Chroma key — better known as green screen — lets you remove a solid-colored background from a video or image so you can place your subject over any scene you want. In Alight Motion, this is done with the built-in Chroma Key effect: you tell it which color to remove, and it makes every pixel similar to that color transparent. This guide walks through the whole process, from preparing your footage to cleaning up edges and exporting — based on the official Alight Motion Effect Guide, so nothing here is guessed.
What chroma key does
The Chroma Key effect (listed under the Compositing effects) keys out all pixels similar to a particular color, making those parts of the layer transparent — which is what makes green screening and blue screening possible. You apply it to a video or image layer, pick the background color with the Eyedropper tool, and the background disappears while the subject stays.
It works best on footage shot against one flat, evenly lit color — usually a bright green, blue, or orange. The cleaner the background in the footage, the cleaner the key. Everything in the next section makes the actual keying step easier.
Preparing your green screen footage
Good keys are made before you open the app. The official guide’s capture-environment advice, condensed:
- Use a solid, vibrant background. A solid bright green, blue, or orange wall, cloth, or pop-up screen covering the whole frame. Avoid white: if your subject is a person, the whites of their eyes would be keyed out too.
- Light it evenly. Light the background as uniformly as possible. Shadows and brightness changes make parts of the background a different color, and those parts will not key out cleanly.
- Flatten wrinkles. If you use cloth, stretch it smooth — folds and wrinkles create color variations that leave patches behind.
- Separate the subject. Keep the subject as far in front of the background as practical. This reduces colored light spilling from the background onto the subject’s edges (the green fringe you often see on hair).
- Dress the subject accordingly. The subject must not wear or contain any color that appears in the background, or those parts will be keyed out as well, leaving “holes.”
Step 1: Add the Chroma Key effect to your layer
Import your green screen footage into the project as a video or image layer and select it. Open the Effects panel, search for Chroma Key, and add it to the layer. Nothing visibly changes yet — the effect starts with a default key color, so the next step is telling it which color to remove.
Step 2: Select the key color
Open the Chroma Key effect’s settings and find the Key Color property — this is the color that will be made transparent, along with similar colors. Tap it and use the Eyedropper tool to pick a representative patch of the background directly from your footage (aim for an average-looking area, not the brightest or darkest spot). If the background is not uniform, picking from the middle of the brightness range usually gives the best starting point.
Step 3: Remove the background with Threshold
Threshold controls how different a pixel can be from the Key Color and still be removed. It runs from 0 to 1 (default 0.1):
- Increase it to remove a broader range of similar colors — this clears away shadowed or uneven patches of the background.
- Go too far and the effect starts eating into the subject, especially areas whose colors resemble the background.
- Note that the threshold is more sensitive to hue and saturation than to brightness, so you may need fairly high values to include noticeably different shades.
The official usage advice is to adjust Threshold first, before touching the other settings. Raise it slowly until the background is gone but the subject is intact, then stop — cleanup belongs to the next step.
Step 4: Clean up the edges with Feather and Defringe
After the background is gone, the subject’s edges are usually the rough part. Two settings handle this:
- Feather (range 0.01–0.75, default 0.05) softens the transition between transparent and visible pixels. Near zero, edges are hard and can look cut out; raising it smooths them. But too much feather lets the background color bleed back in as a colored fringe around the subject — so increase it gradually and check edges at full size.
- Defringe (off by default) specifically tries to remove leftover tints of the key color from semi-transparent edge pixels. Turn it on when the subject has soft or fuzzy edges — hair is the classic case — and the official guide recommends pairing it with Feather for the best result.
There is also an Invert toggle (off by default) that reverses the effect: it keeps only the key color and removes everything else. That is a separate creative trick — for example, isolating one solid-colored object — rather than part of a normal green screen key.
Layering foreground and background
A keyed layer is only half the shot. Build the composite like this:
- Place your new background (another video, image, or scene) on a layer below the keyed layer on the timeline.
- The keyed layer goes on top — its transparent areas now reveal the background beneath.
- Match the two layers: scale and position the subject so it sits naturally in the scene, and check that the lighting direction roughly agrees. A perfectly keyed subject still looks fake pasted over a mismatched background.
- If you want to animate the keyed subject (entrances, moves, zooms), keyframe the layer’s transform properties — our keyframes guide covers exactly how.
Common green screen mistakes
- Uneven lighting. The number one cause of patchy keys. If the background has bright and dark areas, no single threshold cleans all of it without damaging the subject. Fix it at capture time.
- Threshold set too high. Holes appear in the subject (clothing, eyes, reflections). Back the threshold off and clean edges with Feather/Defringe instead.
- Skipping Feather. Hard, crunchy edges scream “green screen.” A small amount of feather plus Defringe usually fixes it.
- Subject too close to the screen. Creates green spill on hair and shoulders. Distance and even lighting prevent it; Defringe cleans up what remains.
- Judging the key on a small preview. Edges that look fine on a phone preview can look rough exported. Zoom in or preview at full resolution before finalizing.
- Wrong-colored wardrobe or props. Anything matching the background color disappears. Check the subject’s full outfit against the screen color before shooting.
Export considerations
Keyed edges are fine detail, so export choices matter more than usual:
- Use a higher resolution. Edge cleanup (feather, defringe) survives better at higher resolutions, where compression has less visible effect on fine edges.
- Match your frame rate. Export at the same frame rate as your footage to avoid judder on the keyed layer.
- Review the full export. Always watch the exported file before sharing — preview and export can differ. See our export settings guide for the available options, and the troubleshooting guide if the export does not match the preview.
Troubleshooting chroma key problems
- Background will not fully disappear: raise the Threshold gradually, or re-pick the Key Color from a different area of the background. If only patches remain, the footage lighting was uneven.
- Green tint or fringe around the subject: turn on Defringe and adjust Feather; if the spill is heavy, the subject was too close to the screen when filmed.
- Holes in the subject: the Threshold is too high, or the subject contains the background color. Lower the threshold; wardrobe problems can only be fixed by re-shooting.
- Edges look choppy or flicker: add a little Feather, and check the result at full preview resolution rather than a downscaled preview.
- The effect seems unavailable or looks different: the Chroma Key effect is documented in the official Alight Motion Effect Guide, but menus and effect availability can change with app updates — check the current official guide if something on your device differs.
Keep learning
Chroma key is one compositing skill; the layering, animation, and export skills around it matter just as much. Next, work through our keyframes guide to animate keyed subjects, review export settings for clean final renders, read the troubleshooting guide for broader fixes, or start from the beginning with our complete how-to-use-Alight Motion hub.
Sources consulted: Official Alight Motion Effect Guide — Chroma Key entry (effect purpose, Key Color, Threshold 0–1 default 0.1, Feather 0.01–0.75 default 0.05, Defringe, Invert, Eyedropper usage, capture-environment guidance); official Alight Motion Help Center (general feature guidance).