How to use green screen in Video by Zaweba
Green screen with spill removal, luma key, colour key and alpha adjust in Video by Zaweba, plus masks and track mattes for a clean cut-out of your subject.
Green screen lets you film someone in front of a plain coloured backdrop and replace the backdrop with anything you like. The technique is called keying, and Video by Zaweba has a set of keyers for it: green screen with spill removal, luma key, colour key and alpha adjust. They are designed to work alongside masks and track mattes, so you can get a clean cut-out even from footage that isn't perfect.
Setting up a green screen shot
Put your background clip on a lower track and the green screen footage on the track above. Apply the green screen effect to the upper clip from the Effects tab. The green is made transparent and the background shows through.
Most of the work then goes into the edges. Hair, fingers and anything moving fast are where a key looks rough. Adjust the effect's settings while looking at the hardest parts of the shot, not the easy middle. Use the preview zoom to look closely at edges.
Spill removal
Green bounces. Light reflecting off a green backdrop tints the edges of your subject, their hair and shoulders especially, and sometimes shiny objects too. Even after the background is gone, that green fringe makes a composite look fake. Spill removal reduces the green in what remains, so the subject's edges look natural against the new background.
After keying, a little colour correction on the foreground often helps it match the background. If the new background is warm and the subject was lit cool, adjust temperature until they agree.
Other keyers for other jobs
Luma key removes parts of the picture by brightness instead of colour. It is useful for footage shot on pure black or pure white, such as smoke, sparks, fire or a logo on a white card.
Colour key removes any colour you choose, not just green. It suits a blue backdrop, a clear sky or any reasonably even coloured area.
Alpha adjust works on the transparency of a clip after it has been keyed, or on footage that already carries transparency. It lets you tighten or soften the cut-out and is a good final step.
Cleaning up with masks and mattes
Real green screens rarely fill the whole frame. Lights, stands and the edge of the studio show at the sides. Rather than push the keyer harder, draw a mask round your subject that throws away everything well outside them. That is called a garbage matte, and it saves the keyer from having to handle all that clutter. Masks in Video by Zaweba can be animated and tracked, so they follow a moving subject.
Track mattes are another tool. A clip can be made to show only inside the picture on another track, using its alpha or brightness, which gives you more ways to combine keyed footage with shapes, text and other clips.
Shooting for an easier key
A good key starts on set. Light the backdrop evenly, without bright spots or deep shadows. Keep the subject a few steps away from the screen so less green spills onto them. Avoid green clothing and very fine, wispy hair if you can. Use the sharpest, least compressed setting your camera offers, because heavy compression smears colour at the edges and makes keying much harder.
Common keying mistakes
Pushing the key too hard to remove the last speck of green tends to eat into the subject, turning hair into a hard helmet. Combine a gentler key with a garbage matte instead.
Ignoring light direction is another. If the subject is lit from the left and the background's sun is on the right, no key will make them look like they belong. Pick or shoot backgrounds that match.
Finally, play the shot at full speed before you export. A key that looks perfect on one frame can flicker on another. Video by Zaweba gives you all the pieces for convincing composites, and taking the extra few minutes on edges is what makes them believable.
