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Video Compositing 101
Posted Jun 2, 2004 Print Version     Page 1of 4 next »
  

Compositing isn't as tough as you might think. With the right tools and some basic skills, creating video overlay effects and even "virtual sets" is easier than ever.

Compositing sounds complicated, but it's really not. Simply put, it's the process of merging two videos (or a single video and still image) to display one combined video. Compositing is also called "overlaying," as in overlaying one video over another, and "blue screen" or "green screen," which are specific compositing techniques used to combine two videos.

The simplest application of compositing is inserting a logo or other bitmap into a video. Much more exciting is video compositing, a technique used to accomplish a variety of goals, including inserting your subjects into videos of different locations, and even placing them in "virtual sets."

The production side of video overlay, both the shooting and editing, may seem functionally demanding, technically complex, and prohibitively expensive, but it's really none of the three, and non-technical, small-outfit users can efficiently and effectively accomplish both.

Key Notes
Video compositing offers far greater creative potential than still image overlay, and with a little coaching, you'll find the process of shooting and compositing video relatively straightforward. Even better, you don't need to spend a lot of money to get a good result. Let's examine what actually happens when compositing one video over another, and then move to the equipment, software, and techniques used to produce a good, clean result.

At a high level, compositing combines two videos by eliminating portions of one video, and placing the remaining portions over the other video. The background behind my image was green, so I used a process called chromakeying (also called color keying) to remove everything that's green in the video and place the remaining portion, just the video of me, over the video of the school.

What are the keys to good keying? There are three worth noting. The first is a simple rule of thumb: the object being keyed can't contain any colors that are similar to the background, otherwise, those will be eliminated along with it. So if you have a bright green background, and wear a bright green shirt, you'll look like the invisible man from neck to navel.

Second, the background has to be relatively consistent. If you have a cloth background, this means it's flat, smooth, and wrinkle free. If the background is painted, this means there are no chips, bumps, or uneven sections.

Finally, you need a clean video signal, which means a decent camera. If you attempt to shoot green-screen video with a $500 single-CCD DV or analog camcorder, you'll likely be disappointed in the results.

Now that we've got the basics down, let's take a closer look at the details.

Back Issues
Backgrounds are typically blue or green. I choose green because blue is plentiful in my wardrobe and green non-existent, but I'm not religious about it. Either color will do just fine, so long as you purchase fabric or paint backgrounds specifically designed for video-compositing applications from a reputable source. Hanging up a blue cotton sheet from Wal-Mart just won't get the job done. As you'll see, you don't have to spend a lot of money—we spent all of $20 for one of our backgrounds—but the color has to be consistent.

Your choice of fabric or paint depends upon application. Paint is probably better if you'll be shooting consistently from one location, and is cheaper from a coverage perspective, though probably not if you have to pay someone to paint the wall. Fabric is faster and more portable, and you can take it down and store it when not in use, which is lovely given that bright green and blue don't feng shui very well.

Most chromakey fabric is made of muslin, a heavy fabric that hangs very smoothly. When hanging fabric, be careful to avoid creases and folds, and if the subject will be moving, or you're shooting outdoors, secure the fabric top and bottom to avoid motion from the wind.

For true portability, consider purchasing a collapsible background. Depending upon where you buy and the size you choose, these can cost anywhere from around $100 to several hundred dollars. Two good places to check for products and prices are www.markertek.com, for name-brand products and prices, and http://stores.ebay.com/J-and-K-Group, for less well-known products and much cheaper prices.

Background Size
The conventional wisdom of chromakey lighting and positioning is that the chromakey background must be lit separately from the foreground speaker and that the speaker should be positioned between six to ten feet from the background. The obvious concern is that using one light source for both subject and background would cast shadows on the background, introducing inconsistencies that complicate the chromakeying process. Unfortunately, forcing the speaker to stand this far from the background significantly increases its required size.

However, you can place the speaker close to the background (and use a smaller screen) without compromising quality if two conditions are met. Specifically, if you have a single light almost directly above both subject and background, and limited motion in the shot, shadows won't be a problem. Since the double fluorescent lamp in my tests lit both the background and me from such a high angle, any shadows are either directly behind me, or so low that they're out of the shot. Either way, the background consistency is maintained for keying purposes.

Conversely, once you introduce motion into the scene, the background size requirements increase very significantly, with precise requirements depending upon the range of motion, height of the subject, and camera location. For most corporate or academic talking head shoots, however, you can produce very high quality with a 3x5' swatch that costs just $20.

Lighting Cues
If you go with a larger background, producing flat lighting over the complete background is critical to achieving a clean chromakey. Typically, you'll need at least three lights, one on each side and one shining down. All lights must be soft lights with no hot spots, so use either fluorescent lights or bounce the lights off of a white surface. Obviously, the lights must be placed behind the subject to avoid shadows on the background.

Then you'll need to light the subject. These lights should not shine on the background, since this could compromise its consistency. Backlighting is critical here because it helps avoid a halo effect where the background color "blooms" around the outline of the subject.

If you're attempting to insert the subject seamlessly into a specific background, consider how the background scene is lighted and attempt to match that in your studio. At the very least, make sure the primary lights are shining from the same direction so that the shadows all point in the same direction.

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