Video compositing is simply the incorporation of multiple layers of visual media elements together to build a scene. These media elements can be videos, still images, graphics, text or 3D animations. In order to create a video composite, your editing software has to be told which parts of the video or image should be seen, which parts should be hidden, and whether there are any underlying elements that are supposed to be showing through. There are many ways this can be done, but three of the most common techniques are through the use of alpha channels, mattes and keys.
Alpha Channels
Alpha channels are additional layers that can be added to a video or an image to make it fully or partially transparent. The channels contain transparency information about the image, which basically allows you to control the transparency of the red, green and blue channel.
Alpha channels are used in a variety of ways. Every time you see a graphic on TV that transitions from fully opaque to transparent; or a set of texts that fades seamlessly into the background video; you’re seeing an alpha channel at work.
There are several benefits of using alpha channels for video compositing, but the biggest one is without a doubt, visual appeal. Without alpha channels, images placed over one another would not be anti-aliased and will have a blocky jagged edge to them, which is not at all attractive.
Mattes
Another important concept to video compositing is the use of mattes. Imagine cutting a shape out of a piece of cardboard, which will leave you with the cutout shape and the hole in the original cardboard. If you place the cutout shape against a picture, the picture is blocked with the shape of the cutout. And if you place the hole in the cardboard against the same picture, you’ll see the picture through the hole. That is how mattes work, just electronically.
Mattes are an important part of compositing because they allow one track to show through another in precisely the way you want, by creating a matte that allows all or part of the background picture to peek through the foreground layer.
There are a variety of mattes available, including geometric mattes of squares, circles and diamonds; and those with customizable shapes, such as four or eight point garbage mattes. They are named as such because they’re particularly useful as quick and ‘dirty’ ways to mask out stuff you want to hide in the underlying video. Text and titles are also considered variations of geometric mattes.
Keys
Chromakeying is a process used to merge one image (often a live one) with a previously shot footage or graphics and make it look completely natural. A chromakey occurs when you tell your software package to take all the pixels of a particular color and replace them with the live video from another shot. Traditionally, the colors used for chromakeys are either bright green or bright blue two of the basic colors that form a typical RGB video signal.
Nearly everyone has seen the effect of keys, as they are used by TV stations for nightly weather reports. On these weather reports are usually animated images of weather information, including clouds, temperatures and rain storms; and these are done using color of chromakeying. What happens is the weather reporter stands in front of a large, evenly lit solid-colored (usually blue or green) screen, and a second video source (in this case, the weather map) electronically replaces that color.
When using chromakeys, it is essential that your talents are not wearing anything that is the same color as the key, because the parts that are of the same color will also become invisible when the key is active and will disappear from the footage when you composite it, allowing the background to essentially show through your talent.
Have Some Fun
Now that you know the essentials of using alpha channels, keys and mattes for, it’s time to let your creative juices flow! Video compositing is a world without creative limitations, so dive right in and let the fun begin!
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