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Computer Editing: Basic Compositing

Computer Editing: Basic Compositing

In a field as rapidly changing as video production, few "buzzwords" generate more interest than "compositing."

Compositing is the process of layering multiple on-screen elements video, still images, text or graphical elements into a single on-screen image.
A classic example of compositing that many people would instantly recognize is the title sequence for the nationally syndicated series Baywatch. The show's graphic designers specified that video clips and graphic elements be combined into multiple layers with images appearing behind, in front of and even within large block letters spelling out the show's title.
But a composite doesn't have to be anywhere near this complicated. In fact, many of the composites seen in video are really just two separate video images combined into a single screen presentation. These simple composites (and even some not-so-simple ones!) are well within the reach of anyone with even the most basic computer-based video production systems.
Before we look at ways to create your own simple composites, let's take a look at some basic compositing terminology.

Talkin' the Talk

In compositing, the term layer refers to a plane of video or graphics. A good way to visualize layers is to imagine them as pages stacked on top of each other on your computer desktop.
In most editing programs these layers correspond to the program's video tracks. The lower tracks appear further back on the screen, and each higher video track is stacked in front.
Another important concept to compositing is the use of mattes (pronounced "mats"). Mattes are the electronic equivalent of cutting a shape out of a piece of cardboard and holding the results over a picture. If you use the hole in the cardboard as the matte, the background image shows through the hole. If you use the cutout piece, you block the background picture with the shape of the cutout. Electronic mattes work the same way.
Most videographers are familiar with wipes as a way to replace one picture with another when editing between scenes. In compositing, a slowly or partially executed wipe can create the same effect as a matte.
Alpha channels are a way to make an image or an area of an image so that it is partially transparent. You can also use alpha channels to define picture borders so that they transition smoothly from opaque to transparent. An alpha channel matte uses shades of gray to represent the amount of transparency between the foreground and background images.
With feathering, an automated alpha channel process, the edges of an image are gently transitioned from opaque to transparent to avoid a hard line of transition showing up between the image and content on an adjacent layer.
Okay, now that you've got some of the basic terminology down, let's look at some examples of simple composites.

Down to Basics

Sometimes beginners get confused when they try to execute a composite by simply stacking two scenes on top of one another in their timeline. The result is typically not a composite but a regular single image. The problem is that, if both scenes are "full screen," the picture on the highest track will totally block any scene behind it.
In order to create a composite, the picture closest to the screen must be manipulated to allow the scene (image) behind (or below) it to show through.
There are dozens of ways to do this. The foreground image can be moved partially aside, cropped, shrunk or made wholly or partially transparent. In one of the computer editing world's most useful tricks, you can create a matte that allows all or part of the background picture to peek through the foreground layer.
That's exactly why mattes are such an important part of compositing. They allow one track to show through to another in precisely the way you want.

Familiar Effect, New Twist

By using the same transitions that you typically use between scenes, but freezing them in a state of partial completion, you can create some simple video composites.
How many times have you seen the traditional split-screen used to bring together two sides of a telephone conversation in a classic movie? That's a basic composite. All that's needed to create this kind of scene is to set your transition controls to execute a common wipe and set the start and stop values to take place at the same physical location. By the way, when you plan to do this kind of effect, it's important to pay close attention to the framing of your two shots when you shoot so that you leave plenty of negative space in the areas that will be covered by the other shot.
A cross-dissolve, stopped when half completed, is an equally effective form of simple composite. It results in a dreamlike combination of the entering and exiting scenes.
Again, you'll need to pay special attention to the framing of your picture when you record each scene. If you do, this simple type of composite can communicate some pretty powerful emotions.
Imagine an establishing shot of a woman washing dishes at the kitchen sink, then move to a head-and-shoulders shot framed with her looking out the window. Now partially dissolve in a scenic shot of a cruise ship sailing the blue waters of the Caribbean! The resulting composite clearly suggests that her thoughts are far away from getting the silverware clean.
Other classic "switcher effects" are equally composite-friendly. Take the picture-in-picture, for instance. With multiple tracks easily accessible in most of today's editing software, it's a snap to composite two scaled-down pictures onto one master shot. If the master shot is a traditional head-and-shoulders close-up, you can use soft-edge oval mattes on two additional shots (placed on higher tracks than your head-and-shoulders shot) to position a pair of "good conscience" and "bad conscience" characters over your main subject's shoulders. You'll have shades of the "devil-and-angel" gag that's been a staple on the Conan O'Brien late-night TV program.
This is also a great place to use the "edge feather" feature that most modern computer editors provide. Instead of oval-shaped insets with hard edges looking like a pair of antique photos, a softly feathered edge will help to integrate the two mattes into the overall composite image.

All Keyed Up

If you want to take the same multiple-picture composite idea a step closer to professional quality, spend some time investigating your software's chromakey and lumakey capabilities.
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.
In blue-screen or green-screen compositing, video scenes are recorded with the characters appearing in front of an evenly-lit green or blue surface, then the software replaces the key-colored pixels with live video or a specified computer-generated background.
Keying is one area where you need to experiment in order to get good results (see Shooting for Chromakey sidebar), but once you achieve a decent key shot, a world of creative options emerges.
For amateur weathercasters or kids who want to suspend their model spaceships over a field of stars, chromakeying or lumakeying can be just the ticket. An effective chromakey setup can take those competing angels in the above example and matte out the backgrounds to let them appear to float over the master shot, without the background oval shapes spoiling the effect.
A close relative to chromakeying is lumakeying, which pulls mattes using brightness rather than color values. A somewhat dark scene with one character appearing in a bright white T-shirt could be a likely candidate for a lumakey composite. Using this technique you can do anything from simply turning the T-shirt the precise color you choose to something as wild as allowing a shot of ocean waves breaking on the shore to replace the area of the T-shirts. Creatively, the sky's literally the limit.
And that's the real magic of compositing. It frees you from the need to show your audience only what your camera can record at one moment in time. Composites can be simple and realistic, or invoke multiple layers of live video, motion graphics, titles and effects.

Have Some Fun

Now that you know some of the secrets of compositing, watch for examples in the shows you see on television. You can expect to find scores of good examples in the openings of network entertainment programs and even in nightly local newscasts. But what you might not expect is that many of the seemingly sophisticated effects you'll see are not far from the capabilities of your own NLE program.
All that's required is that you master the tools you already have! So whether your goal is to get your audience to believe that two or more composited elements are actually part of a single scene, or juxtapose two events so they seem like they're occurring at the same time, or just to have some fun with this cutting-edge technology, compositing is a creative joy. So dive right in, start using these compositing techniques and let the fun begin!

Tags:  August 2001
Wed, 08/01/2001 - 12:00am