Removing a green screen

The colour key route, when one click is not clean enough

The colour key route selects the green itself rather than the subject, and it gives you a slider where the automatic button gives you a result.

1. Duplicate the layer with Ctrl+J (Cmd+J on macOS), because everything below is reversible only if the original survives.
2. Choose Select > Color Range and set Select to Sampled Colors.
3. Click once on the green with the eyedropper, in a mid tone area, not in the brightest or darkest part of the screen.
4. Add the rest of the screen with the plus eyedropper, clicking 3 or 4 more times across the lighter and darker patches.
5. Raise Fuzziness until the preview shows the screen as solid white and the subject as solid black, usually between 30 and 60.
6. Tick Localized Color Clusters and pull Range down, if a green object in the scene is being taken with the screen.
7. Press OK, then Ctrl+Shift+I to invert the selection onto the subject.
8. Click the Add Layer Mask button at the foot of the Layers panel.

What goes wrong at step 5 is chasing a perfect preview. Past about 70, Fuzziness starts eating the subject's own greenish midtones, particularly in skin shadow and in denim. Stop at the point where the screen is white and accept a grey fringe, then clean that fringe with the spill work below rather than with more Fuzziness.

Removing green spill from the subject

Removing green spill is the step that separates a finished key from an obvious one, and there are 4 tools for it, in order of how much control they give you.

• Select and Mask with Decontaminate Colors ticked at around 60%, which rewrites the colour of semi transparent edge pixels and handles hair in the same pass.
• Hue/Saturation as an adjustment layer clipped with Ctrl+Alt+G, set to the Greens channel, with Saturation pulled to -100 and Hue nudged toward yellow by about +15.
• Image > Adjustments > Selective Color on Greens, reducing Cyan and Yellow and adding Magenta, which is finer than a blanket desaturation and keeps real green objects alive.
• Layer > Matting > Defringe at 1 px or 2 px, which replaces the outermost ring of edge pixels with the colours just inside it.

Check the result against a mid grey background, not against the final scene. Green spill hides inside a green scene and reappears the moment the composite goes onto a blue sky, and a mid grey layer at 50% brightness shows it immediately.

Painted wall or projector screen: what to shoot against

A painted wall beats a projector screen for keying, and the difference is not the colour but the surface. A pull down projector screen is built to bounce light back at the audience, so its gain is high and its surface is slightly glossy, which produces hot spots that read as 3 or 4 different greens to Color Range and forces the Fuzziness up until the subject starts to go. Chroma paint on a flat wall is matte by design, so it reflects evenly and the whole screen keys as one value. The practical comparison, for anyone deciding before they shoot rather than after:

Lighting beats surface in every case. Light the screen separately from the subject, keep the subject at least 1.5 m (about 5 ft) off the backdrop, and the spill problem largely solves itself before you open the file.

The channel method, and Photoshop CS6

The channel method builds the key by hand and works in every version of Photoshop ever released, including CS6 from 2012, which has neither the Contextual Task Bar nor Select Subject. It is also the most precise route when a keyed edge has to hold up at print size.

1. Open the Channels panel and click through Red, Green and Blue to find the one where subject and screen differ most, usually Blue for a skin toned subject on green.
2. Drag that channel to the New Channel button to copy it, so you are not editing the picture.
3. Press Ctrl+L for Levels and drag the black and white input sliders inward until the subject is solid black and the screen is solid white, with no grey between them except at the real edge.
4. Paint any remaining grey inside the subject with a black brush at 100% opacity, and any grey in the screen with white.
5. Ctrl+click the channel thumbnail to load it as a selection, click back on the RGB composite, then press Ctrl+Shift+I.
6. Add a layer mask, and finish with Select > Refine Edge in CS6 or Select and Mask in a current version.

What goes wrong at step 3 is pushing Levels so far that the edge becomes 1 pixel wide with nothing in between. That hard binary edge is exactly the giveaway a green screen composite is not supposed to have, and it is unfixable afterwards. Leave a band of grey 2 px to 3 px wide around the subject and let it carry the transparency.

Placing the subject in the new scene

Placing the keyed subject is where a technically clean key still fails, because the mask says nothing about whether the two images belong together.

• Match the light direction first, flipping the background horizontally if that is what puts the key light on the correct side.
• Match the black point with a clipped Curves layer, since a subject shot on a bright screen almost always has lifted shadows compared with a real location.
• Add a contact shadow on its own layer under the subject, painted with a soft black brush at 20% opacity, because a figure with no shadow floats.
• Blur the background slightly with Filter > Blur Gallery > Field Blur if the subject was shot at a wide aperture, so the depth of field agrees.
• Add matching grain last, at the same strength across both layers, since 2 different noise patterns in one frame are visible even at small sizes.

Save the layered PSD and export with File > Export > Export As to PNG if the subject needs to stay transparent, since JPEG has no alpha channel and will fill the cutout with white. Photoshop on a phone handles a simple key with Tap Select in the free app if you are away from a desktop, and Photoshop, operation by operation lists the neighbouring operations, including Removing a background in Photoshop for subjects that were never shot on a screen at all.

Where to go next

PHOTOSHOPPhotoshop, operation by operationPHOTOSHOPRemoving a background in PhotoshopHow to remove a background in PhotoshopPHOTOSHOPPhotoshop on a phoneWhat actually works