Virtual green screen sets are no longer just for the privileged Hollywood bigwigs.
New compositing technology is making Hollywood movies virtually, well, virtual (check out the latest Star Wars flick, Episode III: Revenge of the Sith for a stunning example.) The same technology is available right on your desktop. So, when we talk lighting for compositing, we don’t just mean some jolly guy with a weather map. With today’s post production software, you can do truly cool stuff.
Maybe the biggest innovation is the elastic background. Back in the medieval era (a couple years ago), the colored screen behind your subject had to fill the entire frame, so that the replacement background composited onto it would do the same. Now, most decent editing software packages will let you take an incomplete colored background and digitally stretch it to all four corners. All you have to do is ensure that your subject, at least, is surrounded by the key color. This means you can get great results even with small screens and tight shooting areas.
Even the best-lit backdrops are not perfectly uniform, and the matte quality degrades where the background is slightly lighter or darker. Specifying a range of values reduces the overall quality of the composite. Today, you can move your cursor over specific imperfections in the background displayed on your monitor and instruct the software to ignore them.
Today’s compositing programs (or plugins) are better at handling fiddly bits like the edges of hair and transparency, such as glasses on your subject. With some software, like ULTRA from Serious Magic, you can composite subjects into virtual sets in which they can (seemingly) move from place to place. But even though compositing software is much improved, the process still takes careful lighting technique to work perfectly. Let’s start with the basics.
Good composite lighting still depends on two basic principles: light the background evenly and light the subjects separately.
Big umbrella lights, soft boxes, or pans all deliver broad, even light. Often, one of these large-source lights on either side of the background will do the job. If you do a fair amount of keying, consider fluorescent pans: they’re very large and soft, they emit little heat, and they don’t guzzle power.
You can even mix them with halogen lighting for your subjects because their different color temperature won’t matter if you observe rule number two: light the subjects separately. If you set white balance for 3200K, the background will look bluer; but remember that the software can lock on any tint, so the slight color shift won’t matter.
To separate a subject from the backdrop, spot them as far out in front of the background as the space will allow. Again, if your software is up-to-date, you can shoot off the edges of the backing, as long as the subject is surrounded by the key color.
Spotlights work well with foreground subjects because their directional beams can be aimed away from (or masked off) the color backing. To help do this, place both key and fill lights more to the sides than normal (around 5 and 8 o’clock on the standard lighting floor plan with the camera at 6 and the subject in the center).
How high you set the lights depends on your subject-to-backing distance. If you must keep them close, elevated light positions will tend to splash on the floor rather than the backing. If you have the luxury of good separation, normal instrument heights are fine.
Highly saturated backgrounds are very large sources of reflected light of exactly the green or blue color that you want to keep off your subjects (lest they start turning transparent). If you are shooting close to white or light colored walls, move away if you can and mask them off if you can’t. If the subject is standing on unrolled seamless paper backing, the floor itself can bounce colored light. To solve the problem, mask the floor with a black drop cloth (or paper) placed in front of the subject, as near the feet as possible without showing in the frame. If you move in for closer shots, consider rolling up the part of the seamless backing that has been on the floor.
Another caution: avoid excessive rim lighting, (I try to use little or none at all,) because the resulting hair sparkles can give the keying software fits. If you can feed the camera signal into a computer right there at the shoot, which includes the keyed-in background, you can often move the backlight up or down until it and the software can live together happily.
If you follow these guidelines, your composites will be technically successful, with clean edges and no dropouts. The next step is to make them equally effective as lighting designs. To put the subject into the background instead of just in front of it, you need to match the background’s lighting.
Wherever possible, set up your editing rig at the shoot, so that you can make trial composites in real time. That way, you can look at the background lighting as you adjust the foreground to match it. Before you even start, ask yourself about the basic character of the background lighting. Is it:
- Hard or soft: does it appear to be lit by spotlights or does it have a "natural" look?
- Indoors or outdoors?
- Day or night?
- High key or low (high-key lighting has a low key-to-fill ratio or low contrast between dark/light, low-key lighting is the opposite)?
- What is the color temperature: warm or cool, candles or fluorescents?
When you’ve answered these questions, you’re ready to match the background lighting.
First of all, match the position of the key light source so that the shadows fall in the same direction. Nothing looks worse than a subject with shadows on the right side in a background with shadows on the left.
Next, match the color temperature, as noted. Don’t forget: post production software is great at changing overall color tints but not so hot at adjusting separate parts of an image. If light or lens filters don’t seem to do the job, try adjusting your camcorder’s white balance while observing your monitor.
Finally, add some effects to sell the gag. If the background is a forest dappled with sunshine, move a leaf cookie (a.k.a. cucalorous- random pattern of cut-outs that form shadows when placed in front of a light source,) around in front of the subject’s key light to simulate the effect of leaves. If the background is a highway at night, rake some moving spots across the subject to fake passing headlights.
And what about those virtual sets in which subjects can "move" around in? Typically, the foreground lighting gives them away. Framed in medium shot, a performer simulates walking, while the computer-generated background changes. But if the light on the subject never changes, the effect is bland at best and unconvincing at worst.
To simulate walking in and out of key lights, try this trick: make a good-size (at least 12" square) flag out of neutral density filter gel. Start with ND-3 and add layers until you get the desired effect. As the subject "moves," slowly wave the flag into and out of the path of the key light. If the virtual subject movement is to the right, move the flag from right to left, then bring it back (outside the light path) to the right side and move it across again. The subject will appear to be moving past a succession of lights.
You’ll be tickled with the effect.
Contributing Editor Jim Stinson’s book, Video; Digital Communication and Production, is just out in a second, updated edition.