Chroma Key

More commonly known as green screen or blue screen (though that one also has a different meaning), the process by which a subject filmed on a camera can be seamlessly inserted into a scene generated by other means. It relies on filming the subject in front of a solid-color background—any color will do, so long as it's not used in the foreground—and adjusting the editing system to replace that color with the background signal.

The main methods of controlling the background for live-action shots are, in increasing order of technological sophistication:
 * Finding one that already exists, and film on location.
 * Build a set.
 * Film the background separately, and project it onto a screen behind the actors while filming, typically via rear projection.
 * Double-exposing the film, which results in a slightly transparent foreground but is cheap.
 * The old analog Matte Shot, done with precise blocking of the camera frame.
 * Chroma Key.

The background inserted via Chroma Key can be any visual image. CGI is the most common today, but it can be other live action footage, models, stop motion or cel animation just as easily.

The color used is now entirely arbitrary. Blue was a popular choice in the early days of color motion pictures, because it is complementary to the reds found in human skin. Green became popular because digital editing systems can isolate green with less light in the background, and because lime green is less common than bright blue in costuming Magenta is sometimes used, as is black, but the latter is problematic, as it's almost impossible to shoot a person without having some black visible on their person, in eyes or shadows.

If any part of an actor or prop is colored the same as the background, that part will disappear. Thus, sometimes the background color is chosen because of the colors to be used in the foreground action. The original run of Doctor Who, for instance, used green or yellow backgrounds even when blue was the most common color at The BBC, because a large number of its effects shots involved the TARDIS, a timeship that takes the form of a blue police phone box The problem with using yellow was that foreground objects and actors always had a prominent yellow fringe around them. Normally, wardrobe and prop designers simply avoid using greens in the capture range, but this is not always possible; you'll occasionally see bloopers where weather forecasters have part of the meteorology map show up on their ties, for example.

The invisibility effect can be used intentionally to allow a performer, or part of his body, to interact with props while remaining unseen. A garment that can be used for this purpose is a one-piece jumpsuit in the background color, with a full-face mask, and a mesh eye piece, called a "gimp suit" or, in the case of a blue background, a "blueberry" in the trade. Performers in recent Jim Henson Productions shows have used these suits to perform with puppets without having to raise them above their heads. The suit looks like a Ninja outfit, and that is not a coincidence, as it serves the same purpose as the black outfits traditionally worn by Japanese stagehands. See notes at Ninja.

Almost all productions use Chroma Key at some point, but there are some standout examples. Also notable for causing occasional unintentional hilarity - when background and foreground are poorly matched, or the SFX budget is low, the effect is anything but seamless.

It can be fairly tricky to create a viable Chroma-Key effect, especially with amateur equipment—often, it requires fiddling with hue and saturation, and even then, there is often a faint, tell-tale 'border' around the subject where the green-screen footage and the 'real' actor don't match up.

Commercials

 * Used in Nike's The Lebrons commercials for the eponymous family based on LeBron James, as seen in this one.

Film

 * The first film to use the chroma key process was The Thief of Bagdad back in 1940. It was invented by Larry Butler, who won the Oscar for it.
 * Sky Captain and The World of Tomorrow (2004) had almost no real sets or props, relying on Chroma Key in every shot.
 * As did the film adaptation of Sin City, in order to re-create all those stylistic comic-book-style angles.
 * Mirror Mask does the same, in a very very trippy way.
 * The Movie adaptation of Frank Miller's 300 was filmed almost entirely in Chroma Key. The Movie of Alan Moore's Watchmen (by the same director) uses a combination of chroma key and traditional sets.
 * The live-action movie of Speed Racer was filmed almost entirely on green screen to give it an anime-style effect.
 * The Star Wars prequels helped pioneer the idea of minimal set design through Chroma Key.
 * The fact that only blue screen was available for A New Hope caused Luke's squadron to be changed from Blue to Red to avoid problems with blue markings.
 * Also in Return of the Jedi, the only reason Luke's new lightsaber is green is because the battle that takes place on Tatooine happens to have a bright blue sky. In some early trailers, Luke's saber is blue, but they chose to change it to green so it would show up against the sky properly.
 * In the original editions of the original trilogy, you'll notice that whenever R2-D2 is in space, his panels are painted black instead of blue to accommodate the chroma key effect. This was digitally fixed years later for the special edition versions.
 * In filming the first Superman movie, the costume had to be teal in blue screen effects, and then color corrected after the shots were composited.
 * The opening scenes of Groundhog Day demonstrate this - the woman is wearing a blue blouse when she steps in front of the chroma key camera, and all that can be seen are her head and hands in front of the satellite picture.
 * Quite possibly the best use of chroma key occurs throughout Walt Disney's Darby O Gill and The Little People. Two remarkable examples are the moments where Darby plays "The Fox Chase" on a fiddle to an audience of dancing leprechauns and, most notably, the Banshee sequence.
 * Puma Man has some very unconvincing green screen work, but that's part of its charm.
 * Bad chroma key is deliberately invoked in Cloudy with a Chance of Meatballs, when the mayor appears in front of a freeze frame of the Baby Brent Sardines commercial to promote his unveiling.
 * Tim Burton's Alice in Wonderland used this extensively. The sets seem to consist of nothing but green walls in the proper shape, along with platforms. Only things the human characters touched actually existed on the set, and most seemed to be green and were textured via CG (The Tea Table seemed to be an exception, due to the hatter walking on and knocking stuff off). The staff comments in the "Making Of" stated it was an "Odd Mix" of Full CGI (too many to list), motion capture characters with the actor's head pasted on, normal actors human (mainly Alice), and edited normal actors (the Red Queen and her giant head).
 * The body-suit version was memorably used in Star Trek: First Contact to erase half of the actress playing the Borg Queen during her entrance-in-two-parts.
 * At the Walt Disney Studios, Ub Iwerks developed the sodium vapor process, in which the actors were filmed against a white backdrop lit with powerful sodium lights. A special prism in the camera separated the image and exposed it simultaneously on two different film stocks: regular color film, which did not pick up the sodium light, and black and white film sensitive to sodium light, which created the matte. The process was used for most Disney productions, including Song of the South, Mary Poppins and The Black Hole, and was also used for The Birds and a number of Ray Harryhausen Movies. Although it provided better results than blue screen, and saved time by creating the matte simultaneously with the foreground footage, the process proved too expensive and was discontinued by the 1980s.
 * Used a bit in Who Framed Roger Rabbit?, as you can see here.
 * In the Harry Potter films, the invisibility cloak is, of course, created with a chroma key green cloak. Chroma key is obviously also used for scenes with Flying Broomsticks and so forth. As as far as sets go, the Potter filmmakers tend to prefer building real sets and usually just use chroma key to fill in scenery out a window, for example. However, there have been at least two all-CGI sets in the series, the Hall of Prophecy from Order of the Phoenix (because they couldn't do the scene where all the shelves crash down for real) and the Chamber of Secrets in Deathly Hallows, Part 2 (the original Chamber set from Harry Potter and The Chamber of Secrets was real, but wasn't saved after filming).
 * As mentioned above, Ray Harryhausen used the sodium vapor process on a number of his films. For instance, it's used to achieve some of the scale effects in The Three Worlds of Gulliver. More subtly, in Jason and the Argonauts, in the scene where Jason is talking to Medea at the stern of the Argo, you have to look closely to realize that they were filmed in the studio with location footage of the rest of the ship matted in behind them.
 * In Chitty Chitty Bang Bang, blue screen was obviously used for the Flying Car scenes. Obvious because you can see blue matte lines around the actors in several shots.

Live Action TV
"Eric Idle: That's clever, how did they do that? John Cleese: Colour Separation, you cottonhead."
 * ANT Farm uses this when Olive paints Chyna's bedroom wall green during a slumber party, Chyna uses Stock Footage from a movie for her background while video chatting with Lexi, to prove that her slumber party is cooler than Lexi's. Everyone in the background is at least 10 years older than Chyna and are dressed like they're in a nightclub. It works even when Olive puts on a hoodie the same color as the green wall, which blanks out everything except her head until Chyna covers it up. Lexi doesn't catch on until a ninja drops in and attacks the people in the nightclub.
 * Drew Carey's Green Screen was an improv comedy show that expanded on the green screen antics used on Whose Line Is It Anyway.
 * E!'s The Soup (and its predecessor Talk Soup) is videotaped entirely in front of a green screen. This led to a particularly memorable incident when a guest wore a pair of Italian flag briefs, which made it appear as if a portion of his pelvis was missing.
 * This is also used by a number of networks instead of sets for each of their programs, to save money; instead they just leave one camera rolling and change the background. Game Network and Friendly TV spring immediately to mind.
 * During one Academy Award ceremony, Ben Stiller came out on a full-body green suit to present the Best Visual Effects award, claiming that it made him invisible.
 * In an earlier ceremony, Steve Martin presented the same award wearing a blue mask and his trademark fake arrow through the head. That time the effect was done properly, and the audience saw a headless Martin with an arrow floating over him.
 * This is the method behind Shadow-Rama of Mystery Science Theater 3000 fame, by merit of how cheap the effect is to do.
 * Used in just about every 'field report' on The Daily Show to relocate correspondents to Baghdad, London or outer space.
 * Jon hung a lantern on this in the Crossover with The Colbert Report, Indicision 2008. After Obama won, a Correspondent who was supposed to be in the Middle East appeared in the Studio. His answer was along the lines of "Who cares; Obama won!"
 * The "field report" green screens have been lampshaded a lot ("You see, I'm in Jolly Old England, as you can tell by Big Ben behind me -- [looks over shoulder] sorry, the Houses of Parliament..."/"You know what I love about Sacramento? Their beautiful, stationary sky"), but the best instance is probably the inevitable aversion, where Jason Jones proved he really was in Denmark by... walking over and shoving some guy who was passing by.
 * In another a correspondent who had actually gone to the location had an argument with another, who interrupted their broadcast using Chroma Key to claim to be there as well.
 * The Colbert Report uses it for Formidable Opponent, in which Colbert debates himself; to create the other Colbert, in addition to mirroring the shot, the Chroma Key changes the background he's standing in front of, and the color of his tie.
 * Colbert also makes liberal use of the chroma key in his Green Screen Challenges. The first one involved him fighting whatever the contestants edited in with a lightsaber, the second one was an attempt to make John McCain interesting.
 * The first GSC became awesome when one of the entries was a professional Star Wars job from a "George L." Stephen then had George L make an appearance: The L stood for Lucas. His entry lost to a cheap appeal to patriotism, in keeping with the spirit of the show.
 * Spoofed in an early episode Late Night with Conan O'Brien. Andy Richter was invited to a news set as guest weatherman. Taking off his jacket, he becomes an Oracular Head as his blue shirt disappears into the chroma keyed backdrop.
 * Continues to be subverted in more recent episodes, with guest commentators doing "remotes." When they inevitably anger him, Conan walks over and clobbers them with a chair, revealing the ruse as he "leaves" the studio and appears in the "remote" location, all within the same splitscreen.
 * As mentioned above, old-school Doctor Who used this all the time, and the new series is pretty fond of green-screen too. Since the revival in 2005, it has been lavishly budgeted. Back in the seventies and eighties though? ...Not so much. You'd really have to stretch your suspension of disbelief there.
 * In an interview, Pertwee-era producer Barry Letts said that while the rest of the world calls it Chroma Key, "The BBC always insists upon calling it 'Color Separation Overlay'".
 * You'll hear this a lot on 70s Doctor Who DVDs. It's also referred to in an episode of Monty Python's Flying Circus, in a shot where the frame is split between live action and a Terry Gilliam cartoon.


 * Sanctuary is shot almost entirely in Chroma Key.
 * Seen from time to time on the Show Within a Show segments of iCarly. Relatively light on the Stylistic Suck, considering they're supposed to have been done live by an amateur on a Mac Pear laptop.
 * Lost has used green screens on occasion. In particular, most of the helicopter scenes in season 4 were done this way.
 * Green screens are, of course, commonly used in TV weather forecasts. Woe be to the weatherman who wears a green tie and finds Toronto on his chest, though it can be done for a joke too.
 * The early Disney Channel shows Welcome to Pooh Corner and Dumbo's Circus were made this way.
 * As mentioned above, it's a commonly used feature in Jim Henson productions.
 * The whole concept of Knightmare. The contestants wear a ridiculous helmet with instructions from their friends, because they would otherwise only be able to see a blue or green backdrop with a few physical props and people.
 * Used extensively and to great effect (sometimes) in Filmation's The Ghost Busters for the Ghost Dematerializer visual effects.
 * The British version of MTV 2 has a flagship show called Gonzo, consisting entirely of host Zane Lowe sat on a brown couch in front of a blue screen. The show is "as live" and therefore no corrections are made when items like guests' clothing match the background and disappear.
 * The PBS astronomy show Jack Horkheimer: Star Gazer is done entirely like this.
 * The Irish political debate show "Tonight with Vincent Browne" uses a ridiculously obvious blue screen. It shines on to the faces of guests or, in the case of the host, gives him a very strange blue afro.
 * While a lot of the scenes actually were filmed on location, in some of the cosmic calendar and Library of Alexandria scenes in Cosmos, you can see this effect around Carl Sagan. It's combined with Motion Control to make it look as if he's walking through model sets. Apart from a few full-size props the entire Library of Alexandria is actually a model, and the effect is highly convincing.
 * When Earl's list item of the week involved a television news anchor, he and Randy went down to the studio. Randy wandered into the green corner, wondering why it was there, when he noticed himself on the monitor, standing in front of a weather map. When he unzipped his jacket to reveal a green shirt, he freaked out, seeing himself as just a floating head and hands. He later figures it out enough to do a Pac-Man impression.
 * Super Sentai suffers this in some of the early series, particularly Dengeki Sentai Changeman at the end and sporadically throughout Choushinsei Flashman. By the time Hikari Sentai Maskman aired, the production staff no longer use it.
 * More recent Sentai series abuse this for finisher attacks to give off that anime effect. Most of the explosions and sparks are overlayed through Chroma Key nowadays as well.
 * The Price Is Right uses Chroma Key for mainly Clock Game and a few other parts of the show.
 * Shining Time Station used this for Mister Conductor, keying him in at a smaller scale to give the effect of a miniature man. It worked quite well, too.
 * Wheel of Fortune used to use it: an overhead shot of the Wheel, spinning automatically, would have hosts Chuck Woolery and Susan Stafford (and of course, successors Pat Sajak and Vanna White) chroma-keyed into its green center at the end of the show. Although this shot was long retired, the center of the Wheel is still green.
 * Some criticism of the third and fourth seasons of Chuck focused on less-than-convincing green screen sequences which became more prevalent due to the show's reduced effects budget.
 * On Cougar Town, Travis' college roommates have a green screen set up to make funny videos (and attract the chicks, somehow). It gets used by Bobby and Andy, and later Jules and Grayson.

Music

 * The music video Shine on Me by Chris Dane Owens abuses the hell out of it.
 * Or, as the page puts it, abuses it "like a free bag of heroin."
 * Yes's video for "Leave It", while groundbreaking for its time has some notable Chroma Key issues with the white shirts on the white background. (Most notably at 2:58 in the video)
 * Used in "Friday", as Rebecca Black herself Lampshaded in her appearance on "Funny Or Die". ("I'm talking about riding in a car with a 13-year-old driver, whether on the road or on a windless green screen cityscape.")
 * The video made to promote Strawbs' album Grave New World makes extensive use of Chroma Key, with no pretense at making the compositions look realistic. For instance, one scene shows a dancer performing in front of aerial Stock Footage of clouds, while another has the band hovering over Piccadilly Circus.
 * Tori Amos's videos tend to use a lot of Chroma Key effects, to the point where she once quipped "I seem to live my life on green screen."

Video Games

 * The FMV for Wing Commander III: Heart of the Tiger was filmed exclusively on greenscreen, predating Star Wars by a good half a decade (and stealing Mark Hamill from them while they were at it). WC4 made the shift to actual sets.

Web Animation

 * Homestar Runner makes fun of this in the Strong Bad E-Mail redesign, where Strong Bad imagines what his room would look like if he replaced the walls with a green screen. It backfired when he imagined frolicking through the bread aisle of a grocery store, as due to his eyes also being green, they vanished ("Oh, bread aisle! Warm me with your enriched, bleached bosom! And please, give me back my sight"). It then freaked him out when he saw the green-clad Coach Z walk in, appearing as nothing more than a floating head.
 * Technically, all of this is impossible, but keep in mind, this is also a series where computer viruses can bend "reality" by, presumably, spreading to the creators' machines and messing up the Flash file, not to mention Homestar states he's Behind the Black in almost every Strong Bad Email.

Web Original

 * How the no-budget spoof series The Jerry Seinfeld Program manages to take place in the apartment from the show.
 * Angry Joe uses it as his background.
 * The Spoony One will bring it out every once in a while for their skits in reviews.

Western Animation

 * Parodied on King of the Hill, Luanne is hired as weather girl not because of her acting, but because her conservative blue dress with a white top melds perfectly with the background.

Literature

 * Yep. You read that right. In this case, it's referred to in Tom Clancy's novel Debt of Honor, as a ruse.

Other

 * This video is an example of what can happen when certain clothing color mistakes are made when using this technique.
 * As alluded to above, news folk use this technique when presenting the weather, often using a special pointing device which the computer can track to appear to interact with the weatherboard by drawing lines and arrow. One particular segment has a host who enjoys taking further advantage of the technique: he wears a blue suit skeleton costume on Halloween.