Shooting for Compositing

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At a high level, compositing combines two videos by eliminating portions of one video, and placing the remaining portions over the other video. For example, in Figure 6.1, we'll combine the video on the left over the video in the middle to create the video on the right.

Figure 6.1. Compositing takes one video (on the left), eliminates the chromakey color, and imposes the remaining video over a background video (middle) to produce the composition on the right.


You can't see it in Figure 6.1 (because the book isn't in color), but the background behind the image on the left is solid green. When compositing the two videos, I'll use a chromakey filter to remove everything that's green in the video on the left and overlay the remaining portion- just the video of me, on the video of the building in the middle.

What are the keys to good keying? There are three I'll briefly mention and then cover in detail below. The first is simple: the object being keyed can't contain any colors that are similar to the background video; otherwise, those colors will be eliminated along with the background. 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 use a cloth background, it must be flat against the wall, smooth, and wrinkle-free. If the background is painted, there should be 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 an inexpensive DV or analog camcorder, you'll likely be disappointed with the results.

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

The Background

Chromakey 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 $20 for one of our backgrounds but the color must be consistent.

Your choice of fabric or paint depends on the application. Paint is better if you're shooting consistently from one location, and is cheaper from a coverage perspective, though probably less so 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; this is ideal, 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 your fabric, be careful to minimize creases and folds. If the subject will be moving, or you're shooting outdoors, secure the fabric at the top and bottom to avoid motion from the wind.

For true portability, consider purchasing a collapsible or portable background. Depending on the size of the background and where you buy it, the cost can be anywhere from around $100 to several hundred dollars. Two excellent resources to compare 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 at much lower prices.

Background Size

The conventional wisdom of chromakey lighting and positioning is that the chromakey background must be lighted separately from the foreground speaker and that the speaker should be positioned six to ten feet in front of the background. The concern here 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 10 feet from the background significantly increases the required size of the background.

Figure 6.2 shows the background I used for all of the chromakey tests shown in this book. I bought it at markertek.com for about $20. In most shots, I stood or sat about four feet in front of the wall, and shot with a tight frame to eliminate all but the green screen. I'm assuming that most readers won't have access to a green-screen studio and will have to create a similar ad hoc background.

Figure 6.2. Doesn't matter if your chromakey studio cost $20 or $2,000; if it's done right, it looks like a million bucks!


If this describes you, take heart the size of the background screen isn't anywhere near as important as lighting the screen and the subject correctly. So without further ado, let's move on to lighting.

Lighting

The critical point to remember when lighting for compositing is that there are two distinct issues, as shown in Figure 6.3. First, you have to light the background as evenly as possible to produce the most consistent image; for this, you use background lights, as shown in the example. In most instances, I prefer soft lighting over hard lighting because it's inherently more even.

Figure 6.3. Background lights illuminate the background, while key and back lights illuminate the subject. I like flat lighting for compositing, which is why there are two key lights.


Second, you have to light the subject without casting visible shadows on the background. Unlike background lighting, which has to be flat, you can use three-point lighting (key, fill, and back) on the subject, with the desired shadows and modeling.

However, when shooting in close quarters, it can be challenging to use the hard lights necessary for three-point lighting without some light spilling over onto the background. For this reason, you may find you prefer flat lighting with soft lights (that's why both frontal lights in Figure 6.3 are key lights). In either case three-point or flat effective backlighting is essential to create the crisp edges necessary for a high-quality chromakey.

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 scene's lighting in your studio with the chromakey screen. 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. For example, if the sun is streaming in from the left in the background, makes sure your key light comes from the left.

When I'm producing video for chromakey in my office, I use the fluorescent lights shown in Figure 6.2 to light the background. While the light is slightly brighter at the top of the background than at the bottom, this hasn't been a problem.

I typically sit a few inches in front of the fluorescent lights and use two soft lights at the key and fill position to produce flat lighting. I attach a clamp light with a fluorescent bulb to the shelving on the left and point is at my back as the backlight. Total cost in background and lights is easily less than $100, yet the results are as good as or better than some I've seen shot in expensive studios.

Camera

Even if your background and lighting are perfect, if the image acquired by the camera is noisy, the quality of the overlay will suffer. We used two cameras in our tests, a Sony DCR-VX2000 (a three-chip prosumer), and a very old Sony DCR-TRV9 one-chip consumer camera. We got very good results with the VX2000, while the TRV9 proved totally inadequate.

It's possible that some newer single-CCD camcorders can produce video that's good enough for a clean chromakey, especially if the scene is professionally lighted, which naturally helps to negate the poor low-light performance of most consumer camcorders. Under most real-world conditions, however, consider using a three-chip camcorder in the Canon GL2 or Sony VX2100 class or better.

Before shooting, white balance the camera, or set it to indoor lighting if manual white balance isn't available (even if you're compositing the video over an outdoor scene). We used automatic focus in our shots, but if the subject's sideways movements may cause the camera to go in and out of focus, switch to manual focus.

Use manual exposure controls as well, otherwise the camera will adjust exposure when minor motions like a tilt of the head or waving hands change the balance of light and dark regions in the frame. As we've already mentioned, the chromakey process works by "keying" out a color from the video and overlaying the portion of the video that's not that color on the background video. Chromakeying works best when the background color is as consistent as possible; this limits the range of colors the software has to exclude.

In automatic exposure mode, even visually imperceptible changes in exposure brighten or darken the green background slightly, expanding the range of colors the software has to exclude. This puts a heavy load on the software, and can degrade the results. Using manual exposure avoids this issue.

Otherwise, advise your subjects to avoid wearing clothing with colors similar to the background color. In addition, avoid white clothing, which can easily reflect the background color, and ask the subject to remove watches, bracelets, and other accessories that can also reflect the background color.

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    DV 101. A Hands-On Guide for Business, Government & Educators
    DV 101: A Hands-On Guide for Business, Government and Educators
    ISBN: 0321348974
    EAN: 2147483647
    Year: 2005
    Pages: 110
    Authors: Jan Ozer

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