How Blue Screens Work

Traveling Mattes

Film of the actress dangling from a rope in the studio, shot in front of a blue screen
Film of the actress dangling from a rope in the studio, shot in front of a blue screen

Now let's say that you are a director and you would like to film a scene where the actress is dangling from a rope over a deep river gorge screaming for the hero to come save her. As the director, you have several options:

  • If the actress is up for it, you can actually have her dangle from the rope. Most actors and actresses are too valuable to risk in that way.
  • You can use a stunt person to stand in for the actress and shoot the scene with a long shot so people can't tell. In doing this, you will lose the emotional effect of seeing a close-up of the actress's face as she screams.
  • You can use blue screen photography to make it look like the actress is dangling from the rope. Or, you might use the blue screen for the close-ups and the stunt person for the long shots to get the best of both worlds.

The blue screen technique lets you combine two or more pieces of film into one piece that looks very real.

To use the blue screen technique, you first film the river gorge on location. This shot is called the background plate. You then film the actress dangling from a rope 2 feet off the ground in a studio. Behind the actress in the studio you place a bright blue background screen (hence the name "blue screen"). You end up with two pieces of film that look like this:

Film of the river gorge, known as a "plate" Film of the river gorge, known as a "plate"
Film of the river gorge, known as a "plate"

In the special effects department you can easily use special filters to form two mattes from the shot of the actress. One shows the actress's silhouette in black, and the other is the reverse, like this:

The matte of the actress's silhouette The matte of the actress's silhouette
The matte of the actress's silhouette
The reverse matte The reverse matte
The reverse matte

These mattes are easy to create because the bright blue color, when run through a red filter, turns black. By using high-contrast black-and-white film to create the mattes, you can create the silhouettes. So now you have four pieces of film: the two originals and the two mattes. By combining these pieces of film in layers you can create the final piece of film for the shot. First, you combine the background with the actress's silhouette:

Then, you rewind the film and re-expose it to lay the actress into the "hole" that the matte created. The actress now looks like this:

And the final shot looks like this:

This is called a traveling matte because the matte is different for each frame of the film. In a static matte, you simply tape black paper over the lens and that single matte is the same for the entire shot. In a traveling-matte shot, you need to create a matte that is exactly the same shape as the actress. In each frame, the actress moves, so a new matte is required for each frame. It is possible to create these individual mattes by hand, but it takes a tremendous amount of time. The blue screen behind the actress makes it easy to create all of the mattes automatically using optical or digital techniques.

The blue screen technique is also used extensively in science fiction films such as "Star Wars" and "Star Trek" to make the spacecraft models look real. The models are filmed separately on blue backgrounds and then combined in multiple layers to make the final film. Very complex shots with hundreds of layers have been created.

In order for a blue screen shot to look convincing, several things are important:

  • The actress (or model, in the case of space ships) has to have the right level of diffusion to match the background. You have probably seen bad blue screen techniques used in TV shows where the foreground actor is very crisp and the background plate is diffused. You immediately know it is fake because of the mismatch.
  • The blue screen's color cannot reflect onto the actor or model. If it does, the actor acquires a blue "fringe" around the edges that looks very bad.
  • The actor cannot wear anything blue -- blue will show up as a hole in the actor!

With computers, blue screen shots are even easier because the computer can create the mattes and combine the shots automatically. Many of the links below describe different digital techniques.

The next time you go to a movie, you'll understand how they make some of these impossible shots -- but you can still be amazed at how real they look.

For more information on blue screens and other special effects techniques, check out the links below.

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