I can’t remember how it started, but my attention was caught recently by the idea of slitscans. The term seems to be used indiscriminately for different but related techniques, so I’ve tried to categorise them for my own understanding.
Firstly, there’s the creation of a sequence of images from a backlit static original. That’s how the stargate sequence for ’2001: A Space Odyssey’ was created, as well as the original Dr Who title sequence. It’s a laborious approach, where, for each frame of the sequence, the camera shutter is held open while the camera is lowered towards the static image. Only a thin line of the original is visible through a slit (hence the name), and the light coming through it falls on different parts of the film as the camera’s position alters. For subsequent frames, the camera is raised again, the film wound on, the original image moved slightly so the next part of it is visible through the slit, and the process is repeated.
Intriguingly, someone has reverse-engineered the stargate sequence to produce some of the static images that must have been used to create the effects.
The next version of the technique creates a single image from a sequence of images. A common use is to capture a series of timelapse images of a scene then take adjacent slices from each one and combine them. The result is an image of a scene where different parts of it represent different times. The teeming void has examples of a street scene and the sky.
A variation on this approach captures a single image from a changing scene by using a box with a moving slit in it in front of the camera. Alternatively, though it’s more restricted, you could move things while scanning them.
Finally there is the creation of a sequence of images from a sequence of images. This seems to be particularly popular because of the weird effects you get from simple movement. It’s a development of the previous technique, where each frame of the output sequence consists of slices of different frames in the starting sequence. You can watch a test video to compare the input and output frames and see what’s happening more clearly. Some video editing programmes provide filters to achieve this, and people have supplied code for use with Processing and Quartz Composer. It can be impressive, but the novelty value of this approach wears off very quickly.
I’ll develop this topic further in some way, but in the meantime, if you’re interested in delving deeper into the subject, there’s an extensive collection of examples assembled by Golan Levin.