BEHIND THE CAMERA

How do Green Screens Work?

The green screen is a staple in modern cinematography – you simply can’t miss it. In fact, its bright, atypical colour is what makes it useful to VFX artists. Specifically, computers analyze a shot to remove or make transparent this particular colour, which is later layered with something else through compositing. Using this green screen technique is also called chroma keying. Its most often used when landscapes are too dangerous or far to film in, or simply don’t exist at all. Imagined settings often take shape in matte painting, which are still images that a compositor will put in the green space. If the camera placement is moving in 3D space, however, the background image will have to be tracked (I.e. follow the same motion/angle/position relative to the camera’s motion). Other uses for chroma keying have been gaining traction, such as prop removal and even disappearing limbs.

In Focus film school defines the computer’s chroma keying process below:

1. The new background is composited (i.e. two images or video streams are layered together) into the shot.
2. The chroma key singles out the selected colour (usually the green) and digitally removes it by rendering it transparent. This lets the other image to show through.
3. When used with more sophisticated 3D techniques, this process can add any new element (smoke, fire, rain, etc.) to complex moving shots.

We’ve all seen green screens in work; take the daily weather segment, where the background is swapped out for a map displaying information. Chroma keying is all around us, we just don’t notice it. There are a couple of “rules” when using this helpful tool. Primarily, don’t wear green. This colour green is chosen because of its uniqueness and its absence in most of our wardrobes, nature, props, etc. It also has very little trace in skin tones, making people appear as opaque as possible. If this green isn’t practical for your use, try switching the green out for the second-in-line colour: chroma key blue. The advantage that blue has over green is in the colour spill. Because the green is so bright, it can lead to colour spill which is like a soft green hue reflected onto the object, of which the blue does less of. Blue is also better for nighttime shots. Regardless, chroma keying is a practical and widely used technique in the entertainment industry, and thus a fundamental part of VFX. Whether it’s a sixth grade project or one of Marvel’s many fantasy backdrops, green screens take everything up a level.