Keying

Keying comes in two primary flavors: chroma key (removing a specific color, usually green or blue) and luma key (removing based on brightness). Chroma keying is the backbone of VFX compositing. The principle is straightforward — find all pixels that match the target hue, generate a matte (alpha channel) that's white where the background is and black where the subject is, with gray values at the edges for partial transparency. The execution is where it gets brutal. The three keyers used in professional work are Keylight (After Effects, Nuke), Primatte (Nuke, After Effects as a plugin), and Delta Keyer (DaVinci Resolve Fusion). Keylight is the most widely used — it works by creating a color difference matte based on the distance between the screen color and the subject in color space, then applying edge correction and spill suppression. It excels on well-lit green screen with minimal spill. Primatte uses a proprietary polygon-based algorithm in 3D color space, allowing you to click-drag to define clean background, foreground, and edge zones. It's more manual but handles problematic footage (uneven lighting, semi-transparent fabric, smoke) better than Keylight in many cases. Delta Keyer, introduced in Resolve 17, combines 3D color space isolation with built-in soft edge and matte refinement — it's surprisingly capable for a built-in tool and has become many compositors' first choice for quick keys in Resolve. The green vs blue screen debate has a technical answer: green (typically #00FF00 or thereabouts) is used because the human body contains virtually no green tones in skin or hair, and CMOS sensors have twice as many green photosites in their Bayer pattern (50% green vs 25% red and 25% blue), giving you the highest signal-to-noise ratio on the green channel. Blue screen is preferred when the subject contains green elements (military uniforms, foliage, green costumes) or for blonde hair — green spill on blonde hair is immediately visible as a sickly yellow-green tint; blue spill is much less noticeable.

The single biggest giveaway of a bad key is the edge. Specifically: hard edges with no semi-transparency (hair looks like it was cut out with scissors), color contamination from the screen (green or blue fringe on skin and clothing), and incorrect light wrap (the subject doesn't pick up any environmental light from the background they're supposedly standing in front of). Edge handling requires understanding that real-world edges aren't binary — a strand of hair against a green screen is partially transparent. The matte at that point should be 40–70% gray, not 0% or 100%. Keylight's Clip Black and Clip White parameters control this: Clip Black defines the threshold below which pixels are fully transparent, Clip White defines the threshold above which pixels are fully opaque. The gap between them defines the semi-transparent edge zone. For hair detail, you want a narrow gap (2–5 units) with the screen matte softness adjusted so individual strands are preserved. Spill suppression is the second critical step. Green screen light bounces onto the subject — hair, skin, clothing edges all pick up a green cast. Keylight has built-in spill suppression that desaturates the green component of pixels near the key edge, but aggressive settings will make skin look dead and gray. The better approach is to use a separate spill suppression pass: in Nuke, the H_Spill node or a custom hue-shift node that rotates green spill toward a neutral tone matching the intended background. In Resolve Fusion, the Spill Killer tool handles this. For ultimate control, many compositors use a light wrap technique — sampling colors from the background plate and adding a subtle glow at the subject's edge to simulate reflected environmental light. This is what makes the composite feel physically grounded rather than pasted on.