film from one side to the other over some period of time. Stationary objects look normal, but moving objects become distorted because they are in different places as the slit passes over different parts of the film.
In the past, slit photography could only be accomplished by building an elaborate camera rig. In recent years, though, the same effect has been accomplished by affixing a lens assembly to a flatbed scanner, and then scanning the image being projected on to the bed. (This process is referred to as "scanner photography.")
Moving objects in a slit image are distorted based on two things: the direction they are moving in, and the direction the slit is moving in. If the object is moving in the opposite direction as the slit, the object is compressed. If both are moving in the same direction, the object is stretched.
This relatively simple algorithm can produce some startling results. For example, consider the following pictures, which were all generated from the same movie of an approaching bus. Note the differences that result from the various different scan directions.
When you start to animate the image, things get even weirder. For example, the movie on the left was generated from the same footage using an early version of the Slit-Scan Movie Maker.
In addition to being distorted, moving objects can in some cases be reversed by the process. For example, the following two images were created from the same movie, but one was scanned left-to-right and the other was scanned right-to-left. (In the movie, the cars are traveling left-to-right.)
Again, animation makes things even more interesting. In this movie, cars are passing through an intersection in opposite directions.
Now see what it looks like after processing with the Slit-Scan Movie Maker, with a left-to-right scan.
Note that not only does the process reverse the cars that are traveling from right to left, it also reverses their direction of travel. Here's the same clip scanned right-to-left.
Rotating objects are a lot of fun to work with. Here's a group of bottles rotating on a lazy susan:
Here's the same clip, scanned bottom-to-top:
One of the strangest phenomena I've seen is illustrated by the following clip from a roller derby match. First, the unprocessed footage:
Now, see what happens when this footage is scanned from left to right:
Who are the doppelgangers skating in from the right side? One of the many mysteries you can explore with the Slit-Scan Movie Maker.
