A clean key is a lighting problem solved on set. When the footage arrives with an uneven backdrop, spill on the shoulders and motion blur around the hands, you cannot recover it with a single tolerance slider - but you can get a usable composite if you work in the right order.
Diagnose before you key
Freeze the hardest frame - usually mid-gesture, or the one where the subject is closest to the backdrop - and look at three things:
- Evenness. Sample the backdrop at four corners and the centre. If the green shifts, no single key colour will hold for the whole shot.
- Spill. Zoom on hair and shoulders. Green fringing is easier to suppress before the key than after.
- Edges in motion. Step three frames around a fast hand. If the matte chatters frame to frame, the problem is the source, not the settings.
Order of operations
- Denoise slightly before keying. Compression noise creates speckled mattes.
- Suppress spill with a targeted desaturation of the green hue rather than a global colour shift. Skin tones survive, and the key gets cleaner.
- Key with two passes: a soft key for the overall matte, then a second pass limited to the edges - hair and motion blur - where a harder key fails.
- Clean up with roto on the frames the key cannot hold. Ten hand-drawn frames per shot is normal; a hundred means the shot should be re-keyed from a different angle.
- Composite on a coloured background first - a saturated one, not black. Black hides halos that a real scene exposes.
When to stop keying and matte instead
If the backdrop moves, or the subject crosses in front of a second green surface with different lighting, keying is the wrong tool. Use a matting model that predicts per-pixel opacity: you lose a little control at the edges and gain a matte that holds through motion. Keep the key for touches of hair detail if you want to blend the two.
Render and check
Render to a format that actually keeps alpha - ProRes 4444 for an edit, VP9/WebM with alpha for the web - and verify on a coloured timeline that the transparency survived the export. Then watch the whole shot at speed: a matte that looks perfect frame by frame can still flicker in playback.
Tooling
If you do not want a node graph for a single clip, a browser green screen removal tool handles the common case - upload, key or matte, export with alpha - and leaves the hard frames for your editor. Use it for the bulk pass, then finish the ten stubborn frames by hand.
The short version
Diagnose the frame, denoise, de-spill, key in two passes, roto the rest, composite on colour, and check the export.
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