|Dana Lee stars in the title role of Matt Power’s Throg.|
Are you shooting a scene set in the Louvre museum in Paris, when all you have to work with is a high school corridor? Does your story include characters who fly, mountainous vistas, spaceships or headless monsters? If so, this article could change your life (assuming that moviemaking is your life). If you’re aiming for visual impact at Dollar Store rates, chromakey techniques can help disconnect your checkbook from your imagination. An understanding of the use of “chromakey” to remove backgrounds will let you make the most of your digital footage.
What is chromakeying?
It’s the use of a blue (or green) background behind your talent to create transparent areas in your video footage. That background could be a blue curtain (also known as a bluescreen), or a painted blue wall. You shoot the scene on video, then capture the footage with your computer. Next, you tell your editing program to remove the blue background. Then you replace those missing areas with your own pictures or 3D objects. Voila: virtual reality.
|Using simple techniques, such as hanging blue curtains in an old gymnasium, Power makes digital video look like more than the the sum of its pixels.|
About four years ago, a few talented friends and I set out to make our first feature movie, Throg (www.throgthemovie.com). The movie, which is now entering the festival circuit, follows the bizarre life of an immortal idiot who bumbles his way through the centuries, leaving carnage and disaster in his wake.
When we sat down in the summer of 2000 to outline Throg, we decided not to worry about location cost. Throg had to look and feel like a scaled down version of the visually rich fantasy movies it spoofs, such as Highlander and Excalibur. To pull that off, we came up with an exotic list of locations, including Mount Olympus, a 19th-century insane asylum, castle exteriors, Little Big Horn and Hastings, England.
Given our nonexistent, credit card-based budget, we couldn’t fly an actor to England for a cameo—we had to bring England to the actor. This quickly led us to an interest in chromakeying techniques for creating virtual settings. After a lot of research, we began to collect all the gear we would need.
To get started, we purchased a couple of used Sony prosumer Mini-DV cameras on ebay, a VX-1000 and TRV900. Our non-linear editing suite consisted of a Windows-based computer editing system with a Matrox 2000 capture card, Adobe Premier and Adobe After Effects. We would later add other software to make the chromakey process easier.
10 DV Keying Tips
1. Hair is the enemy. Avoid wispy hair, especially blonde hair, unless you’re shooting extreme closeups. Consider hats or pulled-back hairstyles.
2. Add a fan. Slight air movement in a keyed scene helps sell the illusion, especially if your composite will be an outdoor setting.
3. Use real ground surfaces. Keying out floors is possible, but lighting them without shadows is extremely tricky.
4. Stay medium or close. Reserve long shots for very short cutaways with minimal movement.
5. Avoid camera movement. If you must move, be sure to add white dots to your blue background to provide locking points for motion tracking software in post.
6. Use diffusion gels (translucent material used over theatrical lighting to spread and soften the impact).
7. If you simply can’t achieve a clean key of a certain scene in post, add wild backgrounds and pretend you planned it that way.
8. If using a bright blue sky as your key background, adjust the exposure on your camera down or use a UV filter so you don’t lose the blue color.
9. Think small. Little sections of bluescreen can help you composite smaller effects.
10. Plan costuming. Remember to avoid having variations of the color of your key in each scene.
Next, we purchased a gigantic, seamless 12-by-40 foot piece of blue fabric for about $175 from Film Tools (www.filmtools.com), a mail order film retailer. I have a love-hate memory of that fabric. It was made of a felt-like material that could not be ironed. So when it arrived with deep folds and wrinkles, they never went away. Every wrinkle cost us time later on, because the surface used as a keying background needs to be as smooth as possible in order for the computer to cleanly remove the blue areas. I have since switched over to using hard, painted surfaces for all of my chromakey work.
Lighting the blue fabric proved challenging. It had to be lit evenly, but separately from the talent. For low-cost firepower, we bought three old-fashioned “cyclorama” footlights for about $200 on ebay. (Cyclorama is an old theater term that refers to the big screen behind the stage.) These lights, which hold a series of halogen bulbs, were set at the base of that screen to light it from below. We also used about a dozen standard work lights (the inexpensive kind with the cone-shaped metal reflectors, sold in hardware stores). We hung these lights above the blue curtain, using a temporary grid made from pieces of metal conduit pipe—hollow metal tubing that can be found in the electrical department of any hardware store.
To light the actors, we purchased two T-shaped tripods and equipped each one with two inexpensive theatrical lights (about $50 per fixture). One tripod supplied key lighting (the primary lighting for the scene), the other provided fill light (the soft light used to fill in shadows). I buy all of my lighting equipment by mail from a place in Florida called SpringTree Media Systems (www.springtree.net). Peter, the guy who runs the place, is a charmer and always willing to negotiate price.
The Big Shoot
For our first shoot, we (naturally) decided to tackle a huge chromakey challenge: creating a virtual setting for the nine bored gods of Mount Olympus. We found a gymnasium in the basement of a far-flung town hall in rural Maine and hung our blue curtain in a grand arc, using our metal conduit to create curved curtain rods. We then covered the floor with patchwork pieces of blue canvas remnants from a fabric outlet to complete the blue “stage.”
We placed our actors and set pieces about 10 feet from the blue screen, to avoid blue light spilling back onto their shoulders and hair, and began shooting. (I can’t help wincing now when I think that we tried to shoot a 12-page storyboarded scene in sequence, rather than breaking it down into a shot list.)
For the most part, our hard work paid off. We got some beautifully lit characters and decent lighting on the bluescreen. But we also made some mistakes that cost us weeks of post-production time.
First, we tried to use smoke (from a smoke machine) in front of the blue curtain. One look at the video monitor told us this was a dumb idea. The smoke obscured the blue of the background, so we could not have pulled a clean key, as planned.
Next, we tried a 40-foot dolly shot, moving the camera past the whole pantheon of Gods. In Peter Jackson’s world, $20,000-a-day motion control cameras do this kind of stuff. We thought we could fake it, and add our backgrounds to the keyed out background by using After Effects tracking software later on. But we made a mistake. We forgot to mark the bluescreen with white tracking points (pieces of tape). These would have allowed us to “lock on” our background pictures in post-production so they would move at exactly the right speed to look real. Instead, we had to experiment over and over in post, until we got the illusion right.
We also made another discovery. Lighting a blue floor around nine actors and furniture without creating shadows is virtually impossible. And every shadow makes the chromakey process harder. Solution: Use real surfaces for floors, even if the rest of the set will be virtual.
Into the Studio
Raw footage in hand, the next step was to learn how to use keying software. We chose After Effects as our tool, because it’s the industry standard. But after weeks of experimenting with AE’s built-in keying features, we were still seeing lots of flickering, uneven areas around actors and props in our composites. No good. We tested several programs and settled on Primatte Keyer from Puffin Designs (now owned by Pinnacle), a $700 plug-in for AE. The program nicely compensates for digital video’s inherent low resolution. The only other program I have seen that compares in results is Ultra Key.
One thing I learned late in the game is that DV keying works best if you don’t try to get rid of every smidgeon of blue in the scene. If the background you intend to replace the blue with is dark enough, a little bit of blue haze will hardly be noticed. You’ll end up with a much better looking composite this way, keeping more details in things like hair and transparent objects such as glassware or water.
|To create a virtual scene on Mount Olympus, actors in the foreground were lit separately from the cyclorama with low-cost theatrical lights.|
With our big, fully chromakeyed scene complete, we began to see other uses for keying out small sections of a piece of footage. For example, we needed an office space with a painting hung in a frame above a roaring fireplace. Rather than rent a Victorian building, we borrowed a fireplace mantle frame and punched the glass out of an old painting. Then we stapled blue fabric inside both the painting and the fireplace. By keying out the blue fabric, we could later add a real fire to the mantle, and put a digital photo of one of the actors into the frame.
About half of the scenes in Throg include some chromakey work. To make a singing bird appear in a nest on top of an evil warrior’s head, we attached a nine-inch square piece of blue posterboard to the back of the nest. We then isolated the bird by keying out the blue sky (another handy trick). We also used keying techniques to put our own programs on Throg’s TV, to add psychedelic backgrounds to a dance on Throg’s bed and to depict Throg’s mother being hurled into space by Urshag, the ultimate bad guy. For the movie’s final outdoor scene, we shot footage of the lady of the lake standing on a sailboat in Florida. Months later, we combined that footage with a keyed shot of Throg waking up on the boat.
There’s no doubt that the keying work we did on Throg, a $35,000 movie, might not meet George Lucas’ standards. But we set our quality bar by asking the same question for each keyed scene: Do problems with the backgrounds distract from the story? When we could answer no, we moved on to the next shot. We also had the advantage of making a comedy. If an effect ended up looking a bit cheesy, it sometimes added to the humor. MM