Keye

The whole trick, Keye explains, is that the camera can be taught to be picky about one color.

"Your eye sees the green wall and just thinks 'green wall,'" she says. "But we can tell the computer: find this one shade of green and make it see-through — like a window. Everywhere it's that green, punch a window. Everywhere it's not that green — my face, my vest, my hands — leave it be." Then you slide a second picture behind the window, and suddenly she's standing in a canyon.

The reason green is her favorite window-color is simple and a little funny: "There's almost no green in a person," she says. "Skin is pinkish, hair is brown or black, eyes are brown or blue — barely any green anywhere. So when I tell the camera 'erase the green,' it erases the wall and never once erases me." (She keeps a bright-green scarf in her bin as a warning. "Wear green in front of a green wall," she grins, "and the camera erases your tummy. You walk around with a canyon where your middle should be.")

Keye grew up in a market town of glassblowers and window-makers, where her family made stained-glass panels that layered colored light one sheet over another.

Their whole art was stacking see-through things: a red sheet over a blue sheet made purple light on the floor; a clear pane over a painted one let you look through to whatever was behind. Their lesson, passed down: "A picture isn't one flat thing. It's layers, and the magic is choosing which layer you can see through."

When she was twelve she carried her little swatch-book to EffectsForge, and Render, her mentor, asked, "What is chroma key?" "Filming someone in front of one flat color," Keye said, "and telling the camera to erase exactly that color, so it becomes a window I can put any world behind. Green, because there's no green in a person. It isn't faking — it's stacking one picture on another and letting the chosen color get out of the way." Render nodded slowly. "You are appointed."

In her workshop, Keye ran the demos one after another. She stood against the green wall and tapped a button — and behind her a waterfall appeared, misting down; she stepped left and the waterfall stayed put behind her, exactly as if she were really there. Then she taught the rest as one steady habit: light the green wall evenly, with no dark corners and no bright hot-spots — "a patchy wall gives you a patchy window, and the edges go crumbly." Keep the actor a good step away from the wall, so green light doesn't bounce off it and tint their shoulders. And frame the shot for the world you'll drop in behind, so the horizon lines up and the shadows fall the same way. "The seam is everything," she said. "If the edge around me is clean and the light matches, your eye believes the whole thing — canyon, comet-tail, castle. If the edge is smudgy and the light's wrong, your eye goes, 'hmm, that's a sticker on a photo.'" Her first try that morning had come out ringed with green smudges; she'd just widened the light, nudged herself forward, and gone again. "First keys are always a bit crumbly," she added. "You clean the edge, you don't quit."