Remove a green or blue screen from a photo and keep the subject with real transparency. The key colour, tolerance, edge softness and spill suppression are the four controls that decide the matte; the replace mode decides what goes where the screen was.
Pick the key colour
The key is the colour of the screen as it was lit, which is rarely the colour on the label. Shadows near the floor and the hot spot near a lamp both differ from the middle of the cloth, so set the key from a flat, evenly lit area and let tolerance absorb the rest.
Keying works on chroma, the colour part of a pixel, not on brightness. A dark green shadow and a bright green highlight are both close to the key, which is what makes an uneven screen keyable at all.
Tolerance and softness
Tolerance is the radius around the key colour that counts as screen. Raise it until the last patches of screen go, then stop: past that point green objects, and eventually skin with a green cast, start to disappear.
Softness is the width of the band between fully kept and fully removed. A few percent hides the jagged edge a hard matte leaves on hair. Too much and the subject’s outline turns into a halo.
Spill suppression
Screens reflect. Hair, shoulders and anything shiny pick up a green tint that a matte cannot cut away without cutting the subject. Spill suppression limits the key colour’s channel to the average of the other two wherever it dominates, so the fringe goes neutral and the rest of the picture is untouched. At 100% a green shirt goes grey too; back it off when the subject wears the screen’s colour.
Replace modes
- Transparent writes a PNG with an alpha channel, ready for compositing.
- Fill colour paints the keyed area flat, which is the quick check for holes in the matte.
- Blurred copy keeps the photo behind the subject but defocuses it, a background blur without a second image.
Luma keys
Dark areas and bright areas key on brightness instead. A product shot on black, or a subject against a white backdrop or a blown-out sky, keys cleaner this way than by colour, since black and white have no chroma to measure.
Limits
Motion blur, fine hair against a dark screen and semi-transparent objects such as glass do not key cleanly from colour alone, and no slider changes that. Light the screen evenly and keep the subject away from it; a metre of distance removes most spill before it reaches the matte.