Home Technology Green Screen and Chroma-Keying Technology

Green Screen and Chroma-Keying Technology

It is almost impossible that any of us here haven’t been amazed by Superhero movies/ sci-fi movies. But what is it that makes the movie appear more appealing and mind-blowing? Everyone is aware that the magic ingredient is Visual-Effects (also abbreviated as VFX). So, here a question for you: Do you know how exactly does VFX works?

The technology behind it involves the use of a Green Screen and Chroma Keying. Some of you might have seen actors using green coloured sheets during their behind the scenes clips.

The large Green Screen

The green screen is an integral part of the special effects process known formally as chromakey. Chromakey allows television producers and moviemakers to use advanced technology to superimpose their subjects onto an unlimited number of different virtual backgrounds.

Chromakeying, sometimes known as color keying, is the process of singling out a particular color in an electronic image and then using computer software to make that color transparent. This allows another image, which can be just about anything you can imagine, to show through.

So why is green such a popular color for screens? The answer to that question lies in the concept of contrast. The screen used must be different from — contrast with — the actors being filmed. The shade of bright green often used for green screens happens to be a color that very few actors are likely to wear, so it’s easier to isolate and make transparent.

Chroma-Keying

Chroma key compositing, or chroma keying, is a special effects/post-production technique for compositing (layering) two images or video streams together based on color hues (chroma range). The technique has been used heavily in many fields to remove a background from the subject of a photo or video – particularly the newscasting, motion picture and video game industries. A color range in the foreground footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. The chroma keying technique is commonly used in video production and post-production. This technique is also referred to as color keying, color-separation overlay (CSO; primarily by the BBC), or by various terms for specific color-related variants such as green screen, and blue screen – chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors. No part of the subject being filmed or photographed may duplicate the color used as the background.

Image Courtesy: Virtual Studio

It is commonly used for weather forecast broadcasts, wherein a news presenter is usually seen standing in front of a large CGI map during live television newscasts, though in actuality it is a large blue or green background. When using a blue screen, different weather maps are added on the parts of the image where the color is blue. If the news presenter wears blue clothes, his or her clothes will also be replaced with the background video. Chroma keying is also common in the entertainment industry for special effects in movies and video games.

A chroma key subject must avoid wearing clothes that are similar in colour to the chroma key color(s) (unless intentional e.g. wearing a green top to make it appear that the subject has nobody), because the clothing may be replaced with the background image/video. An example of intentional use of this is when an actor wears a blue covering over a part of his body to make it invisible in the final shot. This technique can be used to achieve an effect similar to that used in the Harry Potter films to create the effect of an invisibility cloak. The actor can also be filmed against a chroma-key background and inserted into the background shot with a distortion effect, in order to create a cloak that is marginally detectable.

In the 2002 film Spider-Man, in scenes where both Spider-Man and the Green Goblin are in the air, Spider-Man had to be shot in front of the green screen and the Green Goblin had to be shot in front of a blue screen. The color difference is because Spider-Man wears a costume that is red and blue in color and the Green Goblin wears a costume that is entirely green in color. If both were shot in front of the same screen, parts of one character would be erased from the shot.

For a clean division of foreground from background, it is also important that clothing and hair in the foreground shoot have a fairly simple silhouette, as fine details such as frizzy hair may not resolve properly. Similarly, partially transparent elements of the costume cause problems.

Blue is generally used for both weather maps and special effects because it is complementary to the human skin tone. The use of blue is also tied to the blue emulsion layer of the film having the finest crystals and thus good detail and minimal grain (in comparison to the red and green layers of the emulsion.) In digital filmmaking, however, green has become the favored color because digital cameras retain more detail in the green channel, and it requires less light than blue. Green not only has a higher luminance value than blue but also in early digital formats, the green channel was sampled twice as often as the blue, making it easier to work with. The choice of color is up to the effects artists and the needs of the specific shot. In the past decade, the use of green has become dominant in film special effects. Also, the green background is favored over blue for outdoor filming where the blue sky might appear in the frame and could accidentally be replaced in the process. Although green and blue are the most common, any color can be used.

Video Courtesy – ” Techquickie ” 

Further Reading and Reference – https://research.ijcaonline.org/gtetc/number1/gtetc1305.pdf

Abhijeet Mahatohttp://scilynk.in
Abhijeet is a 4th-year Undergraduate Student at IIT Kharagpur. His major inclination is towards exploring the science behind the things of our day-to-day life.

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