Yusuf's Thoughts

Viral: Smoky Optical-Flow Particles

3 min read

Ink-in-water smoke that swirls along the motion in a video, hypnotic, endlessly looped, very save-able. It works by reading optical flow (which direction each pixel is moving) from footage or a webcam, then pushing particles along that flow. Move, and smoke trails follow you.

Watch the trend

Technique base: Reaction-Diffusion Living Canvas (feedback) and Motion History & Optical Flow.

1. Get the Flow Field

  1. Add a Video Device In TOP (webcam) or a Movie File In TOP.
  2. Add an Optical Flow TOP. Its output encodes motion direction and speed as colour (a velocity field): red/green channels are X/Y movement.
  3. Smooth it with a small Blur so the field is not noisy.

What optical flow gives you

Every pixel gets a little arrow saying “this is moving up-left, fast.” That arrow field is exactly what you need to steer particles or smoke.

2. Drive the Smoke (two ways)

A) Feedback smoke (no particles, pure TOPs):

  1. Build a Feedback TOP loop holding a “density” image.
  2. Each frame, displace the fed-back density by the optical-flow velocity (a GLSL or Displace TOP), then fade slightly.
  3. Inject density where there is motion. The result smears and curls like smoke.

B) GPU particles:

  1. Use a ParticlesGPU / POP system.
  2. Sample the flow field at each particle’s position and add it to the particle velocity.
  3. Particles stream along the motion, leaving trails.

Start with A: it is fewer nodes and reads as smoke immediately.

3. Make It Beautiful

  1. Run the density through a Lookup TOP gradient (smoke white, or ink blue to violet).
  2. Add bloom and a gentle Blur so it glows and softens.
  3. Slightly zoom the feedback each frame for a drifting, rising-smoke feel.

4. Loop and Post

  1. For a seamless loop, drive motion from a looping clip rather than a live cam.
  2. Output 1080x1920, record 10 to 20 seconds.
  3. Calm, slow footage gives the most elegant smoke; fast motion gives turbulent bursts.

Troubleshooting

  • “The flow is just noise.” - Blur the input and the flow output; raise the Optical Flow smoothing. Good lighting reduces false motion.
  • “Smoke builds up and whites out.” - Increase the per-frame fade (multiply density by ~0.96) so it dissipates.
  • “It looks flat.” - Add the feedback zoom and a colour gradient; flat grey density never looks like smoke.

Parameter Tuning & Behavior

ParameterBehavior
Flow strengthHigher = violent swirls; Lower = lazy drifting smoke.
Density fadeCloser to 1 = thick lingering smoke; Lower = wispy, quick-clearing.
Feedback zoomHigher = fast rising/expanding; Lower = settled, hovering.
Input blurHigher = smooth elegant flow; Lower = detailed but jittery.

Network Architecture

[ SOURCE ]    [ Video Device In / Movie File In TOP ]
                         │
                         ▼
[ MOTION ]    [ Optical Flow TOP ] (velocity field) ──▶ [ Blur ]
                         │
                         ▼
[ SMOKE ]     ┌──▶ [ Displace by velocity (GLSL) ] ──┐
              │            (+ inject density at motion) │
              └──────── [ Feedback TOP ] (fade ×0.96, slight zoom) ◀┘
                         │
                         ▼
[ COLOUR ]    [ Lookup TOP gradient ]
                         │
                         ▼
[ OUT ]       [ Bloom ] ──▶ [ Blur ] ──▶ [ OUT 1080x1920 ]

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