Keying
The general process of generating a mask (key signal) to extract specific elements from video or images. Methods include chroma key, luminance key, and difference key.
Keying is the umbrella term for techniques that generate mask signals (keys) to separate foreground from background in visual media. The key signal functions as an alpha channel controlling transparency during compositing. Accurate key extraction is critical for final composite quality.
Methods are classified by information source. Professional workflows combine multiple techniques in hybrid approaches.
- Chroma key: Detects a specific hue range using green or blue screen footage. Mask boundaries are determined by
ΔE(color difference) thresholds - Luminance key: Generates keys based on brightness values. Effective for white or black backgrounds and highlight/shadow extraction
- Difference key: Computes keys from pixel differences between a clean background reference and subject frames. Effective for fixed-camera shots
- IBK (Image Based Keying): A Nuke-native method estimating clean plates for per-pixel precision keys, handling uneven lighting gracefully
High-quality keying requires two stages: a core matte for definitive foreground regions and soft edges for semi-transparent transitions. Complex edges like hair and smoke demand erosion, dilation, and edge blending for photorealistic integration.