Compositing means “combining images from different sources together” and is a technique that is commonly used today both in still images and video. The old Superman movies used a form of compositing to create the illusion that he was flying through the air, but today’s techniques are far more sophisticated. What was difficult to do in the past is now easily done with computers – and even the most basic software these days, like Apple’s iMovie, allows you to achieve these effects.
Other terms for compositing are “chroma key”, “green screen” or “blue screen” and it usually refers to video production, although it can refer to stills as well.
How it works
A still or video image is shot against a uniform color background. Special software is then used to instruct the computer to remove every pixel of that color from the image and replace the removed pixels with another image. Usually, the computer has to “render” for a while before the process is finished. The more complicated the instructions, the longer the computer will take to render. For example, in a complicated movie sequence with a bunch of special effects, each thing moving on the screen might be a separate layer. When you’ve got multiple elements moving in the same scene, it can significantly increase the render time. For most applications, though (usually 2 layers), the rendering time isn’t that long, especially since computers keep getting faster and faster.
One of the biggest challenges for the software is when there are uneven spots in the background caused by shadows, color differences and lighting issues. This can lead to a “halo effect” and a substandard result. To minimize these problems, make sure that your background is as even as possible in terms of color, that it’s free of wrinkles and that you light your subject and background properly so that you get the best result when compositing.