Fine rhythmic movement of light or shadow across a surface — caused by film grain, compression, or micro-camera motion. Controllable in final pass via LUT or noise reduction.
Crawling effects occur when light or shadow on a static or quasi-static surface begin to flicker and move — not continuously, but in fine, rhythmic patterns frame by frame. You often don't notice it on set; it only becomes apparent in the edit, especially during projection or color grading. The cause is almost always film grain noise in the recording or digital compression (H.264, ProRes, depending on the codec choice). Your eye interprets the noise pixels as movement — a psychovisual effect that is disturbing when the surface should be absolutely still, such as a wall in an interview or a sky in a wide shot.
In practice, this often happens with high ISO recordings or when you have to shoot in low light and push the sensor: the sensor generates noise, the noise is not uniform — it varies pixel by pixel from frame to frame, and the brain reads it as movement. Compression exacerbates the problem. Particularly insidious: you might not see it during normal monitor playback. But on a cinema projector or during 4K grading, it becomes a nuisance. The same applies to post-production — if you blow up a static shot too much (digital zoom), you start to amplify noise crawling.
In editing and color grading, you have several tools: gentle noise reduction (NR) is your first resort, but it also reduces detail. Spatial or temporal filters — the latter work frame-to-frame and smooth out noise flicker over time — often work better without blurring the image. Some DaVinci users specifically apply Temporal Denoise to problematic footage to remove the random variance from the noise. A LUT (Look-Up Table) alone doesn't directly help against crawling effects — but if you lift very dark shadows in your grading curve, you amplify the noise and thus the crawling artifact. So: for critical takes, it's better to add more light in the shadows than to compensate afterward.
Prevention is important: use the lowest possible ISO on your camera (native ISO of the camera) and light the scene adequately — even if it means using an additional HMI or a cinema light. For footage that will be blown up later or graded in 4K, pay special attention to graininess. And if you have to shoot with an older camera (5D Mark III, GH4) anyway, be consciously sparing with high ISO passages on supposedly uniform surfaces — skies, walls, water without structure.