Chroma key is a technique for compositing two images or video streams together based on color hues, or their chroma range. Jim Henson used this method with the Muppets by way of dressing performers all in black to operate characters that would appear to exist unassisted against a backdrop.
On the documentary Inside the Labyrinth, Henson makes note of this style of puppetry, saying that "velvet is probably the blackest fabric that you can use, so that's what they had to wear." If the lighting is just right, the performers dressed in black velvet become almost invisible when in front of a background of black velvet. Often times, this background is then replaced by superimposing another background on top of it. This allows a character to be seen from head-to-toe, and move about freely, without any puppeteers visible. Henson described this process in the same documentary, explaining, "We shot the puppets first with a computerized camera and then we removed the black velvet and shot the background, using the computer to make the camera move at exactly the same speed and so forth. Then all of these elements were put together later in the laboratory."
This style of puppetry is based on Bunraku, which is a type of puppetry founded in Osaka, Japan, in 1684. Traditionally, a Bunraku puppet is operated by three puppeteers. All three are dressed in black, but only one puppeteer shows his face during a performance. This is the performer who operates the puppet's head, a privilege which can take twenty to thirty years to earn. 
In an interview with Jim Henson, it was suggested that Bunraku was an inspiration for the Muppets. Henson corrected the statement: "No, I don't think so. ... Bunraku is a marvelous and fascinating art form and puppetry form but, basically, I knew nothing about it until I had been working for a number of years myself."
A form of Czechoslovakian puppetry also involves performers that dress all in black. They perform in front of a black curtain with brilliantly colored puppets that are lit from the side and the top. This makes the puppeteer almost invisible from the audience. This style of puppetry is part of black light theater, and it has been used various times by the Muppets and The Jim Henson Company.
Both of these forms of puppetry have also been used against a blue or green screen. At times multiple performers are used to operate a single character. Other times, only a single performer is needed to achieve the desired effect.
This form of puppetry is very frequently used on Sesame Street, especially during the show's Elmo's World segments. Dorothy imagines Elmo in various settings where Elmo's full body must be in view. In such instances, the puppet is referred to as "Active Elmo."
It has also been used in creating the Sesame Street openings since 1992, and for a time helped to create the "Journey to Ernie" segments. These techniques are used to help Bert dance when he's "Doin' the Pigeon," and brought Baby Tooth and the Funky Funk to life.
See Also: Elmo Variants
These techniques were used numerous times on The Muppet Show, such as when Gene Kelly taught Kermit how to dance and the "Friendship" number from episode 519. Certain characters on the show such as the Clodhoppers, the Gawky Birds, and the Bossmen are almost always performed against black velvet, enabling them to dance with that week's guest star. Brian Henson discussed the way they're operated in his introduction for episode 424.
On occasion, sketches on The Muppet Show which used “Bunraku” techniques did not always effectively hide the puppeteer. During the “Disco Frog” and “Octopus' Garden” numbers, performers briefly become visible on screen.
Movies and Specials
For The Muppet Movie, an elaborate sequence using Bunraku-style puppetry was staged in which Kermit and Fozzie would dance their way out of a jam at the El Sleezo Cafe. The end result appears almost effortless.
In the documentary Inside the Labyrinth, Jim Henson explained the process involved in developing the wild antics of the Fireys: "When we first came up with these characters, we didn't really know how they would move. We had to put a series of puppeteers working with them just to figure out how they would move, what they were capable of. We tried manipulating them in different ways... We found each way of operating them would create different kinds of movement." At least three puppeteers were used on each of the Fireys to create their movements.
For the Creature Shop film The Adventures of Pinocchio, several different puppets were built to create the illusion of a puppet who had cast off his strings. One of these, considered one of the most difficult puppeteering tasks on the film, was a rod puppet based on Bunraku puppets. Up to five puppeteers would perform the character, oftentimes each puppeteer would be responsible for only one limb at a time. This version of the character was shot against a blue screen with the control rods all painted blue and the puppeteers dressed in blue body suits. Director Steve Barron spoke of the advantages of this puppet, explaining, "John Stephenson told me that we'd have to get the puppeteers as close to the body of the puppet as we could, and he was right. We had them six inches away from a leg or arm, and their performances were superb. But as we gave them longer rods or cables, then we lost all the subtlety." 
The Jim Henson Hour special The Song of the Cloud Forest marked the first time that the Henson Company shot an entire half hour show without a background. Instead, the set and all performers were hidden in black velvet. The velvet backdrop was then replaced, using computer technology, with a bright, colorful rain forest. 
Appearances and Commercials
- ↑ The Muppets Make Puppets!, p. 105
- ↑ Muppet Central Interview with Jim Henson
- ↑ Sesame Street: A Celebration - 40 Years of Life on the Street p. 194, 195
- ↑ No Strings Attached p.161
- ↑ Behind the Scenes of "Song of the Cloud Forest"