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.
Montage, Mon Amore
Classic Hollywood technique pushed this concept to its limit in the montages that were once common in movies. A montage varies the shots in both images at once and sometimes adds a third stream or even more.
Here’s a cliche example: Visual stream A shows successive shots of our heroine performing on the violin, plus audiences applauding, plus heroine bowing. Stream B shows mighty presses spewing out newspapers plus spinning front pages that stop in big closeup to reveal VIOLINIST CHARMS CROWD, and PRODIGY’S CONCERT SOLD OUT! Stream C presents recurring shots of trains roaring past or train wheels churning or station signs for different cities. Stream D shows concert posters and theater marquees, starting with PODUNK ELKS CLUB and escalating to CARNEGIE HALL, NEW YORK.
Whew! In about ten seconds, you’ve told the audience all they need to know about the rise of Helga Hergesheimer from unknown fiddler to world-class star. Though montages fell out of fashion some years ago, they may come back because multiple image streams are so easy to create in digital editing. Even if you have only two visual tracks on your editing timeline, you can combine and save two streams at a time, and then re-combine A+B, C+D, E+F, and so-forth, without losing image quality.
Beam Me Up, Scotty
Superimposition is also the key to special effects using double exposure by means of A/B-roll editing. A/B-roll editing consists of using two separate video tapes, A and B, to edit together a single video. To tape a double exposure, lock the camera very firmly on a tripod and record the shot that will form the A roll of the action. Replace the tape with the B roll cassette being exquisitely careful not to bump the camera. Record the B roll shot. In editing, you align the A and B rolls and then transfer both at once with a video mixer, creating a superimposition.
Here are just a few of the many special effects you can create with double exposure:
- Teleportation: By dissolving from an empty scene on roll A to the same scene with an actor on roll B, you "beam down" the character into the scene. ("Beaming up," of course, uses exactly the opposite technique).
- Ghosting: If you leave the teleportation dissolve at 50/50, the background will seem solid while the actor appears transparent. If the actor lies quite still in shot A, then gets up and moves in shot B, the "ghost" will appear to leave the "body" behind. In this case, it would look more convincing to make shot A, with the body, 75 percent and the ghost in shot B 25 percent. With the 75/25 mix the ghost will be very transparent while the body will appear more solid.
- Cannon shots and earthquakes: You can make the scene shake, rattle, and roll by recording shot A with the camera still and then re-recording it on the B roll while vibrating the camera with one hand. Together, the A and B shots will simulate violent shaking. Rattle the B roll at rhythmic intervals to simulate heavy gunfire or continuously to mimic an earthquake.
Strictly speaking, side-by-side A/B-roll combinations are not supers, but they deserve mention because the techniques are almost identical. For instance, to allow an actor to play a scene with himself, shoot the A roll with the performer doing role 1 on the left side, while a crew member reads the role 2 lines on the right. Then reverse the process and repeat the scene on roll B.
In editing, use a soft-edge wipe to combine the left half actor with the same actor on the right half, eliminating both shots of the crew member in the process. You’ll also need to switch back and forth between the A and B roll sound tracks, so that the actor and not the crew person does all the talking.
You can perform similar tricks by lumakeying, chromakeying and computerized matting, but these are not, strictly speaking, superimpositions. Keep in mind that these techniques do not layer visual B on top of visual A, rather they use digital processing to completely replace the parts of A that lie beneath B.
Compositing like this affords effects that traditional superimposition cannot begin to deliver. On the other hand, inexpensive compositing systems–both stand-alone switchers and computer software–tend to deliver less than perfect results. For a tried and true way to create simple and affordable composite effects, superimposition is hard to beat.