Buzz
Blue and green screens are fantastic tools for crafting diverse backgrounds in videos or films. peng song / Getty ImagesHow do filmmakers achieve the impossible in movies and make it appear absolutely real? For instance:
- In the film ET, how did they create the illusion of reality when the boys' bicycles started soaring through the air?
- In Star Wars, how did they make it seem entirely real when Luke piloted his X-wing fighter through the Death Star's trench, chased by Twin Ion Engine fighters? The X-wing, TIE fighters, and the trench were all meticulously crafted models...
- In Return of the Jedi, how did they achieve the realism of Leia and Luke racing at 100 mph on their speeder bikes through the dense forest?
- In Back to the Future, how did they make it look utterly real when the DeLorean car lifted off and flew down a suburban street?
- Even on the TV news every evening, how do they create the illusion of the weatherperson standing in front of a dynamic, animated weather map filled with computer-generated graphics?
In all these instances, the magic is brought to life through a special effects method called traveling matte or blue screen. This approach enables actors and miniature models to be placed in entirely fantastical scenarios—whether aboard spacecraft, hanging precariously over vast canyons, or soaring through the skies like Superman—while appearing utterly realistic on screen. The technique is so widely used today that it often goes unnoticed. News anchors seem to report from remote locations when they’re actually in a studio, and entire TV show segments can be crafted to give the illusion of on-location filming, even though the cast and crew never left Los Angeles.
This article will delve into the intricacies of the blue screen technique, revealing how these diverse scenes are meticulously brought to life.
Static Mattes
The initial scene: Actors standing on a serene plain on a pleasant day. Not exactly eerie.Mattes have been a staple in the film industry for decades, used to craft stunning special effects. One of the simplest effects can be achieved using a double-exposure matte. Here's how this method works.
Imagine a director wants to create a chilling scene where actors traverse a vast, flat plain under a sky churning with ominous clouds. To achieve this, the cameraman first films the actors on the plain. During this shot, a piece of black paper or tape is placed over the lens to mask the sky area, leaving it unexposed on the film. The scene is filmed normally, but only half of the frame is exposed in the camera. The film is then rewound, and the exposed portion is masked with black paper. Next, the camera captures footage of stormy clouds, possibly filmed at a slower speed to make them appear to boil dramatically when played back at normal speed.
Here’s a breakdown of the process:
The sky is masked using black paper placed over the lens, ensuring the sky portion of the film remains unexposed during the initial shot.
The film is rewound, and a dark, stormy sky is captured with a matte covering the previously exposed section of the film.
Once the film is developed, the two shots merge seamlessly into one, creating an eerie effect.There are two popular variations of this technique:
- The sky could be computer-generated instead of being filmed from an actual outdoor setting.
- The two scenes might be filmed separately on different reels and then sent to the special effects team to be merged onto a third reel using a process called optical compositing. The two reels are projected frame by frame onto the third reel using a precise compositing machine. Alternatively, in a digital studio, the two reels are digitized, combined frame by frame in the computer, and then printed onto a third reel using a film printer.
This type of matting is one of the oldest and most trusted special-effects techniques in the film industry.
Traveling Mattes
Footage of the actress hanging from a rope in the studio, filmed against a blue screenImagine you’re a director aiming to shoot a scene where the actress is suspended from a rope above a vast river gorge, desperately calling for the hero to rescue her. As the director, you have a few choices:
- If the actress is willing, you could have her actually hang from the rope. However, most actors and actresses are too valuable to risk in such a dangerous scenario.
- You could hire a stunt double to replace the actress and film the scene from a distance, making it hard to tell it’s not her. However, this approach sacrifices the emotional impact of a close-up shot of the actress’s terrified expression.
- You could employ blue screen photography to create the illusion that the actress is dangling from the rope. Alternatively, you might use the blue screen for close-ups and the stunt double for wide shots, combining the best of both techniques.
The blue screen technique allows you to merge two or more film segments into one, producing a highly realistic final result.
To employ the blue screen technique, you begin by filming the river gorge on location. This footage is referred to as the background plate. Next, you film the actress hanging from a rope just 2 feet above the ground in a studio. Behind her, you position a vivid blue background screen (which gives the technique its name). The result is two separate film segments that appear as follows:
Footage of the river gorge, commonly known as a "plate"In the special effects department, specialized filters are used to create two mattes from the actress's footage. One matte displays her silhouette in black, while the other is its inverse, as shown here:
The matte showcasing the actress's silhouette
The inverse matteCreating these mattes is straightforward because the bright blue background, when processed through a red filter, turns black. By using high-contrast black-and-white film to produce the mattes, you can generate the silhouettes. Now, you have four film segments: the two original shots and the two mattes. By layering these pieces, you can craft the final shot. First, merge the background with the actress's silhouette:
Next, rewind the film and re-expose it to place the actress into the "hole" formed by the matte. The actress now appears as follows:
The final shot looks like this:
This process is known as a traveling matte because the matte changes for every frame of the film. In contrast, a static matte involves placing black paper over the lens, creating a single matte for the entire shot. For a traveling-matte shot, a new matte must be created for each frame to match the actress's movements. While it’s possible to create these mattes manually, it’s incredibly time-consuming. The blue screen behind the actress simplifies this process, allowing the mattes to be generated automatically using optical or digital methods.
The blue screen technique is also widely used in science fiction films like "Star Wars" and "Star Trek" to make spacecraft models appear realistic. The models are filmed separately against blue backgrounds and then combined in multiple layers to produce the final footage. Some scenes involve hundreds of layers to achieve their complexity.
For a blue screen shot to appear realistic, several key factors must be considered:
- The actress (or model, in the case of spacecraft) must have the appropriate level of diffusion to blend seamlessly with the background. You’ve likely noticed poor blue screen effects in TV shows where the foreground actor appears sharp while the background is blurry, making the scene look obviously fake.
- The blue screen’s color must not reflect onto the actor or model. If it does, a blue "halo" will appear around the edges, ruining the effect.
- The actor should avoid wearing anything blue, as it will appear as a transparent gap in the final shot.
The next time you watch a movie, you’ll know how some of these seemingly impossible scenes are created—but you’ll still be amazed by how authentic they look.
