TouchDesigner Tutorial: Slit Scan using the Time Machine TOP

This tutorial demonstrates how to create a slit scan time warping effect using the Time Machine TOP in TouchDesigner. Slit scan is a classic visual effect that creates fluid, warping imagery by sampling different moments in time across different parts of the image.

Understanding the Time Machine TOP

Basic Setup

To get started with the Time Machine TOP:

1. Add a Movie File In TOP and load a video file (the tutorial uses “count.mov”)
2. Add a Texture 3D TOP - this is required as the first input for the Time Machine
3. On the Texture 3D, toggle on the Pre-fill parameter to fill the texture with a fixed set of images

Pre-filling the Texture

The pre-fill technique ensures your sequence always starts at the same image:

• Without pre-fill, clicking pulse starts at a different image each time
• To fix this, change the Movie File In TOP’s Play Mode to “Specify Index”
• Now clicking pre-fill will always start at frame one

How the Time Machine Works

While the Remap TOP allows you to select pixels from UV coordinates, the Time Machine TOP allows you to select pixels from a W coordinate - essentially selecting pixels from a specific time or history in your cache.

The Time Machine requires a second input:

1. Add a Constant TOP and connect it to the Time Machine
2. Adjust the Color parameter value to scrub through time
3. Values from 0 (black) to 1 (white) map to different frames in your cache

Adjusting Cache Size and Black Offset

To access more frames in time:

• Increase the Cache Size on your Texture 3D (e.g., to 100)
• Remember to click Pulse to refill the texture after changing cache size
• Adjust the Black Offset parameter on the Time Machine to define the boundaries of your time offset
• For example, setting Black Offset to -100 allows you to go 100 steps backwards in time

The mapping works as follows:

• White (1) = most recent frame (e.g., frame 100)
• Black (0) = oldest frame in cache (e.g., frame 1)
• Gray values = frames in between

Creating the Slit Scan Effect

Setting Up the Network

1. Add a Movie File In TOP with your video
2. Add a Texture 3D TOP with cache size set to 100
3. Add a Time Machine TOP
4. For the Black Offset, create a reference to the Texture 3D’s cache size by dragging the parameter, then add a negative sign (e.g., -100)

Using a Ramp for Time Slicing

To create the slit scan effect:

1. Add a Ramp TOP as the second input to the Time Machine
2. On the Ramp’s Common page, set the resolution to 9 by 1 (9 horizontal slices)
3. Set Viewer Smoothness to “Nearest Pixel” to see each step clearly
4. On the Time Machine’s Common page, set Input Smoothness to “Nearest Pixel”

This creates visible slices where each vertical strip shows a different moment in time.

Creating Fluid Warping Movement

For a more dramatic slit scan effect with fluid movement:

1. Use a video with people moving - webcam footage works great
2. The tutorial demonstrates using the Web Browser component from the Palette (Tools > Web Browser) to load reference video
3. Match the resolution: Set the Ramp’s X resolution to match the Texture 3D’s cache size by creating a reference
4. Increase the cache size significantly (e.g., 250 or 450) for more slices

Smoothing the Effect

To remove the hard cuts between slices:

• Change Input Smoothness from “Nearest Pixel” back to “Interpolate Pixels”
• This creates a smooth, fluid time-warping effect instead of distinct bands

Key Parameters Summary

Parameter	Location	Purpose
Cache Size	Texture 3D	Number of frames stored in history
Pre-fill	Texture 3D	Fill cache with initial frames
Black Offset	Time Machine	Defines how far back in time you can go
Input Smoothness	Time Machine (Common)	Controls interpolation between time slices
Viewer Smoothness	Ramp (Common)	Controls visual display of ramp steps

Key Takeaways

1. Texture 3D is essential - It stores the frame history that the Time Machine samples from
2. Black Offset should be negative - Set it to the negative of your cache size for full range
3. Use references - Link parameters like cache size to black offset and ramp resolution for consistency
4. Ramp controls time mapping - The gradient determines which parts of the image show which moments in time
5. Input Smoothness - Toggle between “Nearest Pixel” for distinct slices and “Interpolate Pixels” for smooth warping
6. Higher cache sizes - More frames in cache means more dramatic time warping effects
7. Moving subjects work best - Videos with people or objects in motion create the most interesting slit scan results

This technique creates the quintessential slit scan look - that fluid, warping, time-bending visual effect popular in music videos and experimental film.