Video Editing: Superb Superimpositions

Jim Stinson
February 1999

Combining and manipulating multiple pictures, titles or special effects into a single, composite image is one of the oldest editing tricks in the book, and it's still one of the best. In simpler times, we referred to individual compositing processes such as matting, titling, superimposing and chromakey. In these digital days, "compositing" is a generic name that covers all of these different processes.

In this outing, we'll cover the forms of compositing called "superimposition," reserving titles and mattes for another day. The differences are simple. Superimposition stacks images on top of one another, while matting completely replaces parts of one image with parts imported from another. Of the two methods, superimposition is the older and simpler. Superimpositions, or "supers" as the term is abbreviated for convenience, consist of two or more images combined into one through electronic processing. Unlike audio channels, multiple video signals cannot be combined by simply running them through a common cable.

Because many of us still do analog rather than digital editing, the techniques covered here can be used in both post-production systems. To keep the discussion mostly hardware independent, we'll focus on using superimposition to add style and effect to your video content, rather than on which buttons you push or what menus you pull to make it happen. There are three major uses for superimpositions: transitions, multiple images and special effects.

In linear editing, supering is done by feeding the two images through a switcher. With computer editing software that uses a timeline interface, you can place one shot on one of the video tracks and the other shot on the special effects track. With either system you can control the percentage of each original in the composite.

The most common use of superimposition is to create a transition by fading the first image down from 100 percent to 0 percent and the overlapping part of the second image up from 0 percent to 100 percent. Once called a cross fade for obvious reasons, this is now known as a dissolve or mix. A dissolve signals a change of time, place, or frequently both.

Dissolve transitions offer powerful tools to the editor because they contribute so much to the style of the program. Wham-bam one-second mixes convey speed and energy. Two-second dissolves impose a more dignified pace. Languorous five second transitions impart a dreamy, romantic feel to the proceedings. Carried past that five-second limit, dissolves turn into true multiple exposures, through which the audience receives multiple streams of information.

The second use of superimposition is to enrich the delivery of information by displaying more than one image at a time. If you run the first half of a dissolve--until each image is at 50% strength--and then leave both visuals on screen, you double the amount of information you present to the audience. Each image is communicating its message separately.

However independent, in order to make sense the two streams of information should relate to each other. Combine a shot of a woman's longing expression with a closeup of a baby, and you tell the audience that she is expressing maternal affection. Combine that same shot with a shot of a man in a sailor's uniform, and she is thinking of her lover instead.

Make sure that the two images can be effortlessly related. Presented with multiple topics, the human brain will instinctively struggle to combine them, or at least to connect one to the other. If the images have no intrinsic relationship, the resulting false connection will create a sort of conceptual gibberish.

Suppose, for instance, that one image shows a mother duck leading ducklings across a field while the superimposed image follows seagulls wheeling in circles in the sky. Birds? No. Sea birds? No. Some fly while others walk? No. Seagulls circling over possible road kill? Nah, that's buzzards. What, then? No matter how unconnected the two images may be, viewers will fight to relate them, distracting them from the story in the meantime.

Ah, but replace the gulls with a mama fox licking her kits and you have your relationship: maternal love. Show the fox lurking in the underbrush instead and the ducks suddenly become dinner. Super both fox images over the ducks in succession and you have a complex abstraction like, "mother care is universal, but doesn't extend across species lines."

The point is that supered images can work synergistically, creating a whole that is more than the sum of its parts. The simplest superimposition uses just two images: say ducks plus nurturing fox. The next level of complexity keeps the duck image constant, while supering successive images of the fox.

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