Why does a green screen key flicker when you wave?
A fingertip can vanish for a frame when its edge color gets close enough to the screen color to be keyed out. Step through the wave in the matte view. If the tip alternates between solid and transparent, lower similarity until it stays solid and check that the screen still clears.
A row passes if the tracked fingertip is fully opaque (alpha 255) and the screen shadow at (10, 112) is fully transparent (alpha 0) in all 24 frames.
| Test date | Tool and version | Same source or frame | Setting changed | Result |
|---|---|---|---|---|
| 2026-11-12 | FFmpeg N-126374-g089a48eb36-20260831 | fast-hand-source.mp4, frames 0–23 | 0.04 | Fail. The shadow point remained opaque in all 24 frames. |
| 2026-11-12 | FFmpeg N-126374-g089a48eb36-20260831 | fast-hand-source.mp4, frames 0–23 | 0.09 | Pass. The fingertip stayed opaque and the shadow point stayed clear in all 24 frames. |
| 2026-11-12 | FFmpeg N-126374-g089a48eb36-20260831 | fast-hand-source.mp4, frames 0–23 | 0.13 | Fail. The fingertip point disappeared in every odd-numbered frame, starting at frame 1. |
Source file fast-hand-source.mp4 · Download the original MP4
Get the per-frame data, parameter sheet and reproduction script.
What changes from one frame to the next?
We generated a synthetic hand that moves horizontally, with the tallest fingertip alternating between two green-contaminated colors to model an edge crossing the key threshold. The script followed that fingertip through the move and read alpha at the same point on the finger in every frame. The shadow check used a fixed point away from the hand. We used the same key color and zero blend for all runs and changed only FFmpeg's similarity setting.
The loose setting cleared the screen shadow, but the tracked fingertip alternated between opaque and transparent, with twelve frames losing the point. The tighter 0.04 setting retained the fingertip but also retained the shadow throughout. At 0.09 both measured points behaved as intended. These numbers describe the supplied synthetic clip and two sampled points. Use the visible edge and the matte on your own footage to choose a threshold.
How to check your own wave
Pause near the fastest part of the motion. Follow the same fingertip as you step forward and back, then check a shadow patch away from the hand. If tightening the key brings the finger back but leaves a shadow, isolate that screen area with a separate mask or key. Inspect the whole silhouette and the composite before export. FFmpeg describes how its RGB key applies a distance threshold around the selected color. FFmpeg color key documentation