Yusuf's Thoughts

Slit-Scan Time Warp

5 min read

The slit-scan effect makes your webcam into a time-smear machine. Each horizontal position on screen samples the camera from a different moment in the past - so when you move, you leave an ethereal trail across the frame. Made famous by 2001: A Space Odyssey and widely used in contemporary music video aesthetics.

Operator Families in this Recipe

  • TOPs (Texture Operators): Buffering, time-sampling, and assembling frames.

How It Works

The Texture 3D TOP stores a rolling buffer of recent video frames as a 3D texture (width x height x time). The Time Machine TOP samples from this buffer using a grayscale “time map” - wherever the map is black, it shows the oldest frame; wherever it is white, it shows the most recent. By using a horizontal gradient as the time map, each column of the screen shows a different moment in time.

Part 1: Camera Input

  1. Add a Video Device In TOP and set Device to your webcam.
  2. Add a Blur TOP (Filter Size: 1) to reduce camera noise.
  3. Connect to a Null TOP named VID_IN.

Part 2: Build the Frame Buffer

  1. Add a Texture 3D TOP and connect VID_IN to its input.
    • **Cache Size** → 60 (1 second at 60fps). Oldest frame is at depth 0, newest at 59.

Part 3: Create the Time Map

The time map tells the Time Machine TOP which moment in time to show at each pixel position.

  1. Add a Ramp TOP (separate from the camera chain).
    • Type → Horizontal.
    • Color 0 → 0, 0, 0 (black = oldest frame on the left).
    • Color 1 → 1, 1, 1 (white = current frame on the right).
    • Match its resolution to VID_IN (e.g., 1280 x 720).

Flip the Time Direction

To reverse the effect (current frame on the left, oldest on the right), set Color 0 to white and Color 1 to black. Moving your hand will leave a trail that “follows” rather than “leads.”

Part 4: The Time Machine

  1. Add a Time Machine TOP.
    • Connect the Texture 3D TOP to input 0.
    • Connect the Ramp TOP to input 1 (the time map).
  2. The output is now a slit-scan: the left side of the image shows 60 frames ago, the right side shows the current frame. Moving objects smear across the temporal gradient.

Part 5: Polish

  1. Add an HSV Adjust TOP and animate Hue Offset → absTime.seconds * 3 for a slow color drift.
  2. Add a Level TOP and boost Contrast to 1.3 to make the time smear more dramatic.
  3. Finish with a Null TOP named OUT.

Part 6: Custom Time Maps (Variations)

The time map can be any grayscale texture - not just a linear ramp. Try these alternatives for different effects:

Time MapEffect
Horizontal Ramp (default)Classic left/right temporal smear
Noise TOP (greyscale)Organic, turbulent time distortion
Circle TOP (white center)Objects trail outward from the center
Video Device In TOP (camera as map)Your own movement controls the time warp
Vertical RampTop/bottom temporal smear

Camera-Driven Time Map

Connect a second Video Device In TOP (or a processed version of the camera with high contrast) directly to the Time Machine input 1. Your own brightness and movement now control which frame each pixel samples - where you are bright, you see the current moment; where you are dark, you see the past.

Troubleshooting

  • “Output is static / not changing.” - Check that the Texture 3D TOP has Depth set to more than 1, and that the Ramp TOP is connected to Time Machine input 1 (not input 0).
  • “Effect is too subtle.” - Increase the Depth on the Texture 3D TOP (try 120 or 240) for a longer time trail.
  • “Image looks stretched or wrong resolution.” - Ensure all TOPs (Video Device In, Ramp, Texture 3D) are set to the same resolution.
  • “Black on one side, frozen image on the other.” - The Texture 3D TOP needs a few seconds to fill its buffer after loading. Wait a moment, then the oldest frame will be further back in time.

Next Steps

  • Audio-reactive warp: Use an Analyze CHOP to animate the Ramp TOP’s gradient speed so the time trail stretches on loud beats.
  • Feedback combo: Feed the Time Machine output into a Feedback loop with slight decay for recursive time echo trails.
  • Multiple zones: Use a custom masked time map to apply the effect only to specific regions of the frame (face, hands, etc.).

Parameter Tuning & Behavior

ParameterBehavior
Texture 3D DepthHigher = longer time trail (more past visible); Lower = subtle, quick echo.
Ramp Gradient DirectionHorizontal = left/right smear; Vertical = top/bottom; Custom = sculptural distortion.
Noise as Time Map (Amplitude)Higher noise = more chaotic, fractured time; Lower = gentle ripple.
Hue Offset SpeedHigher = rapid psychedelic color cycling; Lower = slow cinematic mood.

Network Architecture

[ WEBCAM ]
Video Device In ──▶ Blur ──▶ VID_IN
                                │
                                ▼
                         Texture 3D TOP (Depth: 60)
                                │ (input 0)
                                ▼
[ TIME MAP ]           Time Machine TOP ──▶ HSV Adjust ──▶ Level ──▶ OUT
Ramp TOP ──────────────────────┘ (input 1)
(black = oldest, white = newest)

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