Key a clip shot on a green or blue screen and replace the screen on every frame. The matte comes from colour distance, the edge from a softness band, and the green cast on the subject from a spill suppressor; the preview shows the keyed picture as the clip plays, and the export renders every frame through the same step.
Set the key
Play to a frame where the subject stands in front of the screen and pick the key colour from a flat, lit part of the cloth. The presets set a typical green or blue, and the Black background and White background presets switch to a luma key for subjects shot against black or white.
Raise tolerance until the last patches of screen disappear, then stop. Past that point anything in the subject near the key colour starts to go too.
Edges and spill
Edge softness is the width of the half-transparent band around the subject, which hides the jagged edge a hard matte leaves on hair. Spill suppression deals with the green light the screen throws back onto the subject: it pulls the key colour’s channel down to the level of the other two wherever it dominates, so the fringe goes neutral without eating into the matte.
Replace with
Fill colour paints the keyed area flat, which is also the quickest way to spot holes in the matte. Blurred copy keeps the clip’s own background but defocuses it, a background blur with no second file.
Export
The export runs at 720p by default, which keys faster than source size; a lit screen keys the same at either. Check the matte on the preview at the darkest and brightest moments of the clip before exporting, since a screen that changes brightness over a take is the usual reason a key that looked right at the start fails at the end.
Limits
Fast motion blurs the edge of the subject into the screen, and no key recovers it. Fine hair against a dark, unevenly lit screen keeps a visible halo. Both are fixed on set, by lighting the screen flat and keeping the subject a metre or more in front of it.