The transparent export with a black background
Before green screens, alpha. An alpha channel is a fourth channel alongside red, green and blue, storing how opaque each pixel is. Almost every compositing problem beginners hit is an alpha problem in disguise.
Two facts save hours:
H.264 MP4 does not carry alpha. If you exported a transparent animation as MP4 and got a black background, that is why. Formats that do carry alpha: ProRes 4444, PNG sequences, WebM with VP9, and HEVC with alpha (which Safari plays and most other things do not).
Straight versus premultiplied. In premultiplied alpha, the colour values have already been multiplied against a background, usually black. If the software guesses the wrong interpretation, you get a dark or bright fringe on every semi-transparent edge. The fix is to tell the application the correct one — in After Effects, File → Interpret Footage → Main → Alpha — not to choke the matte until the fringe hides.
What a keyer is actually doing
It is not "removing green". It measures, for every pixel, how far that pixel's colour sits from a chosen colour in some colour space, and turns that distance into a matte — a grayscale image of opacity.
Once you hold that in your head, the failures explain themselves.
- Uneven lighting on the screen. The green varies across the frame, so one tolerance setting cannot cover it. On set, light the screen separately and evenly first, then light the subject. That order is the entire craft.
- The subject too close to the screen. Green light bounces off it onto hair, shoulders and the side of the face. That is spill, and spill suppression fixes it by desaturating those areas, which is why keyed people often look slightly grey. Two to three metres between subject and screen removes the problem at source.
- Fine hair and motion blur. These produce semi-transparent pixels that are partly green and partly subject. A keyer must guess. This is why a key looks perfect on a paused frame and falls apart when the person turns their head.
- Chroma subsampling. Nearly every phone and consumer camera records 4:2:0 — full resolution for brightness, a quarter resolution for colour. The matte is drawn from colour information that does not exist at pixel resolution. That is the mechanical reason a phone-shot green screen has stair-stepped edges that no amount of edge refinement fully removes. Shooting 4:2:2 with a capable camera or an external recorder, or simply at a much higher bitrate, is the real fix.
Which is the point of the lesson: the shoot decides whether the key will work. Post-production negotiates with what the shoot handed over.
Free tools that key properly
- DaVinci Resolve (free): the Color page qualifier and Fusion's Delta Keyer are both professional-grade. There is no meaningful keying feature you are missing by not buying Studio.
- Blender (free): the compositor has keying nodes and a good despill.
- OBS (free): a serviceable chroma key for live streaming.
- Kdenlive, Shotcut (free): basic keyers, adequate for a well-lit shot.
- CapCut on a phone: chroma key exists and works about as well as a phone-shot source deserves.
- After Effects with Keylight is the industry default and what job ads name.
When a key will not hold, the answer is rotoscoping — masking by hand, frame by frame or with a tracked shape. Slow, and sometimes the only way.
AI matting has genuinely changed this in the last few years. Resolve Studio's Magic Mask, Runway's background removal and several free web tools will cut a person out of an unprepared shot better than a bad key ever would. Their weaknesses are consistent: the edge wobbles slightly between frames, so a still looks better than the sequence, and fine hair still goes.
Tracking, in three levels
- Point tracking. Follow one high-contrast feature and attach something to it. Good for pinning a label to a moving sign. Needs a distinct, non-repeating feature that stays in frame.
- Planar tracking. Track a flat surface — a phone screen, a poster, a wall — and get its perspective as it moves. Mocha ships with After Effects; Blender's motion tracker is the free route.
- 3D camera solve. Recover the camera's actual path through space so a 3D object can sit in the shot. This needs parallax: nearer things must move differently from further things. A camera that only rotates on the spot gives the solver nothing to work with, and it will fail no matter how good the footage looks.
Rolling shutter, from phone and mirrorless sensors, skews fast pans and breaks solves. Most trackers have a rolling-shutter compensation setting. Find it before you spend an hour wondering why the solve drifts.
Ten minutes today
Shoot ten seconds against any green surface with your phone. Hold your hand up and wave it slowly. Key it in Resolve. Then look at the edge of your moving fingers rather than the edge of your still shoulder. That difference is the honest quality of your setup, and it tells you whether the next thing to fix is the lighting or the camera.
Before you move on