Bluescreen shots are slowly becoming a standard in most movies. In several instances, a set is too expensive to build, impossible to construct, or for some other reason, a matte painting or computer generated scene is used instead. This leaves the filmakers with the task of compositing the actors into the scene, without any traces of the background they're acting against.
In the simplest terms, a bluescreen is just what the name describes; that is, a large blue background set behind an actor as he runs through his motions. With varying televisions displaying what the actor needs to react to, he/she will pretend like they are actually in the scene. The crew needs to be very careful not to get a blue glow on the actor, or to let the actor's clothes match the background. Once the scene is captured on film, it's up to the computer graphics people to do their stuff.
Once the film is transferred to the computer using special hardware, the artists need to do three basic things. The first is the simplest and most straight-forward. In order to replace the bluescreen, the animator must first isolate the actor from the rest of the shot. The programs used are very specific in their tolerance to color, so that only the blue is dropped, and the rest of the shot remains to be worked with.
The second step is also very simple. After isolating the background, the animator can then replace it with the painting, film frame, or CG shot. Rather than pulling the actor from the frame it is much simpler to drop the background down in place of the bluescreen. The programs used by special effects companies will allow the background to remain constant throughout a single camera shot.
The first two steps are nothing special. The real magic comes with making the background and foreground work with one another to make the shot believable for the viewing audience. Little things need to be manipulated to make it appear as though the actor is moving in the background. Little details require special attention. Shadows on the bluescreen and glare coming off the screen can distort the background. The background also needs to appear in focus. If the back is too sharp, it is incredibly obvious that the image is composited.
Lighting is always a large problem. The crew on the set needs to take careful consideration when placing the lighting for the actor. If, when composited with the background, the actor is lighted from below, and the scene is lit by sunlight from above, then the animators have some very serious work ahead of them. Attention to details is crucial when faking reality, and even the smallest things can ruin the illusion.
Examples of bluescreen work can be found in many areas this summer. The best example is Columbia/Tristar's Multiplicity. The special effects in this film center around the creation of multiple clones of Michael Keaton. To accomplish this, Keaton was required to perform the same scene several times from the perspective of each clone. Many bluescreen shots were combined with several takes of Keaton on the set.
Greenscreens, the same as a bluescreen only green, were used to help Micheal bump chests with his clone, a shot that can be seen in virtually every trailer. After performing once on the set, Keaton then jumped to the greenscreen, where he bumped chests with an actor covered in a green spandex suit, the same color as the background. The stand-in and background were removed, the previous take dropped in, and Michael was left bumping chests with his clone.
Bluescreens were also used with another large movie, Forrest Gump, to let Tom Hanks interact with the past. After filming Hanks in front of a bluescreen, his shots were then edited to match the film quality of the old film, then composited with the background. Small changes in the hands and faces of the presidents made it appear as if Hanks was shaking their hands, and even talking with them.
[Image courtesy of the VFXHQ]
Other applications of bluescreen technology exist outside of major motion pictures. Educational shows on TV commonly place a narrator into a picture as he/she describes what is happening in the scene behind him/her. Also, no weather report would be complete without the forecaster pointing out the wind currents on a map. The forecaster is filmed against a bluescreen, and they watch a TV displaying a map to let them know where they're pointing. A computer composites the shots live on the air. However, there is no the level of attention to the details as is displayed in movies, so the shots are very obvious in their origins.
So computers are used in what is a combination of past and present technology. Models and acting, standards for many years, can now be taken to another level with computer graphics and the constant supply of new programs that attempt to falsify reality.
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Last Updated August 22, 1996