Keying a screen properly
A green screen is the easiest background there is, because it is a colour nothing else in the picture should be. The key measures every pixel’s distance from that colour in a perceptual colour space, so the ramp from screen to subject is even across hair, fabric and skin. The uneven-background switch compensates for a screen that is brighter near the lights than at the floor.
What a plain key cannot fix is spill: green light reflected onto the subject. That is a colour problem, not a transparency problem, and the spill control on the Cut panel removes it only near the edge where it lands.
- Green clothing disappears. Anything the colour of the screen is keyed with it. Use the keep brush on the Refine panel to bring it back, or reshoot in a different colour.
- Motion blur and fine hair. A blurred arm is half screen and half arm in every pixel; it comes back partly transparent, which is physically right but can look ghostly on a new background.
Questions about green screens
Does this work on video?
No — still photos only. Video needs every frame keyed and re-encoded, which this site does not do. Export a frame as a picture if you need one still from a clip.
What is spill, and why is there green on my subject's edges?
Light bounces off the screen onto the subject, so hair and shoulders pick up a green tint. Spill suppression clamps the green channel to the larger of red and blue in a band around the edge. Sixty percent is the starting point; raise it if green fringes remain, lower it if a green shirt starts to look grey.
My screen is unevenly lit. Will the key still work?
This page starts with 'uneven background' on, which reads the screen colour as a smooth field across the frame instead of one value, so a darker corner is still recognised as screen. If a patch survives, raise the tolerance a little or add a second key colour with Shift and the eyedropper.
Does it work with a blue screen?
Yes. The screen colour is read from the most common green or blue on the photo's border, and spill suppression works on whichever channel the screen is made of.
Related
- Where colour fringe comes fromWhy an edge keeps the old background's colour, how unmixing takes it back out, and how to read the edge-tint number.
- Remove a background colourPick any flat colour off the photo — up to four of them, black included — and key it out with edges unmixed against it.
- Remove background around hair and furA wider uncertain band and a softer ramp, set that way before you arrive.
- Make a picture transparentThe same studio, with the file-format question answered on the way out.
