When we were approached to do a music video for the electronic band Violet Sands, we wanted to create a visual effect that played off the distorted and chopped up feel of their music. After researching multiple distortion techniques, we discovered slit scanning. We were immediately mesmerized by the twisting and warping nature of the effect. Particularly captivating was the way it distorted images; bodies, objects, and faces would melt in time and become abstract liquid forms.
In contrast to other distortion effects like pixel moshing, slit scanning remains very organic and that might have to do with its origins. The effect was first used in still photography, and then in film by Douglas Trumbull to create the “Star Gate” sequence in 2001: A Space Odyssey. The original process of slit scanning was incredibly time consuming and painstaking, so we had to figure out the best way to replicate it with digital tools.
 
The best digital technique to generate a slit scan-like effect is time displacement in Adobe AE. The time displacement effect uses a displacement map (or gradient map), and it bases the movement of pixels in the layer on luminance values in the map. Pixels in the layer that correspond to bright areas in the displacement map are replaced by pixels in the same position, but at a specified number of seconds forward in time.
 
Likewise, pixels in the layer that correspond to dark areas in the displacement map are replaced by pixels at a specified number of seconds backward in time. Any layer can potentially be used as a displacement map, though using a grayscale image makes it easier to see brightness levels and predict how pixels will be displaced. You might want to start with this very useful tutorial from FilmmakerIQ.

Effect parameters

Once the displacement map is set to affect a specific layer of footage, there are two parameters that can be controlled within the effect:
 

  1. Max displacement time controls how far ahead or behind you want the pixels to be shifted in time. Suppose you specify 2 seconds as the maximum time displacement. After Effects finds the luminance value of each pixel in the displacement map, then it replaces the corresponding pixels at the current time with pixels from another time, based on the maximum time of two seconds. Pushing this parameter with high value would obviously create more extreme and absurd results.

    slit scan

  2. Resolution controls how many frames per second are going to be used in the animation. Typically, this value shouldn’t be greater than the frame rate of the affected layer.

The displacement map determines the way that After Effects assembles the slits in the slit scan effect. After Effects lays the map on top of the footage and examines the displacement map luminance. All the pixels that are white (or brighter than 50% gray) are displaced forward in time. All the pixels that are black (or darker than 50% gray) are displaced backward.

slit scan

It is very important to create the gradients in a 16-bit color system. The AE project itself also needs to be set as 16 bit colors. This way,  the gradients contain information for thousands of levels of gray, rather than 256 levels of gray provided by the 8-bit colors. The gray resolution significantly affects the smoothness of the final animation.

slit scan

The shape and the complexity of the gradient will determinate the kind of distortion. From the more common results given by a vertical gradient…

Img_03

…to more unpredictable figures, generated by more complex gradient geometries.

slit scan

Img_05

Slit scan effect is not a record of spacial relationship but temporal relationship, with lines recorded at different times composing the image. Thus, your footage needs to be shot at high frame rate because once you start applying the slit scan effect, you need as much temporal resolution as possible.
 

Working with footage that has been shot in 24 fps is going to cause a lot of bending in the affected composition, as there aren’t a lot of in-between frames and information for the effect to work with. If you work with footage with a higher frame rate, there’s going to be much more information for the effect to process, and therefore a much smoother result. For instance, the majority of our video was shot in 120fps.

slit scan

Motion:

There are infinite possibilities for results, generated by the infinite kinds of movements and actions the effect is applied to. For instance, the twisting look—one of the most popular—is generated by applying the effect to a spinning object. Keep in mind that for a smooth result, it’s important to work with footage where the actions don’t happen at a fast pace. The slower the action, the more in-between material for AE to process. So in a case where you are filming a very fast action, make sure you are compensating for it with a very slow fps.

Img_07

Horizontal motion is often used to create exaggerated and over-stretched figures.

slit scan

More random motions create more abstract and unpredictable results.

Img_09

Background and Camera Movement:

It is important to consider that everything that contains temporal information in the footage is going to be affected by the time displacement. Applying the effect to footage that contains camera movements or dynamic and complex backgrounds can generate some very random and unpredictable results. Therefore, you may want to stick with no camera movement and static (or neutral) backgrounds, which will make it easier to control the effect and predict how the image will be distorted.

Render issues:

Keep in mind that time displacement is a very heavy effect to process, especially when applied to high resolution footage. Here are some suggestions to make the process smoother and reduce render time:

  • Work on 8-bit colors and set it to 16 just before rendering. 
  • Work with proxies. Never set the maximum displacement time higher than necessary.
  • Increasing time resolution can greatly increase rendering time.
  • Use multiprocessing renders for the more time consuming sequences, when possible.
 Have you tried slit scanning in AE or otherwise? Any tips to add? Let us know in the comments.
Stanley Kubrickslit scan2001vfxMusic Video
Editing & Post-Production